Pesticide Degradation Mechanisms and ... - ACS Publications

Chapter 2

Pesticide Degradation Mechanisms and Environmental Activation

Downloaded by CORNELL UNIV on August 1, 2012 | http://pubs.acs.org Publication Date: March 21, 1991 | doi: 10.1021/bk-1991-0459.ch002

Joel R. Coats Pesticide Toxicology Laboratory, Department of Entomology, Iowa State University, Ames, IA 50011 Pesticides are degraded by many different mechanisms. Physical, chemical, and biological agents play significant roles in the transformation of insecticide, herbicide, and fungicide molecules to various degradation products. Transformation mechanisms include oxidation, hydrolysis, reduction, hydration, conjugation, isomerization, and cyclization. Resultant products are usually less bioactive than the parent psticide molecule, but numerous cases have been documented of metabolites with greater bioactivity. The physical and chemical properties of the degradation products are also different from those of the parent compound, and their fate and significance i n the environment also are altered with the structural changes. The concept of "environmental activation" is introduced, to describe the transformation of a pesticide to a degradation product that i s of significance i n the environment as a result of its environmental toxicology or chemistry. Degradation o f s y n t h e t i c organic p e s t i c i d e s begins as soon as they a r e s y n t h e s i z e d and p u r i f i e d . Formulation processes can i n i t i a t e minor amounts o f decomposition o f a c t i v e i n g r e d i e n t s . During s h i p p i n g and storage o f products, breakdown o f t h e p r i n c i p a l components may occur due t o harsh environmental c o n d i t i o n s (e.g., severe heat) o r prolonged p e r i o d s o f storage. Once a p e s t i c i d e product i s prepared as a tank mix, f u r t h e r degradation can occur because o f chemical i n t e r a c t i o n s between t h e 0097-6156791/0459»0010$06.25Α) © 1991 American Chemical Society In Pesticide Transformation Products; Somasundaram, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


2. COATS

11 Degradation Mechanisms and Environmental Activation

parent molecules and the water used or other p e s t i c i d e s i n the tank mixture. Once a p p l i e d a t a s i t e f o r p e s t c o n t r o l , the p e s t i c i d a l compound i s exposed t o numerous agents capable of transforming i t i n t o v a r i o u s other forms. Once i n s i d e organisms, both t a r g e t and nontarget, they are s u b j e c t t o a t t a c k by d e t o x i f i c a t i o n enzymes. The m a j o r i t y of the p e s t i c i d e a p p l i e d does not immediately enter any organism, but remains i n the environment. These r e s i d u e s i n s o i l , water, and a i r are subject to transformation, to transport t o a d i f f e r e n t l o c a t i o n , and t o uptake by organisms a t t h a t s i t e . Once t r a n s f o r m a t i o n products have been formed i n any of the circumstances d e s c r i b e d , these new compounds are o f r e l e v a n c e , as w e l l , and are s u s c e p t i b l e t o the same f o r c e s of movement and/or f u r t h e r degradation. Agents Involved i n Degradation Processes r e s p o n s i b l e f o r the degradation of p e s t i c i d e s can be c l a s s i f i e d as p h y s i c a l , chemical, and b i o l o g i c a l . A combination of f a c t o r s u s u a l l y i n f l u e n c e the breakdown of a p e s t i c i d a l chemical, over a time p e r i o d of hours, days, weeks, months, or years, and through many of the s i t u a t i o n s enumerated above. F a c t o r s t h a t i n f l u e n c e the r e l a t i v e importance of t h e v a r i o u s t r a n s f o r m a t i o n agents depend t o a g r e a t extent on the chemical's use p a t t e r n , p h y s i c a l p r o p e r t i e s , and chemical s t r u c t u r e . The two primary p h y s i c a l agents i n v o l v e d i n the degradation process are l i g h t and heat. P h o t o l y s i s of p e s t i c i d e r e s i d u e s i s extremely s i g n i f i c a n t on v e g e t a t i o n , on the s o i l s u r f a c e , i n water, and i n the atmosphere ( 1 ) · Thermal decomposition of the chemicals o f t e n occurs concomitantly with the photodegradative reactions; solar radiation, therefore, i s d i r e c t l y r e s p o n s i b l e f o r the decomposition i n two ways, through p h o t o l y s i s and thermal decomposition. Cold, e s p e c i a l l y f r e e z i n g , temperatures can a l s o c o n t r i b u t e o c c a s i o n a l l y to p e s t i c i d e degradation i f c e r t a i n f o r m u l a t i o n s are allowed t o f r e e z e , and the p e s t i c i d e i s f o r c e d out of s o l u t i o n , suspension, or microencapsulation, making i t more s u s c e p t i b l e t o degradative f o r c e s before and a f t e r application. Chemical degradation occurs as a r e s u l t o f the v a r i o u s r e a c t i v e agents i n the f o r m u l a t i o n s , tank mixes, and i n the environment. Water i s r e s p o n s i b l e f o r c o n s i d e r a b l e breakdown of p e s t i c i d e s i n s o l u t i o n , e s p e c i a l l y i n c o n j u n c t i o n with pH extremes. Even s l i g h t v a r i a n c e from a n e u t r a l pH can e l i c i t r a p i d decomposition of p H - s e n s i t i v e compounds. Molecular oxygen and i t s s e v e r a l more r e a c t i v e forms (e.g., ozone, superoxide, peroxides) are capable of r e a c t i n g with many chemicals t o generate o x i d a t i o n products. In a l l but the most oxygen-poor environments, o x i d a t i v e t r a n s f o r m a t i o n s are f r e q u e n t l y the most common degradation pathways observed. Other chemical o x i d a t i o n s , as w e l l as r e d u c t i o n s , can

