11 Novel Polar Xenobiotic Conjugates Gary B. Quistad
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Biochemistry Department, Zoecon Corporation, Palo Alto, CA 94304 This paper summarizes the occurrence of unusual xeno biotic conjugates which are substantially ionized i n animals and plants. Such conjugates generally are deemed "unusual" either because they occur rarely or because their existences seldom are documented, often by virtue of recent discoveries. Although the unique features of polar xenobiotic conjugates often defy a facile assignment of the compounds to general categories, certain classes of conjugates are discussed. For animals, the discus sion focuses on unusual conjugates of urea, carni tine, amino acids, phospholipids, bile acids, and multiple conjugates of foreign compounds. For plants, conjugates containing unusual sugars, amino acids, and malonic acid are featured. There are numerous reviews of conjugation biochemistry available (1-8). These reviews contain miscellaneous unusual conjugates interspersed with a primary discussion of the major conjugative pathways. The bias of most reviews is on conjugations in animals although several reviews of conjugates in plants are available also (9-12). There are even reviews devoted to novel conjugation reactions (13-15). This paper emphasizes unusual conjugates which have not been reviewed adequately elsewhere and provides references to other uncommon, but well-known conjugation pathways. Animals Amino Acids and Peptides. Xenobiotic conjugates have been reported for about two-thirds of the common oramino acids. In general, glycine is the single most common amino acid utilized in metabolic conjugation although certain animal groups exhibit preferential usage of other amino acids (16). For example, ornithine conjugates are common in reptiles and some birds (Galliformes and Anseriformes, e.g. chickens, turkeys, ducks) while other birds (Columbiformes, e.g. pigeons) use glycine (16). Huckle et a l . (17) 0097-6156/86/0299-O221S06.00/0 © 1986 American Chemical Society
Paulson et al.; Xenobiotic Conjugation Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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have shown t h a t the major m e t a b o l i t e of 3-phenoxybenzoic a c i d f r o m c h i c k e n s was an N ^ a c e t y l o r n i t h i n e c o n j u g a t e ( 8 % of the a p p l i e d dose)* M e t h i o n i n e adducts have been p o s t u l a t e d as the source of m e t h y l t h i o groups i n m e t a b o l i t e s of phenanthrene and s e v e r a l o t h e r x e n o b i o t i c s (18 and r e f e r e n c e s t h e r e i n ) . J a g l a n and A r n o l d (19) proposed t h a t n u c l e o p h i l i c d i s p l a c e m e n t of a c h l o r i n e i n d i c h l o r a n by methionine gave a s u l f o n i u m i o n adduct ( F i g u r e 1) which r e p r e sented up t o 50% of the C - r e s i d u e s i n goat m i l k a f t e r dosage w i t h I C ] d i c h l o r a n . T h i s u n u s u a l s t r u c t u r e was s u p p o r t e d by mass s p e c t r a l and a c i d h y d r o l y s i s d a t a . Some A r t h r o p o d s ( e . g . s c o r p i o n s , s p i d e r s ) u t i l i z e a r g i n i n e which can a l s o be d e c a r b o x y l a t e d t o an agmatine conjugate ( 4 , 1 6 ) . There are r e p o r t s of c o n j u g a t i o n w i t h l e s s abundant amino a c i d s such as h i s t i d i n e ; b e n z o y l h i s t i d i n e i s formed i n the f r u i t - e a t i n g bat ( 2 0 ) . V a l i n e and t h r e o n i n e c o n j u g a t e s were found i n r a t f e c e s f o l l o w i n g treatment w i t h f l u v a l i n a t e ( 2 1 ) . Thus, w i t h more s o p h i s t i c a t e d i n s t