Proteins at Low Temperatures - American Chemical Society

4 Parameters of Freezing Damage to Enzymes WILLIAM N. FISHBEIN and JOHN W. WINKERT

Downloaded by CORNELL UNIV on August 23, 2016 | http://pubs.acs.org Publication Date: September 1, 1979 | doi: 10.1021/ba-1979-0180.ch004

Biochemistry Division, Armed Forces Institute of Pathology, Washington, DC 20306 An extensive evaluation of the several parameters involved in freezing damage to catalase has revealed a number of analogies to cellular systems, although the operative mechanisms differ. In all media, damage increased at slower warming rates and on progressive dilution of the enzyme. In phosphate buffer, it increased with faster freezing rates and with lower nucleation temperatures. In phosphate-buffered saline it also increased at very slow freezing rates, thus generating an optimum recovery at cooling rates of 1-20°/min. Here damage was progressive at —40° and was aggravated by low doses of common cryoprotectants, although not by polymers or oligosaccharides. Moreover, the sugars blocked the damaging effects of the cryoprotectants which in turn blocked the protective effects of the polymers, suggesting a hierarchy of biologic interactions by these agents. Several of these features have also been demonstrated with another enzyme, adenylate deaminase. Most of the findings can be explained by pH changes during freeze-thaw and by nonequilibrium phase transitions.

T i T o s t e n z y m e s c a n b e s t o r e d w i t h i m p u n i t y i n a n o r d i n a r y freezer w i t h l i t t l e r e g a r d f o r c o o l i n g a n d w a r m i n g rates, s e e d i n g , storage t e m p e r a t u r e , a n d t h e o t h e r p a r a m e t e r s k n o w n to b e o f i m p o r t a n c e i n t h e storage o f cells a n d tissues.

T h i s r e l a t i v e s i m p l i c i t y o f storage i n t h e

u s u a l case has r e s u l t e d i n a p a u c i t y of d e t a i l e d studies o f f r e e z i n g d a m a g e to e n z y m e s . W e h a v e u n d e r t a k e n s u c h a s t u d y w i t h t h e e n z y m e catalase f r o m the perspective of the cryobiologist. B y careful consideration of a l l of t h e factors w h i c h h a v e b e e n f o u n d t o b e i m p o r t a n t i n c o m p l i c a t e d c e l l u l a r systems, w e a r e s e e k i n g w h a t b e h a v i o r m a y b e o b s e r v e d i n t h e simplest possible biologic system, a n isolated macromolecule.

T h egen-

This chapter not subject to U . S . copyright Published 1979 American Chemical Society

Fennema; Proteins at Low Temperatures Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

56

PROTEINS A T L O W T E M P E R A T U R E S

e r a l i t y o f t h e observations t o f o l l o w is u n c e r t a i n , s i n c e o n l y catalase h a s b e e n s t u d i e d i n d e t a i l ; h o w e v e r , w e w i l l m e n t i o n o t h e r studies as a v a i l able, at appropriate points. T h e materials, methodology, a n d instrumentation used i n this w o r k h a v e b e e n d e t a i l e d e l s e w h e r e (1,2) a n d w i l l b e r e p e a t e d h e r e o n l y w h e n essential to t h e u n d e r s t a n d i n g o f specific e x p e r i m e n t s . T h e catalase, f r o m S i g m a C h e m i c a l C o . , s h o w e d a single b a n d o n a c r y l a m i d e g e l electrop h o r e s i s w i t h a n e s t i m a t e d m o l e c u l a r w e i g h t o f 250,000

(indicating

p e r s i s t e n c e o f t e t r a m e r i c s t r u c t u r e ) a t t h e l o w e s t c o n c e n t r a t i o n tested, 1 / x g / m L . A l l c o o l i n g a n d w a r m i n g rates u p t o 3 0 ° / m i n . w e r e l i n e a r l y


c o n t r o l l e d w i t h t h e a p p a r a t u s p r e v i o u s l y d e s c r i b e d ( I ) ; h i g h e r rates a r e average v a l u e s f o r t h e r a n g e specified. Studies in Neutral

Potassium

Phosphate

W e b e g a n this s t u d y u s i n g catalase d i l u t e d i n l O m M n e u t r a l p o t a s s i u m p h o s p h a t e buffer. E q u i l i b r i u m p h a s e d i a g r a m s f o r this b u f f e r s h o w a e u t e c t i c a t — 1 7 ° a n d n o m o r e t h a n 0.5 u n i t s p H c h a n g e t h r o u g h o u t the freezing range ( 3 ) . This allows us t o presume that a n y damage o c c u r r i n g f r o m f r e e z e - t h a w i s n o t d u e t o alterations i n p H . U s i n g a s t a n d a r d f r e e z e - t h a w p r o c e d u r e , w e first e v a l u a t e d t h e s e n s i t i v i t y o f t h e e n z y m e t o d a m a g e a t different c o n c e n t r a t i o n s as s h o w n i n F i g u r e 1.

