Hepatic Microsomal Enzyme Induction Research Articles

Accelerated elimination of pimonidazole following microsomal enzyme induction in mice: A possible approach to reduced neurotoxicity of the pimonidazole-etanidazole combination

The pimonidazole-etanidazole combination shows promise for improved radiosensitization, although pimonidazole may increase the dose-limiting peripheral neurotoxicity of etanidazole in man. Induction of liver microsomal drug metabolizing enzymes by phenobarbitone resulted in reduced exposure to pimonidazole in plasma, brain, and tumor in mice. Peak tumor concentrations were lowered, but to a lesser extent. Phenobarbitone induction caused no change in the urinary ratio of pimonidazole to its N-oxide metabolite, and in fact, markedly reduced the circulating metabolite concentrations in plasma. There was no effect of phenobarbitone on plasma or tissue pharmacokinetics of etanidazole, which is eliminated by renal clearance. The results suggest that hepatic microsomal enzyme induction may be a possible approach to reducing the toxicity of the pimonidazole-etanidazole combination, and may also provide valuable information on the enzymology of pimonidazole metabolism.

In vivo and in vitro studies of the hepatotoxic effects of 4-chlorophenol in mice

4-Chlorophenol (4-CP) was studied for its toxicological effect on liver by using both in vivo and in vitro approaches. Male mice were administered 4-CP, 1.5 mmol/kg body weight, i.p., and were killed at 10, 20, 30 and 50 min after drug injection. Either i.p. or oral 4-CP administration significantly lowered total liver thiol levels by 20–30% after 30 min and 3 hr respectively. This time-dependent effect of 4-CP after i.p. treatment was enhanced when mice were pretreated with hepatic microsomal enzyme inducers (phenobarbital, 40 mg/kg body weight, b.i.d., 7 days; and β-naphthoflavone, 80 mg/kg body weight once daily, 4 days). Further, the microsomal cytochrome P-450 inhibitor, SKF 525-A, 75 mg/kg body weight injected i.p. to mice 30 min prior to 4-CP administration, blocked the reduction of liver thiol content produced by 4-CP. The results suggest that a chemically reactive intermediate of 4-CP may be formed in liver which is responsible for the observed decrease in liver thiol content. Other investigations were done to characterize the in vitro irreversible binding of [ 14C]4-CP. [ 14C]4-CP was bound irreversibly to mouse liver microsomal proteins in a concentration-dependent manner. Binding was NADPH dependent and gave a maximal binding of 12.0 nmol/mg protein/20 min and an apparent binding constant of 0.222 mM. [ 14C]-Binding of 4-CP was increased by 155 and 127% in liver microsomes of phenobarbital- and β-naphthoflavone-treated mice respectively. By contrast, microsomal enzyme inhibitors [metyrapone, α-naphthoflavone, SKF 525-A, and CO : O 2 (4:1, v/v)] and selected nucleophilic compounds (glutathione, l-cysteine or l-lysine) significantly reduced [ 14C]4-CP binding to mouse liver microsomes. An epoxide hydrolase inhibitor, cyclohexene oxide, did not alter the extent of irreversible binding, whereas scavengers of superoxide anions or agents that are reported to reduce accumulation of active semiquinone and quinone species ( l-ascorbic acid, superoxide dismutase or epinephrine) decreased the binding of [ 14C]4-CP to mouse liver microsomal proteins by 56, 31 and 92% respectively. The data suggest that semiquinone and quinone species of 4-CP may be the chemically reactive intermediates leading to the in vivo reduction of liver thiol levels. Since 4-CP is a minor contaminant and possible metabolite of clofibrate and chemically related hypolipidemic agents, 4-CP and its metabolites may be partly responsible for some of the hepatotoxic effects seen after long-term administration of this therapeutic class of drugs.

