The Metabolism of Cyclic Nitrosamines - ACS Symposium Series

Dec 9, 1981 - Investigations of the in vitro and in vivo metabolism of the cyclic nitrosamines N-nitrosopyrrolidine, N'-nitrosonornicotine, N-nitrosop...
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4 The Metabolism of Cyclic Nitrosamines S T E P H E N S. H E C H T , G . D A V I D M c C O Y , C H I - H O N G B . C H E N , and D I E T R I C H H O F F M A N N

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Naylor Dana Institute for Disease Prevention, American Health Foundation, Valhalla, NY 10595

Investigations of the in vitro and in vivo metabo­ lism of the cyclic nitrosamines N-nitrosopyrrolidine, N'-nitrosonornicotine, N-nitrosopiperidine, N-nitrosohexamethyleneimine, N-nitrosomorpholine, and related compounds are reviewed. Each of these compounds undergoes metabolic α-hydroxylation lead­ ing to electrophilic diazohydroxide intermediates which may act as ultimate carcinogens. Metabolism by β-hydroxylation has been established for N-nitrosopyrrolidine, N'-nitrosonornicotine, N-nitroso­ hexamethyleneimine, N-nitrosomorpholine, and 2,6dimethyl-N-nitrosomorpholine.With the possible ex­ ception of 2,6-dimethyl-N-nitrosomorpholine, a v a i l ­ able evidence suggests that β-hydroxylation is not a major activation pathway . γ -Hydroxylation has been observed i n those cases where it is possible, namely with N-nitrosopiperidine and N-nitrosohexa­ methyleneimine. Its role i n carcinogen activation has not been established. The general patterns of cyclic nitrosamine metabolism in laboratory animals have been established, but little data are a v a i l ­ able on the carcinogen-DNA adducts formed from these compounds or on the mechanisms of their organ specificity. Cyclic nitrosamines are among the most potent and environ­ mentally significant nitrosamine carcinogens. Like the acyclic nitrosamines, metabolism i s necessary for their carcinogenicity. Elucidation of the specific metabolic pathways of cyclic nitros­ amine activation and detoxification is a challenging problem, and considerable progress has been achieved i n recent years. In this chapter, we w i l l review metabolic studies on N-nitrosopyrrolidine (NPYR), N -nitrosonornicotine (NNN), N-nitrosopiperidine (NPIP), N-nitrosohexamethyleneimine (NHEX), N-nitrosomorpholine (NMOR), f

0097-6156/81/0174-0049$06.75/0 © 1981 American Chemical Society Scanlan and Tannenbaum; N-Nitroso Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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N-NITROSO COMPOUNDS

and r e l a t e d compounds and w i l l consider the r e l a t i o n s h i p of metabolic pathways to c a r c i n o g e n i c i t y and mutagenicity i n those cases where data are a v a i l a b l e . Studies on c y c l i c nitrosamine metabolism i n v i v o and in v i t r o are summarized i n Tables I and II.