20

30

40

50

60

70

CATALASE (micrograms per ml) Cryobiology

Figure 1. Effect of enzyme concentration on freeze-thaw inactivation of catalase. Catalase was frozen unseeded at 15°C/min to —78° and warmed at 10°C/minto +5.0° C (1).


4.

FisHBEiN AND wiNKERT

Damage

to

57

Enzymes

S e n s i t i v i t y t o d a m a g e m a r k e d l y i n c r e a s e d w i t h catalase d i l u t i o n as h a s b e e n o b s e r v e d f o r a n u m b e r o f other e n z y m e s

A l t h o u g h not

(4-11).

s h o w n i n F i g u r e 1, w e h a v e tested c o n c e n t r a t i o n s u p to 1 m g / m L w i t h o u t o b s e r v i n g a n y r e t u r n of f r e e z e - t h a w d a m a g e ; so s e n s i t i v i t y appears

to

increase m o n o t o n i c a l l y w i t h e n z y m e d i l u t i o n . W e s e l e c t e d a c o n c e n t r a t i o n of 1.7 / A g / m L , as s h o w n b y t h e a r r o w , for f u r t h e r studies, s i n c e this l e v e l d i s p l a y e d a n a p p r e c i a b l e a n d r e p r o d u c i b l e a c t i v i t y loss a n d w a s c o n v e n i e n t for assay. T o i n s u r e r e p r o d u c i b i l i t y of the s y s t e m the f o l l o w i n g a d d i t i o n a l factors w e r e t h e n e v a l u a t e d , a n d f o u n d t o h a v e n o i n f l u e n c e on the freezing damage:


f r e e z i n g n a d i r of

increase i n buffer c o n c e n t r a t i o n t o

lOOmM,

— 3 0 ° to — 1 9 6 ° , storage at — 3 0 ° or b e l o w f o r

10

m i n u t e s to 24 h o u r s , t h a w i n g t i m e at + 5 ° of 1 / 5 to 10 m i n u t e s , a n d postt h a w s t a n d i n g at 0 ° of 10 to 120 m i n u t e s . T h e next q u e s t i o n w a s t h e n a t u r e of the i n a c t i v a t i o n . A n u m b e r of e n z y m e s h a v e b e e n s h o w n to b e s u s c e p t i b l e t o c o l d i n a c t i v a t i o n o r w h a t t h e c r y o b i o l o g i s t m i g h t t e r m t h e r m a l shock (19,20),

(12-18), resulting

f r o m a f a l l i n t e m p e r a t u r e u n a c c o m p a n i e d b y a n y p h a s e changes.

This

p h e n o m e n o n is r e s t r i c t e d p r i m a r i l y to l a r g e o l i g o m e r s , a n d results f r o m d i s s o c i a t i o n , or r a r e l y , f r o m c o n f o r m a t i o n a l u n f o l d i n g (17).

Thermody-

n a m i c r e v e r s i b i l i t y has b e e n d e m o n s t r a t e d i n t h e case of t w o self-associa t i n g proteins ( n o n - e n z y m a t i c ) , t o b a c c o m o s a i c v i r u s c o a t p r o t e i n a n d c a l f - b r a i n t u b u l i n (22,23).

(21),

D e t a i l e d p h y s i c o c h e m i c a l analyses h a v e

b e e n p r e s e n t e d f o r these e n t r o p i e reactions.

O f course t h e same p h e -

n o m e n o n m i g h t also o c c u r o n f r e e z i n g of o t h e r m a c r o m o l e c u l e s , conseq u e n t to a s t i l l greater f a l l i n t e m p e r a t u r e p l u s t h e a t t e n d a n t increase i n solute l e v e l s ; a n a l y s i s , h o w e v e r , w o u l d b e m u c h m o r e difficult i n t h e presence of phase changes. We

i n t e r p r e t t h e f o l l o w i n g e x p e r i m e n t s to i n d i c a t e t h a t catalase

i n a c t i v a t i o n is i r r e v e r s i b l e , associated w i t h d e n a t u r a t i o n , a n d is t r u e f r e e z i n g d a m a g e n o t c o l d i n a c t i v a t i o n . F i r s t , o u r attempts t o

produce

damage b y dropping or cycling the temperature without freezing have b e e n u n s u c c e s s f u l ( s o m e of these w i l l b e m e n t i o n e d b e l o w ) .

Second,

p o s t - t h a w a c t i v i t y r e m a i n e d u n c h a n g e d after 24 hours s t a n d i n g at 0 ° . T h i r d , a c r y l a m i d e g e l electrophoresis of t h e e n z y m e t a k e n t h r o u g h freeze-thaw

cycles

to p r o d u c e

85%

s m e a r r e p l a c i n g t h e n o r m a l b a n d of subunit bands

damage,

showed

a

molecular weight

w e r e present, a l t h o u g h d i s s o c i a t i o n a n d

five

polydisperse 250,000.

