Document x5DBZ4844rpd1kag82b3ZyagJ

Atfcott. 4 JJ /*7f INSUFFICIENT CQ&UUUA nr-CLUCURONIDATION ACTIVITY: A POSSIBLE FACTOR JN POLYCHLORI NATED UIPHlirfL (PCB) T^KICITY EDWARD J. CALABRESE, Division of Public Health, University of Massachusetts, Amherst, MA 01003, U S.A. SUMMARY Considerable evidence indicates that phenolic and biphenolic compounds are detoxified and excreted primarily via conjugate glucuronidation in man. Since fetuses, neonates and certain enzyme deficient adults have significant functional deficiencies in their capacities to excrete toxic compounds via conjugate glucuronidation, it is predicted that these groups of individuals arc biochemically predisposed to accumulate polychlorinated biphenyls (PCB's), a widely distributed and highly toxic environmental contaminant. INTRODUCTION Since the late 1960's when polychlorinated biphenyls (PClI's) were first recognized as a potentially widespread environmental contaminant, hundreds of research papers have hocn published concerning their structure, industrial In dogs injected with 2.4,4' iricbloro 2'hydroxydphcnyl ether, nearly 100% of the material excreted in their urine and feces over a 5-day period appeared as the glucuronide or sulfate conjugate. Human adults similarly exposed excrete 65% in urine and 20% in feces alter intrasenous uses, presence in the environment, toxicity to numerous injection. It is excreted as a free compound or as .. glu animals from insects to man. tissue storage, metabolism and excretion (1,2). As a result ol such data accumulation, regulatory agencies are now trying to approach the problem of standard setting with respect to PCB's. Standards for air, water and food arc necessary if the total human exposure is to be regulated and controlled. One important component in stundard setting is a consideration of the individuals within the population who may be at high risk to the polliManl because of various genetic, physiological, psycho logical and behavioral traits. Individuals lacking the ability to detoxify and excrete PCB's represent such a high risk group since they will be unable to prevent undue accumu lation of toxic levels of PCB's within the body. It is the intention of this paper to: (1) establish the theoretical basis for why individuals lacking a sufficiently functional conjugate glucuronidation process may (end to accumulate unusually high levels of PCB's; (2) identify and curonidc (1). Other evidence suggests (hat some chlorinated biphenyls arehydroxylatcd by sptcies such as the rat and pigeon. No evidence of reductive dechlorination was observed in either (he trout, rat or pigeon (4). During the excretory process foreign substances (such is PCB's) usually first undergo several metabolic transtorm ations including oxidations, reductions, and hydrolysis. It often happens that following an oxidflme. reductive or hydrolytic reaction, conjugation of die foreign substance with either glucuronic acid, sulfate, glycine, cysteine, main I or acetyl occurs. Of these reactions, glucuronic acid con jugation is probably the most important in the animal kingdom since it occurs extensively in man and all labors tory animals with the exception of the cat. In the eat, glucuronic acid conjugation is not entirely absent, but occurs at a low level compared with other mammalian species. Conjugations are generally regarded as detox quantify (where possible) those groups of individuals com* posing this high risk group; and (3) stimulate clinical trial of the proposed hypothesis. ificadon mechanisms, for compounds which undergo these reactions arc converted into products which arc usually less toxic and more rapidly excreted than their precursors (5 H). The widespread occurrence of glucuronic acid con GLUCURONIDATION IN THE EXCRETION OF TOXIC SUBSTANCES Mammals, including rabbits, dogs, and humans, have been reported to excrete PCB's, in pari, by conjugate glucuronidation and/or sulfonntion (1,3). The urinary excretion of biphenyl and 4 chlorobiphenyl has been studied in rabbits. Biphenyl glucosiduronic acid and 4-hydroxy biphenyl were isolated from urine. Rabbits fed 4-chlorobiphcnyl excreted 4 (p chlorophcnyl) phenol and 4 chlorobiphcnyl glucosiduronide. Twice as much 4-chlorobipheny! ns biphenyl was excreted as the glucosiduronic acid derivative. It was suggested that other low chlorinated biphenyls are excreted in a similar manner (3). jugation may be due to the facility with which glucuronic acid can be produced in the body from carbohydrate sources and the variety of chemical groups to which glucuronic acid can be transferred enzymatically. The other conjugation mechanisms arc more i csiiictcd in their occurrence and this is probably because of the limited availability of the conjugating agents such as ghcim* anu cysteine (via glutathione) and to the small numhci ol chemical groupings to which conjugating agents such a sulfate, glycine, cysteine, methyl, and acetyl can tic tr.mr. ferred. The activity and amount of the transferring cn/\ me concerned in these conjugations arc probably also tumtiny factors. Glycine conjugation is confined mainly to aromutu carboxyl groups, sulfate to phenolic hydroxyl groups an. 162 KQNS 0011*4 ivii.iut'.iiioti m some aromatic Hydrocarbons and halogen mcO hydrocarbons, and mcthylation to certain hydroxyl and amino groups, and acetylation to some amino and hydru/ine groups. However, glucuronic acid conjugation can occur with several types of hydroxyl, amino, and carboxyl groups and with sulfhydryl groups (5-8). The cfTcct of conjugation of a substance with glucuronic acid is to produce a strongly acidic compound which is more water-soluble at physiological pH values than the precursor. The majority of foreign substances Rrc ultimately cleared from the body via their excretion in the urine and bile, most often in the form of polar conjugates such as glucuronides with the avenue of excretion of the glucuronidu probably varying wills the species of animal considered (5, 6). CROUPS AT HIGH RISK TO PCB's A. Fetuses and Neonates: Immature Fnr.yme Detoxification Systems It is apparent Hint individuals lacking the ability to form either conjugate glucuronides or sulfates or both will be at high risk with respect to toxic compounds of a phenolic nature. The occurrence of low levels of glucuronide for mation in the cat has been suggested as an explanation of the widely observed phenomenon in veterinary literature (9 12) that phenolic compounds should not be used on or around cats because of increased incidence of toxicity in this species. Furthermore, intravenous injections of phenol in cnls, pigs, dogs and goats revealed that cats were at least 2 limes more sensitive with respect to fatality caused by the phenol (13). Toxicity in this study was related to the partial deficiency in the cat to conjugate phenol with glucuronic acid. The author concluded that the cat is more likely to he poisoned hy acute doses of phenol and is more likely to become chronically affected with phenol toxicity due to the inability to rapidly form and excrete glucuronides. biochemical studies with the cat have also indicated that conjugate glucuronidntion is the major pathway for the detoxification and excretion of phenol (e g., more than twice i effective ns sulfomition) (1.1). Because cats are hypersusceptible to phenolic like compounds as a result or their diminished capacity to form glucuronides. it is strongly suspected Ihtii human embryos, fetuses, and neonates (2 to 3 months old) which also arc deficient in a functional conjugate giucuronidntion system will also be predisposed to the toxic effects of phenolic compounds including PCB's (*.14,13). Usually by the age of 2 to 3 months, adult levels of most enzyme systems arc achieved (8.14,15). Unfortunately, this "developmental immaturity" in the unborn and the very young may predispose them to the toxic