0

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9 NPYR

N=0

NNN

Ο

NPIP

Ο

NO

N*0

NHEX

NMOR

N - N i t r o s o p y r r o l i d i n e (NPYR) NPYR occurs i n processed meats and, most commonly, i n cooked bacon ( J ^ ^ ) . NPYR i s one of the p r i n c i p a l v o l a t i l e nitrosamines detected i n mainstream and s ides tream c i g a r e t t e smoke (4,_5) . I t induces h e p a t o c e l l u l a r carcinomas i n r a t s , even at r e l a t i v e l y low doses, and tumors of the nasal c a v i t y and trachea i n S y r i a n golden hamsters ( 6 , 7 ^ 8 ) . The metabolism o f NPYR i s summarized i n F i g u r e 1. a-Hyd r o x y l a t i o n ( 2 o r 5 . p o s i t i o n ) leads to the unstable intermediates l_ and _4; decomposition of 4^ gives 4-hydroxybutyraldehyde [6]. The l a t t e r , which e x i s t s predominantly as the c y c l i c hemiacetal 1^ has been detected as a h e p a t i c microsomal metabolite i n r a t s , hamsters, and humans and from lung microsomes i n r a t s ( 9 - 1 3 ) . The r o l e of 1^ and «4 as intermediates i n the formation of 6^ and ^7 i s supported by s t u d i e s of the h y d r o l y s i s of 2-acetoxyNPYR and 4-(N-carbethoxy-N-nitrosamino)butanal, which both gave high y i e l d s of _7 ( 9 , 1 4 ) . I n microsomal i n c u b a t i o n s , 6^ can be r e a d i l y q u a n t i f i e d as i t s 2,4-dinitrophenylhydrazone d e r i v a t i v e ( 1 5 ) . The l a t t e r has a l s o been detected i n the u r i n e of r a t s t r e a t e d w i t h NPYR ( 9 ) . Post-microsomal supernatants convert 6_ and ]_ t o 8^, 9^, and 10; 9^ i s g e n e r a l l y the major m e t a b o l i t e observed from NPYR under these c o n d i t i o n s ( 1 2 , 1 3 ) . Compounds J3 and j ) , as w e l l as s u c c i n i c -

Scanlan and Tannenbaum; N-Nitroso Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Scanlan and Tannenbaum; N-Nitroso Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

N.R.

dimethylamine (1.3-0.5, 24) 16 3-hydroxyNPYR (0.30-0.78, 24) 2 - p y r r o l i d i n o n e (0.04-.0.78, 24) s u c c i n i c semialdehyde (0.1, 24) 4-hydroxybutyric a c i d γ-butyrolactone 2 - p y r r o l i d i n o n e oxime ( 0 . 9 , 2 4 )

N.R. N.R.

49 ( 6 ) 24 (24)

6

6-500

Rat

Rat

NPYR

NPYR

b

4-hydroxybutanal (as 2 , 4 - d i nitrophenylhydrazone) (0.09, 48)

N.R.

18

9

17

N.R.

NPYR

a

582

Rat

NPYR

Rat

20

N.R.

6

Rat

NPYR 11 (24) N.R.

3-hydroxyNPYR ( φ •Η Η •H 1-3 •Η

CO φ Ό •H Φ U Φ d P•4JH CO C

Φ CO 3

CO

Ο u M M 0)

i-l XI Heu

ΌCO α PCS ο Pu S 6 ο ο

Φ

c

•H N CO u φ a •H a ο CO ο U β •H Ό Ν

è Ω C O

Scanlan and Tannenbaum; N-Nitroso Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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N-NITROSO COMPOUNDS

Scanlan and Tannenbaum; N-Nitroso Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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4.

HECHT ET AL.