No

reaggregation

m i g h t h a v e g e n e r a t e d the p o l y d i s p e r s i t y . T h e next factor i n v e s t i g a t e d w a s t h e w a r m i n g rate, w i t h a n d w i t h o u t p r i o r s e e d i n g b y a i r i n j e c t i o n of m i c r o s a m p l e s of f r o z e n b u f f e r at — 1° to — 2 ° . F o r completeness, F i g u r e 2 shows the results i n samples c o n t a i n i n g a d d e d K C 1 a n d a d d e d N a C l as w e l l as i n p h o s p h a t e b u f f e r alone.

The

patterns are essentially t h e same, a l t h o u g h there is greater d a m a g e i n



58


10

0.5

50

100

500

WARMING RATE (°C/MIN)

—ι— 1000

5000

Cryobiology

Figure 2. Warming-rate dependence of catalase inactivation in various solu tions after seeding and quenching in liquid nitrogen. Mean and SE are shown for 4-6 samples in each case. Rates were controlled from —20, —30, and — 50°C or lower for solutions containing phosphate only, KCl, and NaCl, respectively (20).

t h e presence o f a d d e d salt. T h e p a t t e r n w a s also t h e same w h e t h e r o r n o t the samples h a d b e e n seeded.

D a m a g e i n c r e a s e d p r o g r e s s i v e l y as

the w a r m i n g rate was decreased b e l o w 2 0 ° / m i n ;

a n d throughout our

studies this has r e m a i n e d the m o s t i m p o r t a n t e x t r i n s i c f a c t o r i n f r e e z e t h a w d a m a g e t o catalase. S i n c e w a r m i n g rates i n excess of 2 0 ° / m i n p r e v e n t e d e n z y m e d a m a g e , w e u t i l i z e d this feature t o e v a l u a t e the t e m p e r a t u r e r a n g e o v e r w h i c h damage occurred.

I n F i g u r e 3 A samples w e r e q u i c k l y f r o z e n at — 7 8 °

t h e n t r a n s f e r r e d to a — 20° a l c o h o l b a t h w h i c h w a s t h e n w a r m e d s l o w l y to v a r i o u s s u b z e r o t e m p e r a t u r e s , after w h i c h t h e tubes w e r e q u i c k l y


4.

FisHBEiN A N D wiNKERT

Damage

to

immersed and swirled i n a 37° water bath. is a b o u t 5 0 ° / m i n .

59

Enzymes

T h e resulting w a r m i n g rate

Damage was negligible w h e n slow-warming w a s

terminated at — 1 0 ° a n d progressed to 6 0 % w h e n i t was continued to + 2°.

I n Figure 3 B the approach was reversed w i t h slow

warming

initiated at some subzero temperature a n d continued u n t i l thawing w a s c o m p l e t e . T h i s e x p e r i m e n t is less exact b e c a u s e t h e t r a n s i t i o n f r o m r a p i d to s l o w w a r m i n g occurs g r a d u a l l y i n t h e d a m a g e z o n e , b u t i t does s h o w that 6 0 % d a m a g e w a s t h e m a x i m u m o b t a i n a b l e w h e t h e r s l o w w a r m i n g was b e g u n ( n o m i n a l l y ) at — 5 ° , —10°,

or —20°.

W e c a n , therefore,

c o n c l u d e t h a t t h e d a m a g e z o n e f o r catalase i n this m e d i u m extends n o


lower than - 7 ° or - 8 ° .

:— _ 4

-10

-5

Temperature

-10

-5

ο

Temperature °C

+5

Cryobiology

Figure 3. Temperature zone of freeze-thaw damage to catalase. Half-milliliter aliquots of the enzyme (1.7 ^g/ml) were frozen at 900°C/min without seeding and held at -78°C. In both graphs, the arrows show the temperature range over which slow warming (0.6°C/min) was carried out, and the dashed lines show the inactivation signature. (A) The tubes were transferred to a —20°C bath (warming rate ^ 50°C/min) and after equilibration were warmed at 0.6°C/min to various temperatures, after which pairs of tubes were rapidly immersed and swirled in a 30°C water bath (warming rate 30-50°C/min). Negligible inactivation occurred when slow warming was interrupted at — 10°C. Interruption at higher temperatures resulted in progressive inactivation to a maximum of 60% when slow warming was continued to + 2 ° C . (B) Pairs of tubes were transferred to a bath at various temperatures between —20 and + 2 ° C and sub sequently warmed to - f 5 ° C at 0.6°C/min. Average warming rate on transfer was ^ 50°C/min but this dropped sharply as the endpoint was approached, so that rates less than 5°C/min were present for several degrees below the indicated transfer tem perature. The maximum inactivation obtained was 60%, whether slow warming was initiated at +20, —10, or — 5°C. Initiation at higher temperatures resulted in pro gressively less damage (1).