r u m e n t a t i o n i t i s l i k e l y t h a t soon c o n j u g a t e s of a l l n a t u r a l amino a c i d s w i l l be found a l t h o u g h most w i l l be p r e s e n t at t r a c e l e v e l s . T a u r i n e and c a r n i t i n e are two l e s s common amino a c i d s which are becoming more w i d e l y r e c o g n i z e d as important i n c o n j u g a t i v e m e t a b o l i s m . Only a few y e a r s ago t h e r e were few examples of t a u r i n e c o n j u g a t i o n , but now t h e r e are f r e q u e n t r e p o r t s i n v o l v i n g t h i s s u l f o n i c a c i d ( 2 2 ) . S i n c e our r e p o r t s of c a r n i t i n e c o n j u g a t i o n w i t h c y c l o p r o p a n e c a r b o x y l i c a c i d i n r a t s , dogs, m i t e s , and a cow (23-26), s e v e r a l o t h e r a c i d s have been shown t o r e a c t w i t h c a r n i t i n e ( F i g u r e 2 ) . P i v a l o y l c a m i t i n e has been i s o l a t e d as a major m e t a b o l i t e from the u r i n e of humans dosed w i t h a methyldopa p r o d r u g (27). I n t e r e s t i n g l y , t h i s c a r n i t i n e c o n j u g a t e o c c u r s i n humans, but not o t h e r s p e c i e s which e x c r e t e p i v a l i c a c i d as a g l u c u r o n i d e . V a l p r o y l c a r n i t i n e has been i s o l a t e d from the u r i n e of c h i l d r e n t r e a t e d w i t h v a l p r o i c a c i d , a w i d e l y used a n t i c o n v u l s a n t ( 2 8 ) . E x c l u d i n g c o n j u g a t e s d e r i v e d from g l u t a t h i o n e adducts, few x e n o b i o t i c a c i d s are r e p o r t e d t o form p e p t i d e c o n j u g a t e s i n v o l v i n g more than one amino a c i d , and these examples have been reviewed amply (2,16,29). C a t s produce g l y c y l g l y c i n e and g l y c y l t a u r i n e c o n j u g a t e s from both q u i n a l d i c and k y n u r e n i c a c i d s . Rats e x c r e t e a d i p e p t i d e conjugate of DDA (from DDT) c o n t a i n i n g a s p a r t i c a c i d and serine. 3-Phenoxybenzoic a c i d i s e x c r e t e d as a g l y c y l v a l i n e d i p e p t i d e conjugate i n M a l l a r d ducks (29) and c h i c k e n s ( 1 7 ) . The a n t i n e o p l a s t i c analogs methotrexate and a m i n o p t e r i n form p o l y g l u t a m a t e s i n r a t s , mice, and man ( 1 5 ) . R e c e n t l y f o u r p e p t i d e conjugates were i d e n t i f i e d from the u r i n e of c a l v e s dosed w i t h p h e n o t h i a z i n e ( 3 0 ) . A l t h o u g h numerous s t r u c t u r a l types of m e t a b o l i t e s are d e r i v e d f r o m r e a c t i v i t y w i t h g l u t a t h i o n e ( e . g . 31), s e v e r a l r e c e n t examples of n o v e l mercapturates have been r e p o r t e d ( F i g u r e 3 ) . 4-Cyano-^2L" d i m e t h y l a n i l i n e i s the s u b s t r a t e f o r f o r m a t i o n of two u n u s u a l m e r c a p t u r a t e s (32-34). A f t e r monodesmethylation, a s t a b l e t h i o e t h e r mercapturate i s produced (presumably v i a the N ^ m e t h y l o l ) as a major u r i n a r y m e t a b o l i t e by r a t s and mice (15 and 33% of the a p p l i e d dose, r e s p e c t i v e l y ) . Rats a l s o c o n v e r t up t o 10% of the a p p l i e d 4-cyano-NjN--dime t h y l a n i l i n e t o a mercapturate a r i s i n g f r o m the r e a c t i o n of g l u t a t h i o n e w i t h an a c e t y l group ( p o s s i b l y v i a a glycolylaniline). The c o r r e s p o n d i n g m e t h y l s u l f o n y l d e r i v a t i v e from the mercapturate was r e c o v e r e d i n u r i n e a l s o ( 3 3 ) . M e r c a p t u r i c
Paulson et al.; Xenobiotic Conjugation Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
11.
