Xenobiotic biotransformation in livestock: comparison to other species commonly used in toxicity testing

The postnatal development of drug-metabolizing enzymes in hepatic, pulmonary and renal tissues of the goat

It is important to study the development of drug biotransformation enzymes, because from a pharmacological and therapeutic point of view these enzymes are responsible for eliminating most drugs. Their concentration at each age is critical when deciding the dose regimen, particularly in the neonates who are deficient or have very low levels of these enzymes. From a toxicological perspective, the role of these enzymes varies, with some of them being directly responsible for activation of certain chemicals to reactive intermediates with deleterious consequences to the animal. The time course of appearance of these enzymes throughout the life of the animal could be depicted from the study of their ontogeny and therefore the prediction of when the animal would be at risk should be possible. Experiments were designed to measure in vitro, the activity of drug-metabolizing enzymes in liver, lung and kidney of newborn, 1-week-, 4-week and 6-week-old and adult goats. The microsomal monoxygenase activities were measured utilizing substrates designed to characterize the development of the cytochrome P450 (P450). For phase II enzymes, the activity of UDP-glucuronyltransferase towards 1-naphthol and p-nitrophenol was measured in addition to the cytosolic glutathione S-transferase activity towards, 1,2-dichloro 3-nitrobenzene. The results indicated that the newborn goat tissues exhibited very low activity of drug-metabolizing capacity in all pathways studied. These activities increased to the adult values by 6 weeks of age. In general, the development of the mono-oxygenase activities followed the same pattern as the overall P450. The UDP-glucuronyltransferase activity towards both substrates was deficient at birth and surged to above adult values by the first week of age. The toxicologic and pharmacologic implication of the development of these enzyme activities are discussed.

Antipyrine, erythromycin and oxytetracycline disposition in experimental fasciolosis

The effects of fasciolosis on drug disposition were studied by administration of antipyrine, erythromycin and oxytetracycline to sheep and cattle. Fasciolosis was produced by administration of 200 or 400 metacercariae (MC) of Fasciola hepatica to sheep and 500 MC to cattle. The disease was subsequently confirmed by determination of plasma glutamate dehydrogenase and gamma-glutamyl transferase and identification and quantitation of mature flukes in the liver at necropsy. Acute or subacute fasciolosis in sheep was accompanied by a significant decrease in the elimination rate constant (beta) and increase in the elimination half-time (t 1/2) for antipyrine and erythromycin when compared with controls or infected sheep which had been treated with the anthelmintic luxabendazole. An increase in apparent volume of distribution (Vd) was seen only for erythromycin in sheep given 400 MC. There were no changes in the disposition of oxytetracycline in sheep with either acute or subacute infection and no effects on disposition of the three test drugs in chronically infected sheep. With early chronic disease in calves, only the disposition of oxytetracycline was affected; not that of antipyrine or erythromycin.

Microsomal enzymes in lambs and adult sheep, and their possible relationship to alveld

The difference in susceptibility to alveld between lambs and adult sheep may be caused by differences in the microsomal enzyme activities in their livers. There was no difference in NADPH-cytochrome c reductase or 7-ethoxyresorufin-O-deethylase (EROD) activity between ewes, control lambs and phenobarbitone-dosed lambs 3 weeks after dosing ceased. However, aldrin epoxidase activity was at that time significantly highest in the phenobarbitone-dosed lambs and significantly lowest in the ewes. The liver cytosolic glutathione transferase activity was significantly highest in the ewes and significantly lowest in the control lambs at the same time.

Evidence for the absence of cysteine S-conjugate N-acetyltransferase activity in the metabolism of propachlor, naphthalene, and dichlobanil in calves

1. The glutathione conjugate of 2-chloro-N-isopropyl[1-14C]acetanilide (14C-propachlor) was perfused through a calf kidney in situ; 23% of the dose was excreted in the perfused kidney urine as the cysteine conjugate, no mercapturic acid was detected. 2. A 5-day-old calf dosed orally with 14C-propachlor excreted 70% dose in the urine as the cysteine conjugate; no mercapturic acid was detected. Rumen microflora were established in the calf (5 weeks older) and the experiment was repeated with the same results. 3. When the same calf was dosed 1 week later with 14C-naphthalene, 99% dose was excreted in the urine, mostly as the dihydrodiol-glucuronide (34%) and the dihydrohydroxy-cysteine conjugate (47%); no mercapturate was detected. 4. A 9-day-old calf dosed orally with 2,6,-dichlorobenzo[14C]nitrile (14C-dichlobanil) excreted 67% dose in the urine as cysteine conjugates (34%), and products of cysteine conjugate beta-lyase cleavage of cysteine conjugates (30%); no mercapturates were detected. 5. Cysteine S-conjugate N-acetyltransferase activity in calf kidney and liver was about 10% of that in the corresponding rat tissues.