60


T h e effect o f c o o l i n g r a t e w a s n e x t i n v e s t i g a t e d u s i n g a w a r m i n g r a t e o f 0 . 6 ° / m i n . I n t h e absence o f s e e d i n g , a constant l e v e l o f d a m a g e w a s o b t a i n e d at a l l c o o l i n g rates tested. W e n o t e d , h o w e v e r , t h a t s u p e r c o o l i n g i n c r e a s e d p r o g r e s s i v e l y at l o w e r c o o l i n g rates w i t h

spontaneous

nucleation occurring a t progressively lower temperatures.

W h e n this

f a c t o r w a s e l i m i n a t e d b y s e e d i n g a l l solutions at — 2 ° , a c l e a r c u t d e p e n d ence o f d a m a g e o n the c o o l i n g r a t e w a s o b s e r v e d as s h o w n i n F i g u r e 4. D a m a g e i n c r e a s e d w i t h c o o l i n g r a t e u p t o 5 ° / m i n after w h i c h i t p l a t e a u e d . T h i s i n d i c a t e d t h a t t h e r a t e o f f r e e z i n g , i.e., s o h d i f i c a t i o n , w a s a significant f a c t o r i n d a m a g e b u t w a s o b s c u r e d i n t h e absence o f s e e d i n g by

the progressive

supercooling

and lower

nucleation

temperatures


o c c u r r i n g a t l o w c o o l i n g rates. This was verified b y the experiment shown i n F i g u r e 5 where seeding w a s c a r r i e d o u t at p r o g r e s s i v e l y l o w e r t e m p e r a t u r e s . A l t h o u g h a l l samples w e r e c a r r i e d t h r o u g h the i d e n t i c a l f r e e z e - t h a w p r o c e d u r e , those

seeded

at — 1 1 ° s h o w e d t w i c e t h e d a m a g e s u s t a i n e d b y those s e e d e d at — 1 ° . M o r e o v e r , less t h a n 8 %

d a m a g e a p p e a r e d i n samples seeded a n d h e l d

at — 1° t h e n s l o w l y r e w a r m e d , a n d also i n samples s u p e r c o o l e d at — 1 1 ° a n d slowly rewarmed without freezing.

T h i s indicates that a phase

c h a n g e f o l l o w e d b y essentially c o m p l e t e s o h d i f i c a t i o n is i n v o l v e d i n t h e p r o d u c t i o n of d a m a g e to catalase.

60 r

50 Γ-

ΙΟ

h

0 I 0.1

J 0.5

I 1.0

ι

I 5.0

10

Cooling Rate °C/min

J

50

1 100

1

1

500

1000

Cryobiology

Figure 4. Cooling rate dependence of freeze-thaw inactivation of seeded catalase solutions. After seeding at —2°C samples were cooled at the rate noted to —25°C or below, held at -78°C, then warmed at 0.6°C/min from —25 to +S°C (1). 9



4.


01

1

-I

1

"2

1

-3

Damage

1

-4

1

-5

to

I

-6

61

Enzymes

I

-7

I

-8

Seeding Temperature °C

I

-9

L_

I

ÏO

-II

Cryobiology

Figure 5. Effect of seeding temperature on freeze-thaw inactivation of catalase. All samples were cooled at 0.8°C/min to the seeding temperature noted, and thence to —20°C, followed by warming at 0.7°C/min to + 4 ° C . The solid line is least-squares best-fitting parabola (1). T h e possibility of t h e r m a l shock attending the increasing concentrat i o n o f p h o s p h a t e o n f r e e z i n g was i n v e s t i g a t e d b y i n c u b a t i n g catalase i n a saturated solution of neutral potassium phosphate at r o o m temperature (2.2M)

a n d also a t 2 ° w h e r e a c o n s i d e r a b l e a m o u n t o f p h o s p h a t e

p r e c i p i t a t e d . T a b l e I shows t h a t some i n a c t i v a t i o n d i d o c c u r i n s t a t i o n a r y samples b u t d i d n o t differ s i g n i f i c a n t l y a t 2 ° a n d 2 3 ° . I n s t i r r e d s a m p l e s , h o w e v e r , d a m a g e w a s severe a n d s i g n i f i c a n t l y w o r s e a t 2 3 ° , w h e r e n o p r e c i p i t a t e w a s present a n d therefore, e n z y m e a d s o r p t i o n a n d p H changes c o u l d n o t h a v e c o n t r i b u t e d t o a c t i v i t y loss. S i n c e i n a c t i v a t i o n does n o t Table I.

