Effect of development on the activity of microsomal drug-metabolizing enzymes in rat liver

1. The activity of rat liver microsomal drug-metabolizing enzymes was determined at various ages between 6 and 100 days post natum. The enzymes studied were: aromatic hydroxylases by using as substrate biphenyl, which is metabolized by oxidation to 2- and 4-hydroxybiphenyl; nitroreductase by using p-nitrobenzoate as substrate, which is metabolized by reduction to p-aminobenzoate; glucuronyl synthetase by using 4-methylumbelliferone as the substrate, which is conjugated to give 4-methylumbelliferone glucuronide, and cytochrome P-450, which is regarded as the major terminal mixed-function oxidase in hepatic microsomal hydroxylations. 2. The activity of biphenyl 2-hydroxylase reached a peak at 21 days, biphenyl 4-hydroxylase and 4-methyl glucuronyl transferase at 24 days, cytochrome P-450 at 31 days, and p-nitrobenzoate reductase at 38 days of age. After the peak activity had been reached, the activity of each enzyme decreased with age, and in the case of biphenyl 2-hydroxylase the activity fell to a negligible value at 52 days of age. 3. Neither the addition of Triton X-100 to the incubation medium nor the treatment of the animals with phenobarbital resulted in any increase in the activity of biphenyl 2-hydroxylase at 52 days of age. 4. The activity of biphenyl 2-hydroxylase was threefold higher in rats fed on a synthetic diet than in rats fed on a commercial stock diet. 5. These findings are discussed.     

Heterogenous effects of anthraquinones on drug-metabolizing enzymes in the liver and small intestine of rat

The induction of a variety of drug-metabolizing enzymes by six anthraquinones (AQs) has been investigated in the liver and small intestine of rat. In the liver, the intragastric administration for 3 days of 100 mg/kg 9,10-anthraquinone (9,10-AQ). 1-hydroxy-AQ, 1,4-dihydroxy-AQ, but not 1,2-dihydroxy-AQ and 2-carboxy-AQ, resulted in a significant induction of the UDP-GT, DT-diaphorase, P450 1A-linked monooxygenase activities and in particular the methoxyresorufin-O-demethylase (MEROD), an activity dependent on P450 1A2. Immunoblot analysis indicated that 1-hydroxy-AQ and 1,4-dihydroxy-AQ induced P450 1A2 but not 1A1 and 9,10-AQ induced both P4501A2 and P4502B. Northern blotanalysis, using a cDNA probe for CYP 1A1 and CYP 1A2, confirmed that the AQs induce CYP 1A2 but not 1A1 mRNA. In the mucosa of small intestine, none of the above-mentioned enzymatic activities were enhanced following AQ administration. The induction mechanism of the hepatic enzymes by AQs is not known and it deserves a further study as it might be independent from the activation of the Ah-receptor as reported for other tricyclic compounds. The results from inhibition experiments showed that the hydroxylated AQs were strong inhibitors of P450 1A2-dependent monooxygenases. This suggests that long-term ingestion of certain AQs, may affect the toxicity of other components present in the diet through the hepatic induction or inhibition of P450 1A2.     

Effects of Rumex patientia L. extract on some drug-metabolizing enzymes in rat liver

The effect of aqueous extract from the roots of Rumex patientia L. (Polygonaceae) (D-1), a traditional Turkish medicine used as a laxative and cholagogue, on drug-metabolizing enzymes, such as cytochrome P4502E1, NADPH cytochrome c reductase, NADH cytochrome b5 reductase and glutathione-S-transferase (GST); and serum aspartate aminotransferase (AST) and alanine aminotransferase (ALT) were studied in male Wistar albino rat liver. A significant increase was observed in cytochrome P4502E1 and GST activities, but not in NADPH-cytochrome c reductase and NADH-cytochrome b5 reductase activities. Serum AST and ALT activities were found within the normal laboratory range values. The results demonstrated that the aqueous extract of R. patientia triggers induction of cytochrome P4502E1 in liver and cytosolic GST activity.     

Microsomal inducers of drug-metabolizing enzymes suppress cytidine nucleotide biosynthesis in rat liver.

The biosynthesis of cytidine nucleotides of liver rRNA was studied following single administration of seventeen different compounds. Only the substances recognized as inducers of the mixed-function oxidases of liver microsomes decrease the utilization of [2-¹⁴C]orotic acid for the synthesis of rRNA cytidine nucleotides.     

The development of oxidative enzymes in rat liver mitochondria.