In Pesticide Transformation Products; Somasundaram, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

12

PESTICIDE TRANSFORMATION PRODUCTS


progress i n t h e presence of c e r t a i n i n o r g a n i c redox reagents (2). These r e a c t i v e s p e c i e s o f metals f u n c t i o n as h i g h l y e f f e c t i v e c a t a l y s t s which e f f e c t p e s t i c i d e transformations i n s o i l s and i n aquatic and marine environments. B i o l o g i c a l agents a r e a l s o s i g n i f i c a n t degraders o f p e s t i c i d e s . Microorganisms ( b a c t e r i a , f u n g i , actinomycetes) a r e t h e most important group o f degraders, based on t h e i r prevalence i n s o i l and water and on t h e tendency o f p e s t i c i d e s t o c o l l e c t i n those médias ( 3 , 4 ) . Three major degradation s t r a t e g i e s a r e e x h i b i t e d by microbes: 1.

2.

3.

Co-metabolism i s t h e b i o t r a n s f o r m a t i o n o f a p e s t i c i d a l molecule c o i n c i d e n t a l t o t h e normal metabolic f u n c t i o n s o f t h e m i c r o b i a l l i f e processes (growth, reproduction, d i s p e r s a l ) . Catabolism i s t h e u t i l i z a t i o n o f an organic molecule as a n u t r i e n t o r energy source. Some p e s t i c i d e s have been observed t o be s u s c e p t i b l e t o enhanced m i c r o b i a l degradation (EMD) by populations o f microbes ( p r i n c i p a l l y b a c t e r i a ) t h a t have adapted, f o l l o w i n g repeated use, t o u t i l i z e t h e p e s t i c i d e molecule as a s o l e source o f carbon o r n i t r o g e n (5). C o n t r o l o f p e s t s by these products may be s e r i o u s l y compromised under some circumstances (e.g., a need f o r weeks o f p e r s i s t e n c e i n s o i l ( 6 ) . The major f a c t o r s t h a t a f f e c t the s u s c e p t i b i l i t y o r r e s i s t a n c e o f a chemical t o EMD a r e t h e r e l a t i v e ease of h y d r o l y s i s , p l u s t h e n u t r i t i o n a l v a l u e o f t h e h y d r o l y s i s products (7), as w e l l as t h e t o x i c i t y o f those products t o microbes i n t h e s o i l (8). This s p e c i a l i z a t i o n o f biodégradation has a l s o been e x p l o i t e d f o r bioremediation processes ( 9 ) . Microorganisms s e c r e t e enzymes i n t o t h e s o i l f o r digestion of substrates. The enzymes, e.g., phosphatases and amidases, may p e r s i s t i n t h e s o i l long a f t e r the parent m i c r o b i a l c e l l i s dead. The s t a b i l i t y and p e r s i s t e n c e of these e x t r a c e l l u l a r enzymes can p r o v i d e s o i l with v a r i o u s important biochemical c a t a l y t i c c a p a b i l i t i e s ( 1 0 ) .

Other b i o l o g i c a l degradation agents i n c l u d e i n v e r t e b r a t e s , v e r t e b r a t e s , and p l a n t s . In g e n e r a l , v e r t e b r a t e s possess the most s o p h i s t i c a t e d enzymatic a r s e n a l f o r b i o t r a n s f o r m a t i o n o f x e n o b i o t i c s t h a t enter t h e i r bodies. In a d d i t i o n t o t h e broader spectrum o f reactions possible i n vivo, the rates of d e t o x i f i c a t i o n and e l i m i n a t i o n a r e t y p i c a l l y highest i n t h i s group, e s p e c i a l l y mammals and b i r d s . Transformation Reactions Three b a s i c types o f r e a c t i o n s can occur: degradative, s y n t h e t i c (e.g., conjugations), and rearrangements.