Percent Recovery of A c t i v i t y " 2°

Stationary Magnetic stirring

74.4 ± 5.5 34.1 ± 1.1

23° 81.7 ± 4.2 18.5 ± 1.0

" A c t i v i t y recovered from solutions of 1.7 μ% catalase/mL 22M potassium phos phate, p H 7.0 after 2.5 hrs incubation at two temperatures, with or without gentle magnetic stirring, using a large external water bath to maintain constant tempera tures. M e a n ± S E are shown for 3 tubes in each experiment. The solution was clear at 23°, but a heavy precipitate was present at 2 ° . Supernatant aliquots were taken directly from the incubating tubes for assay at 23° in all cases.


62

PROTEINS A T L O W

TEMPERATURES

o c c u r o n s t i r r i n g solutions o f catalase i n d i l u t e p o t a s s i u m p h o s p h a t e , w e c o n c l u d e first, t h a t h i g h p h o s p h a t e levels c a n p r o d u c e significant i n a c t i v a t i o n of the e n z y m e i n s o l u t i o n , p r o b a b l y d u e to the h i g h i o n i c s t r e n g t h ; s e c o n d , t h a t a d r o p i n t e m p e r a t u r e m a k e s n o c o n t r i b u t i o n to t h i s process; a n d t h i r d , t h a t s h e a r i n g o r i n t e r f a c i a l forces b e t w e e n t h e p r o t e i n a n d the s o l u t i o n m a k e a l a r g e c o n t r i b u t i o n to this process.

S t i r r i n g is n o t u s e d

d u r i n g o u r f r e e z e - t h a w p r o c e d u r e s b u t these results suggest t h a t attempts to i n c r e a s e t h e t h a w i n g rate b y s t i r r i n g m a y i n t r o d u c e a n o t h e r c o m p o n e n t of damage. S i n c e t h e s e n s i t i v i t y of catalase to f r e e z e - t h a w d a m a g e increases w i t h d i l u t i o n , i t s e e m e d r e a s o n a b l e t o p r e s u m e t h a t t h e d a m a g i n g effect of Downloaded by CORNELL UNIV on August 23, 2016 | http://pubs.acs.org Publication Date: September 1, 1979 | doi: 10.1021/ba-1979-0180.ch004

r a p i d s o l i d i f i c a t i o n w a s d u e to t h e t r a p p i n g of the m a c r o m o l e c u l e s i n t h e m a t r i x at l o w c o n c e n t r a t i o n s , w h e r e a s s l o w c o o l i n g w o u l d p e r m i t t h e i r c o n c e n t r a t i o n a n d thus r e n d e r t h e m resistant to d a m a g e .

I f this w e r e so,

t h e n sufficiently r a p i d c o o l i n g rates m i g h t b e e x p e c t e d t o p r o d u c e e q u i v a l e n t d a m a g e at h i g h e r concentrations of catalase.

W e w e r e u n a b l e to

d e m o n s t r a t e t h i s , h o w e v e r , o v e r a n 8 - f o l d r a n g e of catalase concentrations w i t h c o o l i n g rates u p to 1 0 0 0 ° / m i n .

A l t h o u g h damage increased w i t h

c o o l i n g rate, i t r e m a i n e d a s t r o n g f u n c t i o n of t h e i n i t i a l catalase c o n c e n t r a t i o n at e v e r y c o o l i n g rate. W e m u s t e m p h a s i z e at t h i s p o i n t , the d o m i n a n t effect of w a r m i n g r a t e as c o m p a r e d to c o o l i n g r a t e i n the p r o d u c t i o n of d a m a g e t o catalase. A l t h o u g h t h e c o o l i n g r a t e p a t t e r n s h o w n is c l e a r cut, i t w a s e l i m i n a t e d i f t h e w a r m i n g r a t e w a s r a p i d , w h e r e u p o n n o d a m a g e is e v i d e n t .

I n con-

trast, a s l o w w a r m i n g r a t e e l i c i t e d d a m a g e regardless of t h e p r i o r c o o l i n g rate. A l t h o u g h extensive studies i n s o d i u m p h o s p h a t e w e r e n o t u n d e r t a k e n , w e d i d t r y , w i t h o u t success, to d u p l i c a t e S h i k a m a ' s studies o n catalase i n l O m M neutral sodium phosphate ( 6 ) .

H e f o u n d a constant loss o f 2 0 %

a c t i v i t y o n q u e n c h i n g samples for 10 m i n or l o n g e r at a n y t e m p e r a t u r e f r o m — 10° t o — 8 0 ° , e v e n i f this f o l l o w e d p r e f r e e z i n g at l o w e r t e m p e r a tures w h i c h d i d n o t cause d a m a g e .

W e are u n a b l e t o a c c o u n t satisfac-

t o r i l y f o r this d i s c r e p a n c y . Studies in Media Containing

Buffer and Salts

T h e rate p a t t e r n of the d a m a g e , n a m e l y , i n c r e a s i n g w i t h c o o l i n g r a t e a n d decreasing w i t h w a r m i n g rate was unexpected a n d w o u l d be

con-

s i d e r e d b y m o s t c r y o b i o l o g i s t s to i n d i c a t e i n t r a c e l l u l a r f r e e z i n g .