The Significance of Hepatic Microsomal Enzyme Induction and Altered Thyroid Function in Rats: Implications for Thyroid Gland Neoplasia

Hepatic microsomal enzymes play an important role in thyroid hormone homeostasis. Glucuronidation of thyroxine is the rate limiting step in the biliary excretion of thyroxine in the rat; the monodeiodinases are important in the conversion of T4 to T3 and reverse T3 and subsequent deiodinations. Phenobarbital is known to affect thyroid function in rats due to an alteration of hormone disposition. We have further characterized these effects and have demonstrated that phenobarbital increases the biliary excretion of thyroxine glucuronide primarily as a result of an induction of hepatic thyroxine glucuronyltransferase. Studies on the mode of action for phenobarbital promotion of thyroid follicular neoplasia were conducted using an initiation-promotion model established by Hiasa et al (35). In this model, we demonstrated that supplemental administration of thyroxine blocked the promoting effect of phenobarbital and furthermore, using various dosages of thyroxine, we observed that the tumor promoting effect of phenobarbital was directly proportional to the level of plasma TSH. The results of these studies support the hypothesis that the tumor promoting effect of phenobarbital in the thyroid gland is mediated via increased secretion of pituitary TSH as a compensatory response to the known effects of phenobarbital on peripheral thyroid hormone disposition. Since a number of microsomal enzyme inducing agents have increased the incidence of thyroid follicular neoplasia in rat carcinogenicity studies, thyroid function should be assessed and a secondary mechanism of hormone imbalance should be considered in the interpretation of the significance of these findings in rodents.

A new method for D-glucaric acid excretion measurement that is suitable for automated instruments.

Urinary excretion of D-glucaric acid (uGA) is an index of type II hepatic microsomal enzyme induction, indirectly revealing possible organic effects of some drugs and environmental pollutants. However, its determination is often cumbersome. We suggest a new, fast microanalytical method for uGA determination in which beta-glucuronidase (BG; EC 3.2.1.31) activity inhibition produced by uGA-derived 1,4-D-glucarolactone is measured. With use of purified BG, the method is suitable for centrifugal analyzers, allowing assay of greater than 100 samples per day. Moreover, the method measures uGA more accurately than other enzymatic methods based on BG inhibition. The within-day CV ranges from 7.9% to 4.6% (uGA 31.55-121.31 mumol/L); the between-day CV ranges from 11.5% to 5.0% (uGA 26.09-124.10 mumol/L). The detection limit is 6.0 mumol/L. The standard curve is linear from 10 to 200 mumol/L. Mean analytical recovery is 100%. Comparison with the method of Simmons et al. (Clin Chim Acta 1974;51:47-51) gave a correlation of r = 0.978, y = 1.40x-2.81. Reference intervals were established in a healthy population sample of 369 people (165 under 14 y), and uGA, expressed in micromoles per gram of creatinine, was higher in women than in girls or in males.

Disopyramide pharmacokinetics and metabolism: effect of inducers.

1. The disposition of orally administered disopyramide was studied in a population of smokers (n = 6) and non-smokers (n = 8) before and during phenobarbitone treatment (100 mg daily for 21 days; Cp 21st day = 13.9 +/- 2.0 micrograms ml-1). The comparative inducibility of these populations by phenobarbitone was assessed as was the inductive effect of cigarette smoking, per se. Furthermore, the determinants of the intensity of the inductive effect were examined, as well as the effect of the barbiturate on the binding of disopyramide to alpha 1-acid glycoprotein (AGP). 2. Smokers and non-smokers exhibited similar half-lives (6.48 +/- 1.49 vs 6.66 +/- 1.02 h), apparent total body clearances (0.100 +/- 0.020 vs 0.117 +/- 0.034 l h-1 kg-1), mean renal clearances (0.043 +/- 0.0093 vs 0.057 +/- 0.013 l h-1 kg-1) and apparent intrinsic metabolic clearances (0.057 +/- 0.015 vs 0.060 +/- 0.024 l h-1 kg-1) before phenobarbitone treatment. 3. Both populations responded comparably to barbiturate exposure in that apparent intrinsic metabolic clearance more than doubled. Interestingly, the magnitude of this increase was highly dependent on the observed baseline apparent intrinsic metabolic clearance, (r' = 0.81; P less than 0.001). 4. Phenobarbitone treatment of non-smokers resulted in an increase in the AUC of the active metabolite N-despropyl disopyramide (MND), but not significantly (3.8 +/- 1.6 vs 4.1 +/- 2.3 micrograms ml-1 h). Similar results were observed in smokers (3.5 +/- 1.4 vs 3.9 +/- 2.0 micrograms ml-1 h, respectively). 5. The percent of administered dose recovered in urine as disopyramide in non-smokers was significantly decreased upon phenobarbitone treatment (43 +/- 6% vs 25 +/- 5%), whereas the percent of dose recovered as MND increased significantly in this group (25 +/- 6% vs 31 +/- 5%). The population of smokers responded similarly. 6. At doses typically used to achieve hepatic microsomal enzyme induction in man, phenobarbitone treatment caused no significant change or trend towards a change in serum AGP concentrations as measured using the radial immunodiffusion method in nonsmokers (67.4 +/- 19.9 mg dl-1 vs 68.0 +/- 40.7 mg dl-1) or smokers (64.5 +/- 15.7 vs 67.9 +/- 14.9). Similarly, when AGP concentration was estimated in serum from non-smokers using a nephelometric method no effect attributable to phenobarbitone was observed (47.9 +/- 1.3 vs 47.9 +/- 16.8 mg dl-1). Consistent with this observation, disopyramide free fraction was not affected by barbiturate treatment.