Metabolism

of Cyclic

Nitrosamines

61

semialdehyde, have been detected as u r i n a r y m e t a b o l i t e s of NPYR (16). Metabolism o f 9^ leads t o CO2, which i s the major metabo­ l i t e of NPYR i n the r a t (9,16,17^ 1£). The h e p a t i c microsomal α-hydroxylase a c t i v i t y f o r NPYR i s i n d u c i b l e i n r a t s by pretreatment w i t h A r o c l o r , and i n hamsters by pretreatment w i t h A r o c l o r , 3-methylcholanthrene, phénobarbi­ t a l , and ethanol (10,15,19). I n c o n t r a s t , pretreatment o f r a t s w i t h 3-methylcholanthrene or phénobarbital causes no change or a s l i g h t decrease i n microsomal NPYR α-hydroxylase a c t i v i t y (19). 3-HydroxyNPYR [2] has been i d e n t i f i e d as a u r i n a r y metabol­ i t e of NPYR i n the r a t (up to 1% of the dose), but was not detec­ ted when NPYR was incubated with s u b c e l l u l a r f r a c t i o n s from r a t l i v e r and lung (13, 20). I t has been proposed that f u r t h e r meta­ bolism of 2_ leads to dime thy lamine, [_5], another u r i n a r y metabol­ i t e of NPYR. 2 - P y r r o l i d i n o n e [3] has a l s o been detected i n the u r i n e o f r a t s t r e a t e d w i t h NPYR. I t s o r i g i n has not been con­ c l u s i v e l y e s t a b l i s h e d , but i t may form from pyrrolidinone-2-oxime (16). Current evidence favors α-hydroxylation as a major a c t i v a ­ t i o n pathway f o r NPYR. The model compounds, 2-acetoxyNPYR and 4-(N-carbethoxy-N-nitrosamino)butanal are both h i g h l y mutagenic towards JS. typhimurium without enzymatic a c t i v a t i o n ( 2 1 , 2 2 ) . This a c t i v i t y i s probably a r e s u l t of conversion to the e l e c t r o p h i l i c diazohydroxide 1_, or to the corresponding carbonium i o n . Inducers which increase the rates of microsomal a-hydroxylation of NPYR a l s o increase i t s mutagenicity toward _S. typhimurium (10, 19) and, i n the case of ethanol, i t s c a r c i n o g e n i c i t y toward S y r i a n golden hamsters (8). I n a d d i t i o n , 2,5-dimethylNPYR i s l e s s c a r c i n o g e n i c than NPYR (23). This observation may support the r o l e o f α-hydroxylation i n the a c t i v a t i o n o f NPYR but i s l i m ­ i t e d by the f a c t that d i f f e r e n t enzymes may c a t a l y z e the hydroxyl a t i o n of secondary and t e r t i a r y α-carbon atoms, as described below f o r NNN. 3-HydroxyNPYR i s l e s s carcinogenic i n r a t s than i s NPYR, which i n d i c a t e s that t h i s m e t a b o l i t e i s a product o f d e t o x i f i c a t i o n (24). I f α-hydroxylation o f NPYR i s i t s mechanism of a c t i v a t i o n , one would expect the formation of carcinogen-DNA adducts c o n t a i n ­ ing a 4-oxobutyl- or r e l a t e d residue. Adducts have been i s o l a t e d from t h e l i v e r RNA of NPYR t r e a t e d r a t s , but t h e i r s t r u c t u r e s have not been determined (25). Carcinogen DNA adducts have a l s o been i s o l a t e d from c u l t u r e d human esophagus, c o l o n , and bronchus (26, 27, 28). These studies on NPYR are t y p i c a l o f the s t a t e of the a r t i n c y c l i c nitrosamine metabolism and a c t i v a t i o n . The major meta­ b o l i c pathways have been rather w e l l c h a r a c t e r i z e d , but data on the r e l a t i o n s h i p of these pathways to carcinogenesis are l i m i t ­ ed. This i s e s p e c i a l l y true of the o r g a n o s p e c i f i c e f f e c t s of NPYR and the other c y c l i c nitrosamines. For example, the main target organs f o r NPYR i n the S y r i a n golden hamster are the trachea and nasal c a v i t y rather than the l i v e r . This i s i n s p i t e

Scanlan and Tannenbaum; N-Nitroso Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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N-NITROSO COMPOUNDS

of the fact that e l e c t r o p h i l i c agents are generated i n the ham­ s t e r l i v e r by α-hydroxylation. To understand the reasons f o r t h i s , more work i s necessary on target organ metabolism of NPYR and on the formation and p e r s i s t e n c e of s p e c i f i c DNA adducts.