Now

t h a t i n t e r p r e t a t i o n is n o t so p e c u l i a r as i t sounds b e c a u s e the e n z y m e is i n d e e d exposed

to a n y i c e t h a t is f o r m e d , just as t h o u g h b o t h

were

p r e s e n t w i t h i n a c e l l . H o w e v e r , i t is also o b v i o u s t h a t n o n e o f t h e u s u a l c o m p a r t m e n t a l explanations f o r this p a t t e r n i n cells, s u c h as o s m o t i c or


4.


Damage

to

63

Enzymes

m e m b r a n e d a m a g e , c a n b e i n v o k e d i n t h i s system.

The

cryobiologist

w o u l d argue t h a t d a m a g e d u e to c o n c e n t r a t i n g solutes d u r i n g f r e e z i n g s h o u l d i n s t e a d p r o d u c e d a m a g e t h a t decreases w i t h i n c r e a s i n g c o o l i n g rate. A c c e p t i n g t h a t a r g u m e n t , w e w o u l d s a y t h a t the d a m a g e to catalase m u s t n o t b e d u e to c o n c e n t r a t i n g solute, a n d f u r t h e r , u p o n a d d i n g this f a c t o r ( b y a d d i n g a n e u t r a l salt to t h e s y s t e m ) w e o u g h t to b e a b l e to i n d u c e a n o p t i m u m c o o l i n g rate c u r v e s i m i l a r t o those c o n s i d e r e d t y p i c a l of c e l l u l a r systems (24).

W e t r i e d a d d i t i o n s of 2 7 m M K C 1 a n d N a C l to

o u r s t a n d a r d p o t a s s i u m p h o s p h a t e buffer.

T h e e q u i l i b r i u m phase

dia-

g r a m s f o r these systems h a v e also b e e n d e s c r i b e d a n d i n d i c a t e , first, t h a t


t h e y s h o u l d b o t h b e f u l l y s o l i d i f i e d b y — 2 3 ° ; a n d second, t h a t t h e N a C l system w o u l d b e c o m e a c i d d u r i n g f r e e z i n g w h e r e a s the K C 1 system w o u l d not

(25). W e d e c i d e d to i n d e p e n d e n t l y test the n a d i r of the d a m a g e z o n e f o r

catalase i n e a c h s y s t e m b y storage a t v a r i o u s s u b z e r o t e m p e r a t u r e s f o r p e r i o d s of t w o t o f o u r days f o l l o w e d b y r a p i d w a r m i n g . I n the p o t a s s i u m c h l o r i d e - p h o s p h a t e system t h e n a d i r f o r i n c r e m e n t a l d a m a g e about

— 2 0 ° , i n agreement w i t h the p h a s e d i a g r a m s .

I n the

was

sodium

chlorides-phosphate system, h o w e v e r , the n a d i r w a s a b o u t — 4 5 ° , m u c h l o w e r t h a n expected.

These nadirs defined the temperature range over

w h i c h w e w o u l d h a v e to c o n t r o l the c o o l i n g a n d w a r m i n g rates to i n v e s t i gate e a c h m e d i u m . T h e p o t a s s i u m c h l o r i d e - p h o s p h a t e m e d i u m w a s e x p l o r e d first, since i t i n v o l v e d no p H c h a n g e that m i g h t c o n t r i b u t e to d a m a g e . U s i n g a fixed w a r m i n g rate ( 0 . 5 ° / m i n ) , d a m a g e w a s h i g h e r at a l l c o o l i n g rates t h a n i n p h o s p h a t e a l o n e b u t t h e increase w a s greater at t h e l o w e r c o o l i n g rates t o y i e l d a flat response.

T h u s d a m a g e a p p e a r e d to b e i n d e p e n d e n t o f t h e

c o o l i n g rate despite s e e d i n g . W e p r o c e e d e d n e x t to t h e s o d i u m c h l o r i d e - p h o s p h a t e m e d i u m , t h e results of w h i c h are s h o w n i n F i g u r e 6. T h e s e results p r o v i d e a n u n e q u i v ocal demonstration of a n o p t i m u m recovery cooling rate curve for

a

s i m p l e s o l u b l e p r o t e i n . I n t h e absence o f salt, d a m a g e i n c r e a s e d i n t h i s e x p e r i m e n t as the c o o l i n g rate w a s i n c r e a s e d f r o m 1° to 2 5 ° / m i n a n d w a s r e l a t i v e l y constant outside this r a n g e .

