Antipyrine metabolism in the rat by three hepatic monooxygenases

The relative contribution of 3 oxidative reactions of antipyrine to its metabolism in vivo were assessed by comparing male and female rats, and by studying the effects of phenobarbital (PB) and 3-methylcholanthrene (MC). After [N¹⁴CH₃] antipyrine, 11–15% of the dose was excreted as ¹⁴CO₂ in both sexes as a consequence of N-demethylation. PB pretreatment had no effect but MC doubled ¹⁴CO₂ excretion. The other metabolites, 4-hydroxyantipyrine (4OHA) and 3-hydroxymethylantipyrine (3HMA), in urine were determined by thin-layer chromatography. Of the total activity in the urine, 4OHA represented 30% in male and 40% in female rats; 3HMA represented 35% in male and 20% in female rats. The ratio of 4OHA to 3HMA in both sexes increased after PB and MC, the effect being more pronounced with the latter. The results show that the 3 major oxidative pathways of antipyrine metabolism are mediated by different enzymes, almost certainly different forms of cytochrome P450.     

Influence of tauroursodeoxycholic and taurodeoxycholic acids on hepatic metabolism and biliary secretion of phosphatidylcholine in the isolated rat liver

Studies were carried out using an isolated rat liver system to define: the contribution of exogenous phosphatidylcholine (PC) to biliary phospholipid secretion; and its hepatic metabolism during perfusion of the livers with conjugated bile salts with different hydrophilic/hydrophobic properties. A tracer dose of sn-1-palmitoyl-sn-2-[14C]linoleoylPC was injected as a bolus into the recirculating liver perfusate, under constant infusion of 0.75 mumol/min of tauroursodeoxycholate or taurodeoxycholate. The effects on bile flow, biliary lipid secretion, 14C disappearance from the perfusate and its appearance in bile, as well as hepatic and biliary biotransformation were determined. With both the bile salts, about 40% of the [14C]PC was taken up by the liver from the perfusate over 100 min. During the same period less than 2% of the given radioactivity was secreted into bile. More than 95% of the 14C recovered in bile was located within the identical injected PC molecular species. The biliary secretion of labeled as well as unlabeled PC, however, was significantly higher in livers perfused with taurodeoxycholate than tauroursodeoxycholate, while the reverse was observed with respect to bile flow and total bile salt secretion. The exogenous PC underwent extensive hepatic metabolization which appeared to be influenced by the type of bile salt perfusing the liver. After 2 h perfusion, the liver radioactivity was found, in decreasing order, in PC, triacylglycerol, phosphatidylethanolamine and diacylglycerol. In addition, the specific activity of triacylglycerol was significantly higher in tauroursodeoxycholate than in taurodeoxycholate-perfused livers (P less than 0.025), while the reverse was true for the specific activity of hepatic PC (P less than 0.01). Because taurodeoxycholate and tauroursodeoxycholate showed opposite effects on both biliary lipid secretion and hepatic PC biotransformations, we conclude that the hepatic metabolism of glycerolipids is influenced by the physiochemical properties of bile salts.     

The effect of environmental lighting on porphyrin hepatic metabolism in the rat

It has been shown that light may stimulate hepatic porphyria in the rat. This is evidently due to a neuro-endocrine pathway involving retina, nerve pathways in the brain and including the sympathetic chain, the pineal gland and the gonads. This light effect in porphyria appears to be via a circadian rhythm in the liver.     

Investigations into the hepatic metabolism of dimethylnitrosamine in the rat.

In this paper we report the detection and identification of methanol as an intermediate formed during both the $\text{in vivo}$ and the $\text{in vitro}$ metabolism of dimethylnitrosamine (DMN) in the rat. Methanol was formed in both hepatic 10,000 $\text{g}$ av. supernatant and washed microsomal fractions over a wide range of nitrosamine substrate concentrations. Furthermore the total amounts of methanol and formaldehyde formed largely accounted for the metabolic fate of both methyl moieties of DMN. Although a number of inhibitors of alcohol metabolism profoundly inhibited the hepatic metabolism of DMN they had little effect on the activities of two mixed function oxidase dependent enzymes. The results suggest that DMN and possibly other dialkylnitrosamines are degraded by enzymic pathway(s) not dependent on cytochrome P-450.     

Effects of dieldrin on hepatic carbohydrate metabolism in the suckling and adult rat

Hepatic carbohydrate metabolism was studied in adult and suckling rats given age-specific LD50 doses of dieldrin po. These doses in 5-, 10-, and 60-day-old Wistar rats were 38, 28, and 63 mg/kg, respectively. Plasma glucose and free fatty acids (FFA), and hepatic glycogen, phosphoenolpyruvate carboxykinase (PEPCK), fructose-1,6-diphosphatase (FDP), and glucose-6-phosphatase (G6P) were measured 1 and 3 h after administration of the insecticide. Plasma glucose concentrations were elevated (17%) in some 5-day-old rats after 1 h and in all adults after 1 and 3 h (45 and 30%, respectively). Plasma FFA concentrations were decreased (9%) in the 5-day-old rat 1 h after dieldrin. Hepatic glycogen content was reduced in both 5- and 10-day-old pups at 1 hour (22 and 17%, respectively). Hepatic FDP activity was elevated in the 5-day-old rat at 1 h (17%) and was decreased (10%) in the 10-day-old rat at 3 h. Hepatic PEPCK activity was increased in adult animals by 30% 1 h after dieldrin. Furthermore, PEPCK activity was increased at 3 h in rats of all ages (76%, 5-day-old pup; 115%, 10-day-old pup; 56%, 60-day-old adult). Hepatic G6P activity was unaltered by dieldrin. Thus only the activity of hepatic PEPCK is consistently elevated by dieldrin exposure. However, this enhanced PEPCK activity is associated with dieldrin-induced hyperglycemia only in the adult rat.     