effects of certain Mihsiances since they may be unable to detoxify and excrete them as quickly ns necessary (16). It has already been pointed out that human adults do excrete, in part. PCB's via conjugate glucuronidntion. Clinical experience has shown that infants may respond differently to doses of drugs which arc easily tolerated by older children and adults. Presoni ably this is because older children and adults have fully functioning enzyme detoxification systems while the mechanisms in the \ery young have broad significance since the absence of such mechanisms prevents the prompt elimination of toxic substances from the body. A further problem for the very young with regard to PCB's is that in addition to having a difficult time excreting PCB's they may also consume more PCB's per unit of body weight than at any other time during their life span. For example, samples of human milk from (wo cities in California contained aver age PCB levels of 60 ppb and 100 ppb. respectively. Based on a daily milk intake of 150 ml/kg, breast fed infants in California would ingest about 9 ^g/kg/day of PCB's. A range of 1-3 /rg/kg/day has been suggested as a "reason able" level for an Acceptable Daily Intake (2.16). One ug/ kg/day has been reported as being 100 times less than the lowest "no effect" level in animal studies (17). Thus, in children the margin is reduced to about a factor of 10 if excretory activity is equivalent with adults. However, ns a result of the immature enzyme systems, the safety factor of 10 may be considerably reduced. How much is presently unknown. A tragic example of toxicity arising from the inability to form glucuronides has been reported. The drug chlor amphenicol, which is known to be metabolized in humans by (he action of the glucuronic pathway, caused the death of more than 30 premature babies who had been treated with the antibiotic for infectious diseases. Theoretically, the premature babies were not able to conjugate the drug with glucuronic acid and thus the drug could only be slowly excreted and so tended to accumulate in the body to toxic proportions eventually leading to death (15). An additional problem encountered by many neonates is that approximately five percent of the mothers of normal infants secrete milk which inhibits the activity of glucuronyl transferase (and thus the giucuronidation process) by more than 20 percent via the action of a steroid present in the breast milk. Inhibition of this glucuronyl transferase has been reported for up to 49 days after birth. Clincally these children have been reported to develop unusually severe neonatal jaundice. This condition develops because glu curonide formation which assists in the elimination of bilirubin (breakdown product of hemoglobin) is partially inhibited. Cow's milk does not contain sufficient amounts of this steroid to affect a noticeable inhibition of (lie glu curonidation process. Consequently about 5 .per, cent of the neonates breast fed would he expected to have their ability to excrete PCB's impaired (18). Administration of the antibiotic novobiocin has also been associated with unconjugafcd hyperbilirubinemia in infants (19). Novobiocin is a noncompetitive inhibitor of glycuronyl transferase activity in vitro (20). Thus, children receiving concomitant exposure to novobiocin and PCB's would be expected to have their ability to detoxify and excrete PCB's impaired. B. Individuals with Genetic Deficiencies and Modified Liver Function After the neonatal period, a broad range of conditions is associated with the improper or incomplete development of (he glucuronide conjugation system. The usual physio logical problem associated with these conditions is the inadequate -detoxification and excretion of bilirubin. This HONS 0011*5 HONS 0 0 1 1 4 6 spectrum extends from the frequently occurring "mild" condition known ns Gillrcrt's syndrome to Ihc very rare, but severe and often fatal Crigler