During early postnatal development there was an increase in the specific activity of a number of oxidative enzymes localized on the outer and inner mitochondrial membrane. The succinic oxidase complex of the inner mitochondrial membrane, whose activity in 1-day-old rats was 50% of the value in adult animals, attained the maximum on about the 10th day after birth. Activity of the choline and the proline oxidase complex, both of which are also localized in the inner mitochondrial membrane, was minimal in 1-day-old rats and went on rising after the 10th day. Rotenone-insensitive NADH-cytochrome c reductase activity, which is localized on the outer mitochondrial membrane, remained stable up to the 10th day, and rose between the 10th and the 90th day. Developmental changes in monoaminooxidase activity, which is likewise localized on the outer mitochondrial membrane, followed a similar course to the choline and proline oxidase complexes. The amount of cytochromes a+alpha3 and cytochrome b in isolated mitochondria did not alter during development. The protein spectrum of the mitochondrial particles, determined by polyacrylamide gel electrophoresis in sodium dodecyl sulphate, likewise displayed no marked changes during postnatal development. The above findings show that the metabolic functions of the mitochondria mature during development and that changes in the different enzymes have their own characteristic time course.     

Regulation of drug-metabolizing enzymes during the perinatal period in rat and human liver

The importance of drug-metabolizing enzymes in developing mammals has been recently reevaluated in view of the activities and potential inducibilities of these enzymes. The role of endogenous factors raises the question of whether there is a positive regulation of the expression of drug-metabolizing enzymes by hormones. In humans, among the different isoenzymes of cytochrome P-450 described in adult liver, only one is absent in 20-week-old fetuses. Epoxide hydrolase and glutathione S-transferases are active while UDP-glucuronidation develops postnatally. The consequence of this asynchronous rise of activities is briefly discussed.     

The influence of phenobarbitone on maternal and perinatal hepatic drug-metabolizing enzymes in the rat.

Phenobarbitone (PB) (75 mg/kg) was administered orally for three consecutive days to pregnant or lactating rats at different pre- and postnatal stages in order that the perinatal animals would receive the agent either by transplacental passage or via the milk. Control animals received equivalent volumes of saline. The dams, fetuses, and pups were killed 24 h after the last dose. Hepatic p-nitroanisole O-demetnylase (OD), carboxylesterase (CE), and bromosulfophthalein-glutathione (BSP-GSH) conjugating enzyme activities in a 12 100 g - 20 min supernatant of a 20% w/v homogenate were measured. The morphology of the developing rat liver in the absence and presence of PB was examined by electron microscopy. The results demonstrated that the transplacental passage of PB to rat fetuses at term or 3 days prepartum had no effect on either the hepatic drug-metabolizing enzyme activities or on the ultrastructural appearance of the liver. Increased hepatic OD activity was observed in the pregnant animal but no effect was observed in the lactating dam. Phenobarbitone received by the suckling rat had two distinct effects. Compared to control activities, twofold increases in hepatic OD activity were observed in rat pups as early as 4 days after birth, associated with a marked proliferation in hepatic smooth endoplasmic reticulum. In contrast, PB-related significant increases in neonatal hepatic CE and BSP-GSH conjugating enzyme activities were not observed until 21 days of age. In the 4-day-old treated pups, characteristic morphological changes included numerous small membrane whorls in addition to increased smooth endoplasmic reticulum and microbodies in the liver.     

The effects of the continuous administration of N,N-dimethyl-4-phenylazoaniline (DAB) on the activities and the inducibilities of some drug-metabolizing enzymes in rat liver.

(1) The effect of feeding a relatively low-protein diet containing 0.06% DAB for 29 weeks on the activity of DAB-azoreductase, nitroreductase (p-nitrobenzoic acid), N-oxidase (N,N-dimethylaniline), N-demethylase (DAB), cytochrome P-450, NADPH-cytochrome c reductase, beta-glucuronidase and arylsulphatase A were studied. Rapid decreases occurred in the activities of the first six enzymes, reaching minimal values at between 4 and 8 weeks. Activities then increased in all cases to control or nearly control levels. This rate of increase was least for cytochrome P-450. At 4 weeks azoreductase activity with the chemotherapeutic agent CB10-252 (I) as substrate was significantly higher than in control rats. Early increases occurred in the activities of beta-glucuronidase and arylsulphatase A and the activity of the latter never dropped below the control level. (2) An investigation was made of the differential effects of dye feeding on some of the enzyme activities in the two major liver lobes and differences were found. (3) The effect of phenobarbital (PB) pretreatment on the DAB-fed rats was studied at 4-week intervals. The activities of DAB-azoreductase and of nitroreductase increased throughout the whole period, while the activities of the lysosomal enzymes were decreased. (4) After feeding DAB for 4 weeks the effect of PB and 3-methylcholanthrene (MC) on the activities of DAB-azoreductase, CB10-252-azoreductase and components of the azoreductases-cytochrome P-450, NADPH-cytochrome c reductase, the CO-CB10-252-azoreductase was not induced by PB or MC, and CO did not inhibit its reduction. Its reduction depended only slightly on NADH. CO caused a greater relative decrease in the activity of DAB-azoreductase in dye-fed animals and also in animals following PB and MC pretreatment, implying a greater role of cytochrome P-450 in dye-fed animals.     