Toxicity of Sevoflurane in Rats

Sevoflurane, an experimental potent volatile anesthetic with a low blood/gas partition coefficient, degrades in the presence of soda lime to products the toxicity of which is unknown. We tested whether toxic products were produced by the passage of sevoflurane through soda lime, and a comparison was made of the toxicity of sevoflurane passed through soda lime with the toxicity of other potent volatile anesthetics in current clinical use. Halothane, isoflurane, sevoflurane (all in 1-MAC concentrations), or no anesthetic (control) were passed through soda lime for 4 hr with 12, 14, or 300% oxygen to groups of rats with hepatic microsomal enzyme induction. Separate groups of 12–13 rats were given 1 MAC of sevoflurane that had not passed through soda lime and either 14 or 100% oxygen. Sevoflurane was no more toxic than isoflurane and both of these anesthetics were less toxic than halothane. Soda lime was not a factor in any toxicity produced. Hepatic injury with all agents varied inversely with the oxygen concentration administered during anesthesia.

Discovery and development of a novel class of nonsteroidal aromatase inhibitors

Efforts to develop a novel class of nonsteroidal aromatase Inhibitors began with the discovery that the infertility in male rats exposed to high levels of the agricultural fungicide, fenarimol (α-(2-chlorophenyl)-α-(4-chlorophenyl)-5-pyrimidine- methanol), was attributable to the inhibition of aromatase activity within the central nervous system during the critical neonatal period. Although fenarimol was not particularly potent in inhibiting rat ovarian microsome aromatase activity in vitro (50% inhibition (IC 50) -4.1 μM). Subsequent testing of a number of analogues led to the identification of LY56110 (α, α-bis(4-chlorophenyl)-5-methylpyrimidine) which exhibited an IC 50 of 29 nM. LY56110 was orally active, blocking the testosterone-induced increase of uterine weight and ovarian estrogen biosynthesis in immature female rats. In rats with established DMBA-induced mammary carcinoma, complete tumor regression was observed in 80% of the animals. Development of LY56110 was, however, stopped because of its effects on hepatic microsomal enzymes and an unacceptably long half-life. Structural modifications resulted in the development of the indenopyrimidines. LY113174 (8-chloro-5-(4-chlorophenyl)-5H-indeno<1, 2D>pyrimidine) was highly effective in vitro (IC 50 - 24 nM) and in vivo but was far less potent than LY56110 with respect to induction of hepatic microsomal enzymes. LY113174 exhibited an acceptable biological half-life and had no effect on cholesterol side-chain cleavage. The indenopyrimidines appear to be a novel class of nonsteroidal aromatase inhibitors which may prove useful in the treatment of estrogen-dependent diseases.