Najjar syndrome (21). Since the clinical cITccts associated with these syndromes nrc considered to be caused by metabolic disturbances oi the glucuronidc conjugation scheme, it is expected that PCB elimination in these individuals would be impeded. The population incidence of Gilbert's syndrome has been variously reported as I in 200 males (23), 7 percent based on examinations of 100 medical students (24) and 0 percent of 297 healthy individuals 1252 healthy National Blood Transfer Service donors, nnd 47 healthy medical students (197 males and 102 females)! with no difference in frequency between males and females (25). Concomitant exposure to PCB's and other drug-like chemicals may potentiate the toxic cITccts of PCB's. For example, rats and monkeys given SKF525A (/1-diclhylaminocthyl 2, 2 diphcnylpcntanoatc, a non-specific inhibitor for many of the microsomal enzymes especially hepatic microsomal enzymes) during the initial 24 hours of ex posure to PCB succumbed rapidly as compared to the control group (22). Of possible significance is the fact that SKF525A inhibits the proper functioning of the glucuronic pnthwny (15). Individuals with liver infections may also be at high risk with respect to PCB's. For example, depression of glu ctironidc synthesis has been observed in humans with infectious hepatitis (15). liven (hough PClI Jikc poJfulanls require a functional conjugate gtucuronidaiion system for their excretion, this docs not imply that such substances arc merely passive molecules in this process. For example, synthesis of micro somal enzymes such as glucuronyl transferase can be stimulated by drugs such ns phenobarbetal. Phenobarbital treatments have been successfully employed with patients with a partial defect in bilirubin conjunction in order to reduce the levels of bilirubin in the blood (26,27,28,29). Thompson ct nl (30) demonstrated similar results with oral administration of dichlorodiphcnyl trichloroethane (pp'DI)T). Although it has been suggested that the ben eficial cITccts of phenobarbital or DDT arc caused by their Induction of glucuronyl transferase in the liver (31), con clusive evidence still remains to be demonstrated (28,32,33). Finally, supportive evidence by Allan et al (22) has recently showed that phenobarbital treatment prior to PCB exposure may reduce PCB loxicity by stimulating liver enzyme induction in rats. With regard lo public health implications, it is still not resolved whether or not people with a partial glucuronyl transferase deficiency may accumulate significant quantities of PCB like substances. These substances seem capable of inducing microsomal enzymes which lead to their (PCB's) metabolism nnd excretion. This constitutes a built-in safety feature which should help to prevent these individuals from nccmmilating PCII like chemicals in their bodies. In contrast, individuals with the Criglcr-Najuar syndrome-- seem to have an absolute deficiency of glucuronyl trans ferase. nnd thus do not have such a built-in compensatory safety system. However, it is not known how effective such a system is in those with the partial enzyme deficiency. It shook! also be noted that Mibslnnccs which require (he activity of glucuronyl transferase for metabolism and excretion and which do not afTcct microsomal cn/ynn- induction would be expected lo accumulate within the hoJ\ of individuals who have any type of deficiency of glucuronyl transferase. Rlih'F.RLNCl.S 1. Goldberg l- (fid) The tonicity of polychlorinated poluulir com pounds nnd related chemicals. Critic al Reviews m 2, (4). 445, 1974 2. SelikotV U. (I;.d) Polychlorinated htphcnylenMfoitim-ni.il impact a review of the panel on hazardous trace substances l-.uvnoti Research J, 249, 1972 3. Block WD. Cornich Hit Metabolism of biphenyl ami 4 chltir obiphcnyl in rabbil. J Biol Chart. 234 ( 12). 