The effect of selenium on the hepatic drug metabolism and inducibility in rat

1. Rats were fed either a normal or selenium-deficient diet for 4 weeks. The subgroup on selenium deficient diet had selenium supplementation as 3 ppm Se in the drinking water. Benzo(a)pyrene was given intraperitoneally as an inducer. 2. Se deficiency decreased glutathione peroxidase and cytochrome c-reductase activities while other activities were unchanged as compared to normal diet. 3. Selenium deficiency was a prerequisite for the induction of glutathione peroxidase, S-reductase and S-transferase enzymes. 4. Benzo(a)pyrene increased hepatic microsomal cytochrome P-450 content in rats on normal and selenium supplemented diet but not in the selenium deficient group. 5. The 7-ethoxyresorufin and 7-ethoxycoumarin deethylase, aryl hydrocarbon hydroxylase and cytochrome c-reductase activities were increased by benzo(a)pyrene in all the dietary groups. 6. The UDP-glucuronosyltransferase activity was also increased by benzo(a)pyrene in all the experimental groups and this was true with p-nitrophenol and phenolphthalein as aglycons.     

Influence of glucogenic amino acids on the hepatic metabolism of threonine.

The supplementation of a low-protein diet with L-threonine leads to a marked accumulation of threonine in plasma and liver, whereas increasing dietary protein generally leads to an induction of threonine dehydratase in the liver, hence depressed availability for extrasplanchnic tissues. The aim of the present study was, thus, to further investigate the factors which control the utilization of threonine by the liver. Increasing the dietary supply of threonine led to parallel increases in the afferent and hepatic concentrations and in the rate of utilization by the liver; however, the fractional extraction tended to decrease. It appears that the addition of a mixture of glucogenic amino acids to the diet prevented the accumulation of threonine in plasma induced by exogenous threonine. The glucogenic amino acids increased the fractional hepatic uptake of threonine, and counteracted its accumulation in the liver. These effects reflect the fact that the glucogenic amino acids elicited a potent induction of the threonine dehydratase, whereas threonine alone was uneffective. Our results suggest that, besides the well-established effect of glucogenic conditions, the availability of some glucogenic amino acids is an important factor in the control of threonine catabolism.     

Cobalt-protoporphyrin, a synthetic heme analogue, feminizes hepatic androgen metabolism in the rat

A single injection of cobalt-protoporphyrin (50 mumol/kg) produced marked changes in the metabolism of 14C-labeled testosterone and 4-androstenedione by male rat liver microsomes and this effect was maintained for at least 3 weeks. The rate of 3 beta- and 5 alpha-reduction was increased to levels observed in untreated adult female animals and cobalt-protoporphyrin altered the metabolic profile of testosterone towards that observed after infusion of growth hormone whereas hypophysectomy produced a more general inhibition of androgen metabolism. The reduction of testosterone or 4-androstenedione by liver microsomes was also increased when cobalt-protoporphyrin (10-30 microM) was added in vitro but a higher concentration (100 microM) led to inhibition of androgen metabolism. The identity of the main androgen metabolites was established by TLC, HPLC and mass spectrometry and the role of 5 alpha-reductase was demonstrated using a specific inhibitor of this enzyme. The possible sites of action of cobalt-protoporphyrin are discussed in relation to its in vivo effects on serum testosterone and LH concentrations.     

Influence of valproic acid on hepatic carbohydrate and lipid metabolism

Valproic acid (dipropylacetic acid), an antiepileptic agent known to be hepatotoxic in some patients, caused inhibition of lactate gluconeogenesis, fatty acid oxidation, and fatty acid synthesis by isolated hepatocytes. The latter process was the most sensitive to valproic acid, 50% inhibition occurring at ca. 125 microM with cells from meal-fed female rats. The medium-chain acyl-CoA ester fraction was increased whereas coenzyme A (CoA), acetyl-CoA, and the long chain acyl-CoA fractions were decreased by valproic acid. The increase in the medium chain acyl-CoA fraction was found by high-pressure liquid chromatography to be due to the accumulation of valproyl-CoA plus an apparent CoAester metabolite of valproyl-CoA. Salicylate inhibited valproyl-CoA formation and partially protected against valproic acid inhibition of hepatic metabolic processes. Octanoate had a similar protective effect, suggesting that activation of valproic acid in the mitosol is required for its inhibitory effects. It is proposed that either valproyl-CoA itself or the sequestration of CoA causes inhibition of metabolic processes. Valproyl-CoA formation also appears to explain valproic acid inhibition of gluconeogenesis by isolated kidney tubules. No evidence was found for the accumulation of valproyl-CoA in brain tissue, suggesting that the effects of valproic acid in the central nervous system are independent of the formation of this metabolite.