The use of porcine hepatocytes for biotransformation studies of veterinary drugs

1. Cultures of porcine hepatocytes with high viability were isolated from a liver sample by a simple procedure. In ageing monolayer cultures the cytochrome P-450 content and 7-ethoxycoumarin-O-deethylase activity decreased gradually while glutathione levels increased. 2. The nitrofuran, furazolidone, was rapidly metabolized, partly resulting in the formation of 3-(4-cyano-2-oxobutylidene amino)-2-oxazolidone. 3. Acetylating and deacetylating activities towards sulphadimidine and its N4-acetyl metabolite were present in porcine hepatocytes. Relative and absolute levels of these activities varied in different batches of hepatocytes. 4. No differences were seen in a number of enzyme activities measured in cytosolic and microsomal fractions isolated from different lobes of one liver. Differences between livers from different animals were marked.

The activities of aminopyrine N-demethylase, aniline 4-hydroxylase and UDP-glucuronyltransferase in tissues of camels, desert sheep and Nubian goats

1. The activities of the drug-metabolizing enzymes, aminopyrine N-demethylase, aniline 4-hydroxylase and UDP-glucuronyltransferase have been measured in vitro in the liver, kidney and duodenal mucosa of camels, sheep, goats and rats. 2. Enzyme activities were generally higher in the liver, followed by the duodenal mucosa, then the kidney in all species. 3. Male kids had much lower enzyme activity in the liver when compared to adult goats, and in the former animal, no measurable activity could be detected in the duodenal mucosa or kidney. 4. In general, goats seemed to have the highest and camels the lowest enzyme activity when compared to the other species. 5. Some sex differences were noted in the three enzymes studied. In sheep duodenal mucosa and rat liver and duodenal mucosa, males had higher aminopyrine N-demethylase than females. In rat liver and goat duodenal mucosa males had higher aniline 4-hydroxylase than females. Male rats had higher UDP-glucuronyltransferase in liver when compared to females.

Comparison of hepatic drug metabolizing enzyme activities in several agricultural species

1. Several pathways of drug metabolizing enzyme activity were measured in hepatic fractions of cattle, sheep, goats, chickens, turkeys, ducks, rabbits and rats. The pathways examined included the O-demethylation of p-nitrophenol, microsomal ester hydrolysis of procaine and glucuronidation of p-nitrophenol, and the cytosolic acetylation of sulfamethazine and sulfation of 2-naphthol. 2. For most enzymatic pathways measured, goats were more similar to sheep (wether) than to cattle (steers). The exception was UDP-glucuronyltransferase activity, which was significantly higher for the goat than for any other species studied. 3. Within the avian subset, the chicken and turkey were usually the most similar species. 4. The activities of arylsulfotransferase isozymes III and IV were particularly low for the duck compared to the chicken and turkey. 5. N-acetyltransferase activity was very high for rabbits and very low for sheep and goats.

Application of isolated hepatocytes to studies of drug metabolism in large food animals

A definitive hazard assessment of xenobiotics translocated through food animals into edible products such as meat or milk requires a complete analysis of metabolism in food animals. However, large animal metabolism studies present many experimental difficulties. None of several in vitro alternatives such as subcellular fractions has been established as an acceptable predictor of in vivo metabolism. The feasibility of using isolated hepatocytes to predict the metabolism of xenobiotics, both quantitatively and qualitatively, in large ruminant animals (e.g. cattle) is being studied in our laboratory. A procedure was developed for isolating hepatocytes aseptically from the caudate process of the liver which was obtained surgically from 100-125 kg calves. A modified two-step vascular perfusion procedure provides hepatocyte suspensions that are typically greater than or equal to 85% viable and greater than or equal to 1 X 10(7) viable hepatocytes/g of liver (wet wt). Xenobiotic metabolism has been evaluated in suspensions and primary cultures using aldrin epoxidation, ethoxycoumarin O-deethylation, and 7-hydroxycoumarin glucuronidation and sulfation. Metabolic activities are relatively short-lived in suspensions less than or equal to 4 h, but quite stable up to 10 h when cultured on collagen-coated plates in chemically defined medium. Bovine hepatocytes behave similarly in culture to rodent hepatocytes. Although primary culturing of hepatocytes is more difficult than suspensions, primarily due to the asepsis requirements, it is the method of choice for xenobiotic metabolism determinations in isolated hepatocytes of cattle.

Xenobiotic biotransformation in livers and lungs of adult black-tailed deer: comparison with domestic goat and sheep

1. The capacity of liver and lung tissue of black-tailed dear (Odocoileus hemionus columbianus) to biotransform xenobiotics was compared in vitro to the domestic sheep and goat. Donor animals were all females of varying ages. Tissues from the black-tailed deer were collected in the wild. A variety of biotransformation enzymes were measured in both microsomal and cytosolic fractions. 2. Deer liver was lower in total cytochrome P450 concentration, but mono-oxygenase activities were greater compared to sheep and goat. The opposite was true for the lung. 3. Epoxide hydrolase activities were significantly different in deer vs sheep and goat. 4. In general, both hepatic and pulmonary activities were more similar between sheep and goat than either species compared to the deer, however, the magnitude of the hepatic differences did not exceed 5-fold. 5. Based on these limited results, there is no reason to discredit the sheep or goat as a toxicity testing model for deer.