TOXICITY OF SEVOFLURANE IN RATS

Sevoflurane, an experimental potent volatile anesthetic with a low blood/gas partition coefficient, degrades in the presence of soda lime to products the toxicity of which is unknown. We tested whether toxic products were produced by the passage of sevoflurane through soda lime, and a comparison was made of the toxicity of sevoflurane passed through soda lime with the toxicity of other potent volatile anesthetics in current clinical use. Halothane, isoflurane, sevoflurane (all in 1-MAC concentrations), or no anesthetic (control) were passed through soda lime for 4 hr with 12, 14, or 100% oxygen to groups of rats with hepatic microsomal enzyme induction. Separate groups of 12-13 rats were given 1 MAC of sevoflurane that had not passed through soda lime and either 14 or 100% oxygen. Sevoflurane was no more toxic than isoflurane and both of these anesthetics were less toxic than halothane. Soda lime was not a factor in any toxicity produced. Hepatic injury with all agents varied inversely with the oxygen concentration administered during anesthesia.

Clinical implications of hepatic microsomal enzyme induction by antiepileptic drugs

Most of the drugs used for the treatment of the major epilepsies are known to be potent inducers of the hepatic microsomal enzymes (1). The inducing potencies of several of these compounds have been compared in a recent study (2). Chronic treatment with approximately equieffective doses of phenytoin, phenobarbital, carbamazepine and primidone (which is partly converted to phenobarbital in the organism) was associated with marked and quantitatively similar increases in both antipyrine clearance and urinary D-glucaric acid excretion, two markers of enzyme induction (Fig. 1). In the same study, evidence was provided that the stimulation of antipyrine clearance and D-glucaric acid excretion was significantly related to the dosage of the inducer, thus confirming that enzyme induction is a dose dependent phenomenon (3). Among the major antiepileptic drugs, only valproic acid (Fig. 1), ethosuximide and benzodiazepines (1) appear to be largely devoid of enzyme inducing properties at therapeutically prescribed dosages. Since anticonvulsant drugs are prescribed for prolonged periods of time, sometimes in combination, it is not surprising that epileptic patients as a group show one of the highest degrees of enzyme induction ever described in humans. Indeed, these patients represent an ideal population for studies on the clinical implications of induction. The latter are traditionally classified into two major groups: (i) those resulting from stimulation of the metabolism of endogenous compounds or vitamins and (ii) those resulting in accelerated metabolism of exogenous agents, especially drugs.

The effect of low-dose phenobarbitone on three indices of hepatic microsomal enzyme induction.

The effects of low-dose phenobarbitone on three indices of hepatic enzyme induction were studied. Eight healthy volunteers took phenobarbitone 7.5 mg daily for 4 weeks followed by 15 mg daily for 4 weeks; five subjects took 30 mg daily for a further 2 weeks. Phenobarbitone 15 mg daily produced a significant rise in antipyrine clearance (P less than 0.05). Phenobarbitone 30 mg daily produced a further rise, but probably because of the reduced numbers of subjects, this did not achieve significance (P = 0.06). Urinary 6-beta-hydroxycortisol and D-glucaric acid levels did not change significantly and remained within the range seen in subjects not taking enzyme-inducing drugs. We conclude that phenobarbitone 7.5 mg daily produces little (if any) enzyme induction whereas 15 mg, or more, may have the potential to produce drug interactions through enzyme induction.

Effect of pretreatment with hepatic microsomal enzyme inducers on the toxicity of diazinon in calves

The pretreatment of calves with a single dose of 10 mg kg-1 dieldrin or 21 daily doses of 10 mg kg-1 phenobarbitone increased the toxicity of diazinon as reflected by the development of more severe clinical signs and greater depression in whole blood cholinesterase activity in the pretreated calves. Induction by dieldrin or phenobarbitone of the hepatic microsomal enzyme amidopyrine-N-demethylase was also accompanied by a concurrent rise in the liver carboxylesterase activity.