Early effects of aminonucleoside on enzymes in the Golgi apparatus of rat liver.

Rats were injected with a single intravenous dose of aminonucleoside (AMN) and sacrificed 1-48 h later. The activity of several enzymes was assayed in the Golgi apparatus isolated from the liver. Galactosyltransferase activity showed little changes after the AMN, but both acid (EC 3.1.3.2) and alkaline phosphatase (EC 3.1.3.1) activities increased within the first hour and reached control levels only 5-24 h later. Thiamine pyrophosphatase and arylsulfatase A (EC 3.1.6.1) activities also increased and stayed at higher levels for the duration of the experiment. Arylsulfatase B (EC 3.1.6.1) activity decreased shortly after the AMN but later increased to above control levels. These findings support earlier results in which liver ultrastructural and biochemical changes were observed early before renal lesions and proteinuria.     

Postnatal development of peroxisomal and mitochondrial enzymes in rat liver.

Subcellular organellles from livers of rats three days prenatal to 50 weeks postnatal were separated on sucrose gradients. The peroxisomes had a constant density of 1.243 g/ml throughout the life of the animal. The density of the mitochondria changed from about 1.236 g/ml at birth to a constant value of 1.200 g/ml after two weeks. The peroxisomal and mitochondrial fatty acid beta-oxidation and the peroxisomal and supernatant activities of catalase and glycerol-3-phosphate dehydrogenase were measured at each age, as well as the peroxisomal core enzyme, urate oxidase, and the mitochondrial matrix enzyme, glutamate dehydrogenase. All of these activities were very low or undetectable before birth. Mitochondrial glutamate dehydrogenase and peroxisomal urate oxidase reached maximal activities per g of liver at two and five weeks of age, respectively. Fatty acid beta-oxidation in both peroxisomes and mitochondria and peroxisomal glycerol-3-phosphate dehydrogenase exhibited maximum activities per g of liver between one and two weeks of age before weaning and then decreased to steady state levels in the adult. Peroxisomal beta-oxidation accounted for at least 10% of the total beta-oxidation activity in the young rat liver, but became 30% of the total in the liver of the adult female and 20% in the adult male due to a decrease in mitochondrial beta-oxidation after two weeks of age. The greatest change in beta-oxidation was in the mitochondrial fraction rather than in the peroxisomes. At two weeks of age, four times as much beta-oxidation activity was in the mitochondria as in the peroxisomal fraction. Peroxisomal glycerol-3-phosphate dehydrogenase activity accounted for 5% to 7% of the total activity in animals younger than one week, but only 1% to 2% in animals older than one week. Up to three weeks of age, 85% to 90% of the liver catalase was recovered in the peroxisomes. The activity of peroxisomal catalase per g of rat liver remained constant after three weeks of age, but the total activity of catalase further increased 2.5- to 3-fold, and all of the increased activity was in the supernatant fraction.     

The effects of benzopyrene and safrole on biphenyl 2-hydroxylase and other drug-metabolizing enzymes.

A study was made of the nature and specificity of the increase in biphenyl 2-hydroxylase activity after preincubation of liver microsomal preparations with various carcinogens in vitro. This enhancement of enzyme activity in vitro was investigated in mouse, hamster and rat, and although the rat appears to be atypical in the variation of the pattern of 2- and 4-hydroxylation with age, similar enhancements were detectable in each species examined. An increase in biphenyl 2-hydroxylase activity was apparent 2h after intraperitoneal administration of safrole or benzopyrene to mature Wistar albino rats and appeared to be similar in nature to that observed after preincubation of liver microsomal preparations with the same chemical in vitro. Investigation of other drug-metabolizing enzyme systems suggests that the enhancing effects of carcinogens in vitro are specific for biphenyl 2-hydroxylase. No correlation between the enhancement of biphenyl 2-hydroxylase and inhibtion of biphenyl 4-hydroxylase was apparent.     