O n a d d i t i o n of

3-9mM

NaCl,

d a m a g e i n c r e a s e d at a l l rates b u t e s p e c i a l l y so at v e r y l o w rates to y i e l d a n o p t i m u m r e c o v e r y o r m i n i m u m a c t i v i t y loss at 0 . 5 ° / m i n . W i t h 2 7 m M N a C l , d a m a g e i n c r e a s e d f u r t h e r b u t m o s t s t r i k i n g l y at l o w c o o l i n g rates a n d the o p t i m u m became more p r o n o u n c e d a n d shifted to 5 ° / m i n . 81mM

NaCl,

added

damage

b e c a m e q u i t e severe

but the

With

optimum

r e m a i n e d d i s t i n c t a n d s h i f t e d f u r t h e r to 2 0 ° / m i n . A s w i t h c e l l u l a r systems, w e w o u l d a r g u e t h a t t h e presence of a n o p t i m u m i n d i c a t e s t w o factors i n v o l v e d i n f r e e z i n g d a m a g e , one o p e r a t i n g at h i g h c o o l i n g rates a n d t h e o t h e r at l o w c o o l i n g rates. T h e n a t u r e o f



64


COOLING RATE ( c/MIN) e

Cryobiology

Figure 6. Cooling-rate dependence of catalase inactivation in lOmM neutral KHPO4 solutions containing various concentrations of NaCl. All solutions were seeded, cooled at stated rates to — 50°C or lower, and then warmed at 0.5°C/ min from — 50°C. Mean and SE are shown for four to six samples in each case (20). t h e h i g h c o o l i n g rate f a c t o r is s t i l l o b s c u r e b u t the l o w c o o l i n g r a t e f a c t o r m u s t b e salt or s o m e c o n s e q u e n c e o f salt, since w e c a n a d d or r e m o v e i t at w i l l . A s w e freeze w i t h h i g h e r s t a r t i n g levels o f salt, t h e i n t e r m e d i a t e l i q u i d u s c o n c e n t r a t i o n s m u s t increase, p r o d u c i n g m o r e d a m a g e at l o w c o o l i n g rates a n d s h i f t i n g t h e o p t i m u m o r m i n i m u m to t h e r i g h t . N o t e , h o w e v e r , t h a t d a m a g e at h i g h c o o l i n g rates i s also i n c r e a s e d i n t h e presence o f salt so t h a t i f a separate f a c t o r i s i n v o l v e d , i t i s a c c e n t u a t e d nevertheless b y t h e p r e s e n c e o f salt.

A t l o w c o o l i n g rates

d a m a g e to catalase w a s m u c h greater i n N a C l t h a n i n K G , a l t h o u g h t h e eutectic c o m p o s i t i o n s o f t h e t w o salts differ l i t t l e . T h e d a m a g e therefore m a y be d u e n o t solely to c o n c e n t r a t i n g salt b u t to e i t h e r of t h e a d d i t i o n a l factors o p e r a t i n g o n l y i n N a C l s o l u t i o n s : a c i d i f i c a t i o n d u r i n g f r e e z i n g a n d the m u c h broader temperature zone of damage. Cryoprotectant

Effects

W e w i l l first e x p l o r e i n m o r e d e t a i l t h e u n e x p e c t e d l y b r o a d d a m a g e z o n e , a g a i n m a k i n g use o f t h e f a c t t h a t r a p i d w a r m i n g p r e v e n t s d a m a g e . A l i n e a r r a t e - c o n t r o l l e d a l c o h o l b a t h is w a r m e d at 0 . 5 ° / m i n s t a r t i n g a t


4.


Damage

to

65

Enzymes

— 7 0 ° . S e e d e d samples q u e n c h e d i n l i q u i d n i t r o g e n a n d s t o r e d at — 8 0 ° are t r a n s f e r r e d to t h e b a t h at 10° i n t e r v a l s , w a r m e d s l o w l y f o r 1 0 ° , a n d t h e n t r a n s f e r r e d to a 3 7 ° w a t e r b a t h f o r r a p i d t h a w i n g . T h u s w e c a n e v a l u a t e t h e r e l a t i v e effect of a n a d d e d c r y o p r o t e c t a n t i n v a r i o u s s u b z e r o t e m p e r a t u r e zones.

T h i s c o n s i d e r a t i o n h a d b e c o m e i m p o r t a n t since i n

p r e l i m i n a r y e x p e r i m e n t s w e h a d o b s e r v e d a n e t increase i n d a m a g e at l o w , b u t n o t at h i g h e r , levels of cryoprotectants.

A similar observation

h a d b e e n m a d e i n 1973 b y W h i t t a m a n d R o s a n o d u r i n g f r e e z i n g studies of α-amylase, also i n p h o s p h a t e - b u f f e r e d s a l i n e (26)

but their study d i d

n o t address t h e z o n a l l o c a t i o n of t h e d a m a g e .


F i g u r e 7 shows

the effect of

adding diglyme

(diethylene

glycol

d i m e t h y l e t h e r ) to the l O m M p h o s p h a t e - 2 7 m M N a C l m e d i u m . T e n m M d i g l y m e w a s a n effective p r o t e c t a n t of catalase i n a l l zones except t h a t near — 4 0 ° , where it p r o d u c e d m a r k e d enzyme damage.