3301, 1959 4. Hul/ingcr O. Nash DM, Safe S. Dcfrcilas ASW. NoMsiutm RJ, Wildnth PJ. 7-itko V. Polyctilorinmed biphenyls: mccilmlu. behavior of pure isomers in ptpeons. tats and brook trout Science 178, 312. 1972 5. Adamson Rll, Davies DS. Comparative aspects >! absorption, distribution, metabolism and rucreltoit of drupe Omi/vim/mPharmacology. Vol 2. p831. Pergamon Picss. be* York. M Micltclson MJ. (l-d) 1973. ft. Dutton GJ. Glucuromdr cnniugaiinti. I'roc 1st Ini I'hurmtiau Meeting. Vol (i. p.39. Pergnmnn Press. London. Dll Urmlte am) I:G Ridas (I ds) |9(>l. 7. Dutton GJ. The mechanism of glucuronidc formation. Biochnr Pharni. 6, 63. 1961 8. Gillette JR. Individually different responses to drops according i<> ape. sex and functional or pathological states. Drug Re\/ximet in Man. p24. Churchill. London. G Wolstenholtne and R Porter, (lids) 1967. 9. Clarke FGC. Clarke ML. Garner's I ctcrimtri lii\nitli>gy 3rd Ldition. pp!56 158. Williams and Wilkins Co. Baltimore. 1967. 10. Jones LM. Veicrinary Parmacolngy anti Therapeutics. 3rd Lilition. pp442-447. Iowa Slate University Press. Ames, 1965 11. Steeber PO. (fid) The Merck Index. 8th Fdilion. pH 10. Merck and Co, Rahvetty, NJ, 1968. 12. Wilkensnn TT. A review of drug toxicity in the cat. J Small Animal Pract. 9, 21, 1968 13. Ochinc i:M A comparative study of the bintransfornialioii and excretion of phenol. PhD Dissertation. (. of Missouri. 1969 14. Nyhan WL. Toxicology of drugs m the neonatal period Jnur Pediatries. 39 ()), I. 1961. 15. Smith KL, Williams RT. Implication of the vonjugiunm of Jrugv and other exogenous compounds Glucuronic Aetd. j*457. Academe Press. NY. GJ Dutton (lid) 1966. 16. Rcrglund F. Levels of polychlorinated biphenyl in lood in Sweden Knviron Hllh t'erspec. 1, 67, 1967 17. Vos Jl). Toxicity of PCD on non human mummals and on budi linviron tilth I'crspcc I, 120. 1972 18. Garthcr LM. Arias IM. Studies of prolonged nennaial jaundice m the breast-fed infant. Jour Pediatries. 68 (I). 54. 1966 (9. Sutherland JM. Keller Wit. Novohicin and neonatal hyperhilt rubmcmia. Am J Dis Child. 101, 4-17. (961 20. Lokeitr II. Dowhen KM. Hsia DY. Studies on the efleci of novobiocin ami glucuronyl transferase Pediatries 32, 47. 1961 21. Lester R. Schmid K. Bilirubin metabolism. New Lng J Med. 270 (15). 779 1964 22. Allen JK. Cnrlcns LA, Abrnhamson I.J. Marini RJ. Response* of rats and non human primates to 2,5,2'.5 ' ictrachlorohtphcnyl. linviron Research 9, 265. 1975 23. Billing 1)11 Bilirubin metabolism nnd jaundice with special refer enec to unconjugated hypcrbilirubmcmia. Annals of Clin lltochen. 7, 69. 1970 24. Kornberg A. I.atcul liver disease m persons rcct>vcred from catarrhal jaundice ami in otherwise normal medical students n revealed by the bilirubin excretion test. Jour Clm Invest 21, 299. 1942 25. Owens D. I vans J Populotinn studies on gilbert's syndrome Jour Med Genetics. 12, 152. 1976 26. Yafi'c SJ. Levy G, Matsueawa T, Baliah 1. I nhaiucmciii of glu curoniilc conjugating capacity in n hyperhiliritbtnemic ml:mi dut lo apparent en/yme induction by phenobarbital New lug J Med. 275, 1461. 1966 27. Smith PM. Middleton JU. Williams R. Studies on die bumlii! incidence and clinical history of patients with chtoinc unconjug.UA} hyperhilirubinemia_Gut. R. 469. 1968 ^ 28. Arias IM. Gunner LM. Cohen M. issci JB. Levi AJ Chtoru. non hcmoiyiic unconjugated hypcrbilirubmcmi.i wnh glucutonyl transferase deficiency. Amer J Med. 47, 395. 1969 29. Kreck MJ. Slciscngcr MIL Reduction of scrum unconjugaled bilirubin with hyperhilirubinarmia. Lancet I. 73. 1968 30. Thompson RPO. Slathers GM. Pilcher CW1, McLean M:M. Robinson J. Williams R. Treatment of untoniugated juundicr with iticopbiinc. l.nncri 1, 4. 1969 >}. NVhdum NH. KruMcv LP. Hiding HH. Reduction in scrum bilirubin by phcimbnrbiinl in nriult imconjupntcd hyperbilirubinemia. Is fnryrnc imluiMion rc,\|>im.\iblc? Amcr J Med. 45, 160. 1968 33. Marvcr IIS. Schmid R. IlKtlnmsfnrmntion in the liver: implications .foi human danise. Onsiroenlcrolopy 55. 7*?. IU6B HONS 001147 165