Factors influencing the development of urea-synthesizing enzymes in rat liver

1. The activities of enzymes of the urea cycle, carbamoyl phosphate synthetase, ornithine transcarbamoylase, argininosuccinate synthetase, argininosuccinase (the last two comprising the arginine synthetase system) and arginase, were measured in the liver during development of the rat. All five enzymes exhibited relatively low activities in foetal liver and a rapid postnatal increase was found. The rate-limiting enzyme of urea synthesis in the rat, the condensing enzyme of the arginine synthetase system, showed the lowest activity at birth and the most rapid postnatal increase, a fivefold increase within 24hr. after birth. A second increase of activity was noted after the tenth day. These results suggest that the postnatal increase of arginine synthetase activity initiates the ability for urea synthesis in the rat. 2. Some factors influencing the development of the rate-limiting arginine synthetase system were studied in more detail. (a) Intraperitoneal administration of puromycin inhibited the postnatal increaseof the enzyme activity. (b) Starvation of newborn animals for 24hr. after birth had no effect on the postnatal development of the enzyme. (c) Bilateral adrenalectomy at birth caused a marked diminution in the postnatal increase of the enzyme activity and injections of triamcinolone were effective in preventing the effect of adrenalectomy. (d) Administration of triamcinolone alone had a marked stimulatory effect on the postnatal development of this enzyme. (e) Premature and postmature birth had virtually no effect on the developmental pattern of the arginine synthetase activity, suggesting that the increase of this enzyme activity after birth is not initiated by the birth process.     

The effects of N-alkylmaleimides on the activity of rat liver glucose 6-phosphatase.

A series of N-alkylmaleimides has been synthesized and used to investigate the thiol groups that are essential for the activity of rat liver microsomal glucose 6-phosphatase. All of the N-alkylmaleimides inactivated glucose 6-phosphatase when preincubated with microsomes (microsomal fractions) at pH 6.5 and 30 degrees C. When enzyme activity was assayed in intact microsomes, the inactivation was non-linear with respect time, showing an initial rapid phase followed by a slower secondary phase. During the initial rapid phase the inactivation may apparently be completely reversed by disrupting the microsomal membrane with detergent. However, after longer exposure to N-alkylmaleimides the reversal is no longer complete. This observation was explained by the results obtained from studying the inactivation in detergent-disrupted microsomes. In this case glucose 6-phosphatase was also completely inactivated, but much more slowly than was seen in intact microsomes, and the process was linear with respect to time. When assayed in both intact and detergent-disrupted microsomes, glucose 6-phosphatase inactivation was dependent on the number of carbon atoms in the alkyl side chain of the N-alkylmaleimides; this dependence was much more marked in disrupted microsomes. Analysis of the data showed that in neither case was there a saturating effect at high concentrations of maleimide. The data have been interpreted to suggest that there are are least two thiol groups essential for activity located in two separate non-polar regions of the membrane-enzyme system. The conclusions are discussed in the light of the current model for the microsomal glucose 6-phosphatase system.     

Alterations in rat liver microsomal and lysosomal β-glucuronidase by compounds which induce hepatic drug-metabolizing enzymes

Chlordane, dieldrin, piperonyl butoxide, and benzpyrene, which induce the hepatic microsomal mixed function oxidases and UDP glucuronyltransferases, decreased activity of smooth and rough endoplasmic reticulum β-glucuronidase. The reduction occurred when either p-nitrophenyl β-D-glucuronide or phenolphthalein mono-β-glucuronide was used as the substrate. Chlordane or dieldrin pretreatment of rats for 3 days resulted in a 2.5-fold reduction in endoplasmic reticulum activity while the reduction was less for piperonyl butoxide or benzpyrene. On the other hand, aminopyrine demethylase and UDP glucuronyltransferase were increased 2-fold by chlordane or dieldrin pretreatments. Decreases in microsomal β-glucuronidase activity might be directly or indirectly involved in the induction process since decreases in β-glucuronidase activity are quantitatively similar to increases in activity of the drug-metabolizing enzymes. Lysosomal β-glucuronidase also decreased following pretreatment of rats with inducing agents, but the reduction was less than that observed in the endoplasmic reticulum fractions. Analysis of pH optima, temperature optima, K_m values, heat denaturation data, and effects of Triton X-100 on activities of various liver fractions suggests that β-glucuronidase from the endoplasmic reticulum and lysosomes have similar properties.