The

weight

r a t i o of c r y o p r o t e c t a n t to salt is less t h a n 1, so i t s h o u l d n o t s i g n i f i c a n t l y affect

the eutectic

temperature according

to

ternary phase

diagram

studies. M o r e o v e r the d a m a g e m u s t not b e a d i r e c t t o x i c effect of d i g l y m e because a 5-fold h i g h e r dose e l i m i n a t e s t h e d a m a g e l e a v i n g i n s t e a d a z o n e of w e a k p r o t e c t i o n .

T h e same

findings

were obtained w i t h

the

same c o n c e n t r a t i o n of d i m e t h y l s u l f o x i d e ( D M S O ) a n d of g l y c e r o l , t h e t w o most w i d e l y u s e d c r y o p r o t e c t a n t s . O n l y o c c a s i o n a l l y w a s the d a m a g e n e a r — 4 0 ° severe e n o u g h to y i e l d n e t d a m a g e i n a f u l l - r a n g e f r e e z e - t h a w e x p e r i m e n t ; i n the u s u a l case the net effect w a s s l i g h t p r o t e c t i o n .

We

e m p h a s i z e , therefore, t h a t ineffective c r y o p r o t e c t i o n m a y r e s u l t f r o m t h e i n t e r p l a y of t w o o p p o s i n g effects p r o d u c e d b y t h e same agent, at least i n phosphate-buffered

saline: protection i n one temperature zone

and

d a m a g e i n another t e m p e r a t u r e z o n e . C o m p a r i s o n w i t h a m a c r o m o l e c u l a r c r y o p r o t e c t a n t is s h o w n i n F i g u r e 8. P V P ( p o l y v i n y l p y r r o l i d o n e ) is m a r k e d l y p r o t e c t i v e i n a l l t e m p e r a t u r e zones w h e r e a s d i g l y m e p r o d u c e s m a r k e d d a m a g e i n t h e z o n e n e a r — 4 0 ° . I f t h e t w o agents act v i a the same m e c h a n i s m , t h e y s h o u l d b e f u n c t i o n a l l y a d d i t i v e a n d a m i x t u r e s h o u l d s h o w n o z o n e of excess d a m a g e

whereas

i n f a c t d i g l y m e p r o d u c e d almost as m u c h d a m a g e i n t h e presence of P V P as i n its absence.

P o l y e t h y l e n e g l y c o l at m o l e c u l a r w e i g h t s 20,000 a n d

4 m i l l i o n b e h a v e d q u i t e a n a l o g o u s l y to P V P , w h i l e D M S O a n d g l y c e r o l behaved like diglyme. T h u s macroprotectants w e r e rendered inoperative i n t h e presence o f l o w - m o l e c u l a r - w e i g h t agents. N o w i f the d a m a g e z o n e i n v o l v e s a persistent l i q u i d u s , as seems l i k e l y , t h e n w e s h o u l d b e a b l e to s i m p l i f y f u t u r e experiments b y q u e n c h i n g s a m p l e s a n d s t o r i n g t h e m at — 3 5 ° to — 4 0 ° f o r p r o g r e s s i v e i n t e r v a l s b e f o r e fast w a r m i n g , thus y i e l d i n g greater d a m a g e w i t h less effort.

We

also w i l l scale u p o u r c o n c e n t r a t i o n s to m a i n t a i n t h e same ratios w h i l e p r o v i d i n g t h e 1 % s a l i n e l e v e l t h a t is u s e d i n m o s t p h a s e d i a g r a m studies.


66

PROTEINS A T L O W

TEMPERATURES

100-


90H

-60H -70-80-90-100"70

40

=60 =50 =30 =20 TEMPERATURE DECADE OF SLOW WARMING

=10 (°C )

Figure 7.

0

Effect of diglyme on recovery of catalase activity after slow warming through discrete subzero temperature zones. Tubes contained 1.7 μ% catalase I ml lOmM KPO* (pH 7.0) + 27mM NaCl ± lOmM or 50mM diglyme, and were seeded at —1.5° and quenched in alcohol at —80° before warming. Pairs of tubes with and without diglyme were transferred to a linear-rate warming bath (0.5°/min) at the initial temperature of each decade and after the 10° interval was traversed, they were removed and thawed rapidly in 37° water bath. The % effect was calculated as 100 (A — B)/A, where A = activity lost with diglyme absent, and Β = activity lost with diglyme present.


4.

FisHBEiN AND WTNKERT

Damage

to

67

Enzymes

100 908070-


605040301

20 104

g

04

a-

= §-2CH β Q.

Diglyme 1.34 mg/ml

5

PVP 10pg/ml

g g

-30

83

Expected Summation

-40-1

•

Actual for Combination

Ξ

-50

-80 -90-100 - 7 b — = * -20 4Ô—3

Proteins at Low Temperatures - American Chemical Society

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