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Ν'-Nitrosonornicotine (NNN) NNN i s f o r m a l l y a d e r i v a t i v e o f NPYR, but the p y r i d i n e r i n g has a marked e f f e c t on i t s metabolism and c a r c i n o g e n i c i t y . NNN induces lung adenomas i n mice, esophageal and nasal c a v i t y tumors i n r a t s , and t r a c h e a l and nasal c a v i t y tumors i n S y r i a n golden hamsters (29, 30, 31). I t s tumorigenic a c t i v i t y i n Syrian golden hamsters i s only s l i g h t l y l e s s than that of NPYR (8). NNN i s important because of i t s r e l a t i v e l y high concentrations i n mainstream and sidestream tobacco smoke and i n unburned tobacco 032). The occurrence and c a r c i n o g e n i c i t y o f NNN and r e l a t e d tobacco s p e c i f i c nitrosamines i s reviewed i n another chapter of t h i s volume. The metabolism of NNN i n the F-344 r a t i s summarized i n F i g u r e 2. L i v e r microsomal α-hydroxylation o f NNN leads t o 2 -hydroxyNNN [2] and 5 -hydroxyNNN [5]. These unstable intermediates open to diazohydroxides &_ and 9^ which undergo s o l v o l y s i s to keto a l c o h o l JO and l a c t o l _12. I n a d d i t i o n , 2 gives r i s e to myosmine [7]. The chemistry of the intermediates _2, _5, and 9^ has been e s t a b l i s h e d through studies of the corresponding model compounds, 2 -acetoxyNNN, 5 -acetoxyNNN, and 4-(N-carbethoxy-N-nitrosamino)-l-(3-pyridyl)-l-butanone (14). Keto a l c o h o l JLO and l a c t o l 12^ can be assayed i n microsomal incubations as t h e i r corresponding 2,4-dinitrophenylhydrazone d e r i v a t i v e s (33, 34). α-Hydroxy l a t ion o f NNN has a l s o been demonstrated i n human and S y r i a n golden hamster l i v e r microsomes (11, 35). Species differences i n rates o f 2'-, and 5 - h y d r o x y l a t i o n and i n t h e i r i n d u c i b i l i t y have been observed. In the r a t , 2 - h y d r o x y l a t i o n but not 5*-hydroxylation i s induced by pretreatment with phénobarbital or 3-methylcholanthrene whereas i n the S y r i a n golden hamster 5 - h y d r o x y l a t i o n i s induced by phénobarbital but 2'-hydroxylation i s unchanged by pretreatments (35). E v i d e n t l y , d i f f e r e n t enzymes c a t a l y z e the 2 - and 5 - h y d r o x y l a t i o n s of NNN and the d i s t r i b u t i o n of these enzymes d i f f e r s i n the l i v e r s of r a t s and hamsters. There may a l s o be d i f f e r e n c e s i n d i s t r i b u t i o n between organs of a given species. For example, i n p r e l i m i n a r y s t u d i e s , we have observed that i n c u l t u r e d r a t esophagus, a target organ, 2 - h y d r o x y l a t i o n i s the major metabolic pathway, i n contrast to r e s u l t s obtained i n l i v e r , a non-target organ f o r NNN. β-Hydroxylation o f NNN by r a t l i v e r microsomes to g i v e 3 -hydroxyNNN [3] and 4 -hydroxyNNN [4] has a l s o been observed, but the rates are lower than those o f a - h y d r o x y l a t i o n (36). An­ other microsomal metabolite of NNN i s NNN-l-N-oxide [I] (36). f

1

1

f

1

1

1

f

1

1

1

1

Scanlan and Tannenbaum; N-Nitroso Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

HECHT E T AL.

Metabolism

of Cyclic Nitrosamines

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4.

Scanlan and Tannenbaum; N-Nitroso Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

63

64

iV-NITROSO COMPOUNDS

The rates of formation of these metabolites in l i v e r microsomes from Aroclor pretreated rats is summarized i n Table I I I . Table I I I .

Rates of Formation of NNN Metabolites by F-344 Rat Liver Microsomes rate, nmol min""* (mg of protein)"* 3

product

1

4- hydroxy-1-(3-pyridy1)1-butanone [10] 3 -hydroxy-N -nitrosonornicotine [3] 4 -hydroxy-N -nitroso­ nornicotine [4] 5- (3-pyridyl)-2-hydroxytetrahydrofuran [12] N -nitrosonornicotine 1-N-oxide [1]

2 -hydroxylation

0.40+0.02

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b

1

1

1

3 -hydroxylation