2. COATS


Although most o r g a n i c p e s t i c i d e s must undergo many r e a c t i o n s before they are completely degraded, o r m i n e r a l i z e d , one o r two t r a n s f o r m a t i o n s a r e f r e q u e n t l y s u f f i c i e n t t o a l t e r the b i o l o g i c a l a c t i v i t y d r a s t i c a l l y . The change u s u a l l y i s a d e t o x i f i c a t i o n , but numerous examples o f i n t o x i c a t i o n , o r a c t i v a t i o n , have been r e p o r t e d as w e l l (11). The evolved biochemical s t r a t e g i e s o f organisms f o r d e t o x i f i c a t i o n o f o r g a n i c f o r e i g n substances a r e f o u r f o l d . They transform the x e n o b i o t i c molecule t o a more water-soluble, more p o l a r molecule t o f a c i l i t a t e e l i m i n a t i o n o f i t from the organism. Oxygen o r oxygenc o n t a i n i n g m o i e t i e s l i k e sugars, amino a c i d s , s u l f a t e s , and phosphates serve t h i s purpose w e l l . Secondly, they a l t e r f u n c t i o n a l groups i n an attempt t o render t h e molecule l e s s t o x i c (e.g., amines and s u l f h y d r y l s ) . The t h i r d approach e n t a i l s breaking o f the p o t e n t i a l l y t o x i c molecule i n t o two o r more fragments t o decrease t h e p r o b a b i l i t y o f t o x i c impacts o f t h e chemical on v i t a l organs o r o r g a n e l l e s . In a d d i t i o n , t h e breaking down process a l s o c o n t r i b u t e s toward p o s s i b l e use o f t h e component p a r t s o f the molecule as n u t r i e n t s , as mentioned r e g a r d i n g m i c r o b i a l catabolism.

Degradative

Reactions

O x i d a t i o n s . A wide spectrum of o x i d a t i v e r e a c t i o n s occur i n organisms and i n the g r e a t e r environment. Nearly a l l impart some degree of i n c r e a s e d water s o l u b i l i t y t o t h e p e s t i c i d e molecule. Most a l s o a l t e r the b a c t i v i t y o f t h e parent compound. For t h e i r impact on the b i o a c t i v i t y and m o b i l i t y , and, hence, t h e environmental s i g n i f i c a n c e o f the chemicals, t h e o x i d a t i o n pathways a r e extremely important t r a n s f o r m a t i o n r e a c t i o n s . The mechanisms o f o x i d a t i o n vary: p h y s i c a l and chemical o x i d a t i o n s i n v o l v e molecular oxygen o r more r e a c t i v e s p e c i e s i n c l u d i n g v a r i o u s a c i d s , peroxides, o r s i n g l e t oxygen, and are o f t e n enhanced by l i g h t , heat, o r o x i d i z e d metals i n s o i l o r water. B i o l o g i c a l o x i d a t i o n s are accomplished e n z y m a t i c a l l y , p r i m a r i l y by the mixed f u n c t i o n oxidases ( u t i l i z i n g cytochrome P ) (12,13), the flavin-dependent monooxygenases (14,15), and monoamine oxidases (12). The r e a c t i v e forms o f oxygen t h a t are i n v o l v e d i n the v a r i o u s o x i d a t i v e b i o t r a n s f o r m a t i o n s have been r e p o r t e d t o be hydroxyl r a d i c a l , superoxide anion, and hydrogen peroxide. S p e c i f i c oxidative reactions of significance i n pesticide degradation are presented below. 4 5 0


14


ping

PESTICIDE TRANSFORMATION PRODUCTS Hydroxylation

T h i s r e a c t i o n occurs on aromatic r i n g s i n p e s t i c i d e s o f s e v e r a l types, as w e l l as p o l y c y c l i c aromatic hydrocarbons (PAH). I t i s e a s i l y accomplished on s i n g l e or m u l t i p l e - r i n g molecules t h a t a r e u n s u b s t i t u t e d o r a c t i v a t e d . S u b s t i t u t i o n with bulky o r m u l t i p l e groups can s t e r i c a l l y i n h i b i t t h e r e a c t i o n , as can t h e presence of d e a c t i v a t i n g s u b s t i t u e n t s such as halogens.