QUISTAD
Novel Polar Xenobiotic Conjugates
223
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a c i d s u l f o x i d e s are produced by r a t s f r o m p r o p a c h l o r (35,36) and N7 a c e t y l - j r - ( 2 , 3 , 5, 6 - t e t r a c h l o r o p h e n y l ) c y s t e i n e ( 3 7 ) . The f i r s t a c y l 1 i n k e d m e r c a p t u r a t e found i n human u r i n e was r e p o r t e d f o r c l o f i b r a t e ( 3 8 ) . A d i s u l f i d e w i t h c y s t e i n e was the major m e t a b o l i t e o f c a p t o p r i l i n humans ( 3 9 ) . F o r many y e a r s the f o r m a t i o n of p r e m e r c a p t u r i c a c i d s has been r e c o g n i z e d f o r the m e t a b o l i s m of a r o m a t i c compounds ( 1 , 3 ) . These h y d r o x y d i h y d r o d e r i v a t i v e s of a r o m a t i c systems are a c i d l a b i l e and r e v e r t t o more s t a b l e a r o m a t i c m e r c a p t u r a t e s u n l e s s p r e c a u t i o n s a r e taken during i s o l a t i o n . Work w i t h phenanthrene (18) and n a p h t h a l e n e (40,41) has demonstrated t h a t pre m e r c a p t u r a t e s are m e t a b o l i z e d f u r t h e r i n a manner s i m i l a r t o m e r c a p t u r a t e s ( i . e . t o me r e a p t o a c e t i c , m e r c a p t o l a c t i c , and m e t h y l t h i o compounds, each r e t a i n i n g the h y d r o x y d i h y d r o c h a r a c t e r of one of the o r i g i n a l a r o m a t i c r i n g s ; Figure 4). In r a t s these secondary m e t a b o l i t e s of p r e m e r c a p t u r a t e s a r e produced p r i m a r i l y by the i n t e s t i n a l m i c r o f l o r a ( 4 1 ) . Amides and Urea A d d u c t s . There are o n l y a few examples of amide f o r m a t i o n from x e n o b i o t i c a c i d s ( F i g u r e 5 ) . In 1969 i t was r e p o r t e d t h a t humans and r a b b i t s c o n v e r t m e t i a z i n i c a c i d t o an amide ( 4 2 ) . S u b s e q u e n t l y , amide m e t a b o l i t e s have been i d e n t i f i e d f o r c l o f i b r a t e (43), a n t h r a n i l i c a c i d (44), and an a n i l i n o a c i d (21). I t i s not s u r p r i s i n g t h a t such amides are i s o l a t e d c o n s i d e r i n g that n i c o t i n i c a c i d i s converted r e a d i l y to nicotinamide i n h e a l t h y a n i m a l s and numerous n a t u r a l p e p t i d e s c o n t a i n amide f u n c t i o n a l i t y at the C - t e r m i n u s . I t i s s t i l l u n c l e a r whether amides are produced e n z y m a t i c a l l y or merely c h e m i c a l l y . I f ammonia i s r e s p o n s i b l e f o r amide f o r m a t i o n , i t may a l s o c o n t r i b u t e t o the p r o d u c t i o n of an u n u s u a l a c e t y l a t e d amino a c i d f r o m n - b u t y l g l y c i d y l e t h e r (45, F i g u r e 5 ) . The a c e t y l a t e d amino a c i d r e p r e s e n t e d 23% of the u r i n a r y m e t a b o l i t e s f r o m r a t s and presumably arose by ammonoly s i s of the e p o x i d e f o l l o w e d by o x i d a t i o n and a c e t y l a t i o n . Recently i t was shown i n earthworms t h a t spermine, a polyamine,forms an amide w i t h 3-phenoxybenzoic a c i d f r o m c y p e r m e t h r i n (46). S e v e r a l u r e a adducts have been i d e n t i f i e d i n m e t a b o l i s m s t u d i e s of x e n o b i o t i c s ( F i g u r e 6 ) . The a d d i t i o n of u r e a t o i s o p r o t u r o n (47), formaldehyde (48), and oxazapam (49) i s c h e m i c a l l y mediated and o c c u r s when p r i m a r y m e t a b o l i t e s remain i n u r i n e f o r prolonged periods. In the case of caroxazone (50), i t i s u n c l e a r whether the u r e a adduct i s a c h e m i c a l a r t i f a c t or i s p r o d u c e d enzymatically. The s t r o n g e s t e v i d e n c e t h a t u r e a adducts can be genuine m e t a b o l i t e s comes f r o m work w i t h r o s a r a m i c i n ( 5 1 ) . This m a c r o l i d e a n t i b i o t i c c o n t a i n s an aldehyde group t h a t r e a c t s w i t h two e q u i v a l e n t s of u r e a t o f o r m 2 0 - b i s u r e i d o r o s a r a m i c i n which i s excreted i n urine. The b i s u r e i d o m e t a b o l i t e d i d not f o r m when r o s a r a m i c i n was i n c u b a t e d w i t h u r i n e i n v i t r o . One may e x p e c t u r e a a d d u c t 8 from c e r t a i n k e t o n e s a l s o s i n c e 1 1 - $ - h y d r o x y - 4 - a n d r o s t e n e 3,17-dione i s c o n v e r t e d by humans t o a u r e a s t e r o n e . The 3orureido s t e r o i d forms n o n e n z y m a t i c a l l y from the g l u c u r o n i d e i n u r i n e a t pH 5 (52). Glycosides. S i n c e g l u c u r o n i d e f o r m a t i o n i s o f t e n the predominant m e t a b o l i c pathway i n a n i m a l s , t h i s p r o c e s s has been r e v i e w e d f r e q u e n t l y ( e . g . _3 and r e f e r e n c e s t h e r e i n ) . The l a s t decade has
Paulson et al.; Xenobiotic Conjugation Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
XENOBIOTIC CONJUGATION CHEMISTRY
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Downloaded by UNIV OF BATH on July 4, 2016 | http://pubs.acs.org Publication Date: January 24, 1986 | doi: 10.1021/bk-1986-0299.ch011
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