Side-chain

oxidation

CH

CH

3

Y - O H

\£H CH

dîazinon

3

3

CH

3

hydroxy diazinon

(17)

Any methylene group i n an a l i p h a t i c s i d e - c h a i n i s susceptible t o oxidation. Carbofuran undergoes h y d r o x y l a t i o n o f a methylene i n t h e furan r i n g t o form 7hydroxycarbofuran, followed by f u r t h e r o x i d a t i o n t o 7ketocarbofuran (18). Terminal methyl groups a r e a l s o susceptible t o hydroxylation.


2. COATS



Epoxidation

dieldrin

aldrin

(19)

The conversion o f alkenes t o epoxides does not r a d i c a l l y a l t e r t h e p o l a r i t y of the s u b s t r a t e , nor t h e b i o l o g i c a l a c t i v i t y i n some cases, e.g., a l d r i n s e p o x i d a t i o n t o dieldrin. Epoxide formation may be, i n f a c t , c r i t i c a l t o b i o a c t i v i t y , e.g., precocenes r e q u i r e a c t i v a t i o n t o be c y t o t o x i c t o the corpora a1latum of i n s e c t s t o i n h i b i t the p r o d u c t i o n o f j u v e n i l e hormone ( 2 0 ) . Epoxides can be hydrolyzed t o form d i o l s ; the net e f f e c t o f the arene oxide formation f o l l o w e d by h y d r o l y s i s , i s r i n g hydroxylation. 1

0-dealkvlation

CH O 3

\

c -/

methoxychlor

y

0CH3

(O)

CH O-^ 3

y- O H

desmethyl methoxychlor (21)

D e a l k y l a t i o n o f groups l a r g e r than methyl a l s o occurs, p r i m a r i l y v i a an α-hydroxylation, f o l l o w e d by t h e cleavage of the ether bond. R e l a t i v e r a t e s o f O - d e a l k y l a t i o n can be q u i t e s p e c i e s - s p e c i f i c ( 2 2 ) .


16


N-dealkvlation

Cl

v

N ^ N

I

II

r

w

3

9^

Ν ΙΟΙ


atrazine

„

JL 11

C

H

I

desethyl atrazine

(23)

In t h i s example, e i t h e r or both of the a l k y l groups can be o x i d a t i v e l y removed, N-Demethylation i s an important step i n t h e a c t i v a t i o n o f chlordimeform (24). S-dealkylation

CI

CI (O)

prothiophos

despropyl prothiophos

(25)

D i - and t r i - a l k y l thiophosphate (and thiophosphoramidate) e s t e r s a l s o a r e s u s c e p t i b l e t o i s o m e r i z a t i o n which e n t a i l s m i g r a t i o n o f an a l k y l group (12). Oxidative

desulfuration

fonofos

fonofoxon

(26)

T h i s r e a c t i o n i s r e q u i s i t e of n e a r l y a l l thionophosphates for a c t i v a t i o n t o highly e f f e c t i v e i n h i b i t o r s of a c e t y l c h o l i n e s t e r a s e (AChE). I t occurs i n v i v o p r i m a r i l y , through an o x i d a t i v e a t t a c k on t h e s u l f u r atom, as demonstrated with t h e metabolism o f the R and S o p t i c a l isomers o f fonofos, s i n c e the oxons (P=0) formed r e t a i n e d the stereochemistry o f t h e parent phosphorothion (27).


2. COATS

Degradation Mechanisms and Environmental Activation

Sulfoxidation

C s H

'°)p-S-CH -S-C(CH3>3 2


terbufos

lo] C

Î

H

P

^-S-CHJ-C(CH;

3

CJH Ο " terbufos sulfoxide

[O]

> -

S- C H - S- C(CHJ 2

terbufos sulfone

3

(28)

The o x i d a t i o n o f s u l f i d e s t o t h e s u l f o x i d e and s u l f o n e i s often rapid. S u l f o x i d e formation i s t h e r e s u l t o f FAD monooxygenase enzymes i n v i v o , w h i l e s u l f o n e s a r e formed by t h e cytochrom P monooxygenase system. The two forms can be generated c h e m i c a l l y by peroxides and peracids, respectively. I t i s o f t e n t h e path o f l e a s t r e s i s t a n c e t o degradation, and i t r e s u l t s i n o x i d a t i v e metabolites that are often of b i o l o g i c a l s i g n i f i c a n c e . Aromatic a l k y l t h i o e t h e r s , e.g., measurol (17) and methiochlor (29), a r e a l s o s u s c e p t i b l e t o s u l f o x i d a t i o n . Thiomethyl m e t a b o l i t e s o f c h l o r i n a t e d aromatic hydrocar bons, formed v i a a mercapturic a c i d i n t e r m e d i a t e , can a l s o be o x i d i z e d t o t h e s u l f i n y l and s u l f o n y l d e r i v a t i v e s (30). 4 5 0



18 Amine o x i d a t i o n


CI

CI

CI

Arylamine N-oxidation can occur v i a monoamine oxidases or the CytoP-450-dependent monooxygenases (12). The hydroxylamine form i s very r e a c t i v e i n v i v o (31) and can undergo deamination or f u r t h e r o x i d a t i o n t o form the more s t a b l e phenol or nitrobenzene, r e s p e c t i v e l y . H y d r o l y s i s r e a c t i o n s . For many p e s t i c i d e molecules, h y d r o l y s i s i s a primary route of degradation. Many types of e s t e r s are h y d r o l y t i c a l l y cleaved, y i e l d i n g two fragments with l i t t l e or no p e s t i c i d a l a c t i v i t y . The products do possess b i o a c t i v i t y of v a r i o u s other types, however, e.g., m i c r o b i a l t o x i c i t y (8) or i n d u c t i o n of m i c r o b i a l growth (7). H y d r o l y s i s of e s t e r s can occur by chemical means; even m i l d l y a l k a l i n e s o l u t i o n s can cause h y d r o l y t i c decomposition of some e s t e r s (organophosphorus, carbamate, p y r e t h r o i d ) , w h i l e a c i d c a t a l y z e d h y d r o l y s i s t y p i c a l l y i s induced o n l y by s t r o n g l y a c i d i c s o l u t i o n s (e.g., pH 3-4). H v d r o l v s i s - of phosphate e s t e r s

parathion

diethyl thiophosphate

p-nitro phenol

(12)

B i o l o g i c a l l y , the h y d r o l y s i s of p a r a t h i o n has been shown t o proceed by two mechanisms, one o x i d a t i v e , as a result of the a c t i o n of mixed-function oxidases or by h y d r o l a s e s (33,34).


2. COATS

Degradation Mechanisms and Environmental Activation

Hydrolysis

S CH O II 3

X

θΗ θ'

- o f carboxvl

Ο il I Ch^-C-O-C^ II * Ο

3

esters

c 9

*

^P-S-CH-C-O-C^ CH 0 I * CHr-C-OH il Ο carboxy malathlon /

3

5

3

2

malathion


Ο

3

(12)

Carboxyl e s t e r a s e enzymes o f v e r t e b r a t e s r a p i d l y render malathion non-toxic through t h i s h y d r o l y s i s . Malathionr e s i s t a n t i n s e c t s a l s o have been demonstrated t o possess the enzymatic c a p a b i l i t y (25). Carbamate e s t e r s , e.g., N-methylcarbamate i n s e c t i c i d e s , are a l s o degraded by e s t e r h y d r o l y s i s (35). Hvdrolvsis

- o f amides

F 2,6-diflurobenzoic

acid

H Q 2

diflubenzuron 2

N-C-N-^

^Cl

p-chlorophenylurea

Hvdrolvsis

- o f halogens Cl ι"

H Ο

CH hydroxyatrazine (23) 3

atrazine


(17)

20



Chemical h y d r o l y s i s of c h l o r i d e s and bromides can occur, y i e l d i n g a hydroxylated product. H y d r o l y s i s of an epoxide, by an epoxide hydratase, produces a d i o l (36). Oximecarbamates can be hydrolyzed t o an oxime and a carbamic a c i d (35). Reductions. Under c e r t a i n c o n d i t i o n s , r e d u c t i o n r e a c t i o n s are commonly observed, y i e l d i n g products with lower p o l a r i t y g e n e r a l l y . T h e i r b i o l o g i c a l a c t i v i t y may be concomitantly a l t e r e d as w e l l . P e s t i c i d e s undergo r e d u c t i o n r e a c t i o n s i n reducing environments, c h a r a c t e r i z e d by low oxygen c o n c e n t r a t i o n s , low pH's, and anaerobic microorganisms. Examples of s i t u a t i o n s i n which p e s t i c i d e s are reduced i n c l u d e stagnant or e u t r o p h i c ponds and l a k e s , bogs, f l o o d e d r i c e f i e l d s , the rumens of ruminant l i v e s t o c k , and lower i n t e s t i n e s of some s p e c i e s , i n c l u d i n g man. Reductive dehaloaenation

c

s

OO

a

a-c-a ι CI

a

Pesticide Degradation Mechanisms and ... - ACS Publications

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