Diabetes Mellitus and Primary Hepatocellular Carcinoma

In view of the increasing incidence of primary hepatocellular carcinoma in western Europe and concern that this may in part be related to long-term use of drugs which cause hepatic microsomal enzyme induction, we undertook a comparison of long-term drug use in 105 patients with hepatocellular carcinoma and equal numbers of age and sex-matched patients with colorectal tumours and with fractures of femur. We found no patients with hepatocellular carcinoma who were long-term anticonvulsant users and only one who used oral contraceptives. However, we observed a four-fold excess of diabetic patients among the group with hepatocellular carcinoma. This association did not appear to be due to pre-existing haemochromatosis, alcoholic cirrhosis or viral hepatitis. The association was strongest in patients receiving drug treatment for diabetes, but the data, although suggestive, were insufficient to determine whether any specific anti-diabetic agent could be responsible. Further studies are required to elucidate the nature of this unexpected association. An association of this magnitude with diabetes mellitus could account at least in part for the increasing incidence of hepatocellular carcinoma in western Europe.

Alcohol and cancer.

Alcohol and cancer.

A Possible Diurnal Variation in Trazodone Clearance

Diurnal variations in drug metabolism that have clinical consequences have been described for several drugs used in internal medicine but have not been reported for antidepressants. The long half-lives of most antidepressants may obscure such variations. Trazodone hydrochloride has a short half-life compared with other commercially available antidepressants. The authors measured serum levels of trazodone 11 hours after both 9 a.m. and 9 p.m. equal doses of the medication in individuals who had been on these doses for more than 3 days. For patients on other medications in addition to trazodone, serum levels were significantly higher at 8 a.m. than at 8 p.m. (N = 7, P less than 0.01) and approached significance for patients on trazodone alone (N = 8, P = 0.06). Induction of hepatic microsomal enzymes by the other medications may have exaggerated the tendency for trazodone to show diurnal variations in serum levels.

Pathways for the bioactivation of aliphatic nitriles to free cyanide in mice

Reports from several laboratories agree that many, but not all, aliphatic nitriles undergo hepatic biotransformation in mice and rats to release free cyanide, but the mechanisms at work in these reactions remain in doubt. We have used primarily n-butyronitrile, propionitrile, and their respective α-carbon-hydroxylated homologs, propionaldehyde cyanohydrin and lactonitrile, to examine this question in mice. Pretreatment of mice with the hepatic microsomal enzyme inducers, pregnenolone-16α-carbonitrile, troleandomycin, and isosafrole, or with the cytochrome P-450-depleting agent, cobaltous chloride, did not influence the mortality of mice given single doses of nitriles. Repeated injections of aspirin or sodium salicylate in water failed to protect mice against death by the nitriles. Dimethyl sulfoxide, however, was effective in reducing mortality after nitrile administration. Repeated injections of 4-methylpyrazole or disulfiram protected mice against death after nitriles. Most of the treatment regimens successful against the nitriles also protected against death due to the cyanohydrins. The cyanohydrins were more acutely toxic than their parent nitriles, produced death much more rapidly, and resulted in the same toxic signs, suggesting that they are intermediates in the bioactivation pathway leading to free cyanide. The cyanohydrins appeared to serveas weak substrates for yeast alchohol dehydrogenase, however, incubation of them with either yeast or horse liver alcohol dehydrogenase did not increase the rate of cyanide release over that in incubations where the enzymes were absent. The slow rate of cyanide release due to spontaneous hydrolysis interfered with the determinations of alcohol dehydrogenase activity, but it cannot account for the rapid action and high toxicity of the cyanohydrins in vivo, or for the efficacy of the treatment regimens which protected against death. It appears unlikely that prostaglandin synthetase or alcohol dehydrogenase are importantly involved in nitrile bioactivation. The same active process, however, appears to be responsible both for α-carbon hydroxylation and for the subsequent degradation of the resulting cyanohydrins to release free cyanide. It is far more efficient in mediating the latter reaction than the former.

Evidence that methylsulfonyl metabolites of m-dichlorobenzene are causative substances of induction of hepatic microsomal drug-metabolizing enzymes by the parent compound in rats

This study was undertaken to clarify the relationship between the formation of 2,4- and 3,5-dichlorophenyl methyl sulfones, metabolites of m-dichlorobenzene (DCB), and their inducing effect on hepatic microsomal drug-metabolizing enzymes in rats. When m-DCB was injected ip into bile duct-cannulated rats, little or no methyl sulfones were detected in blood, liver, kidneys, adipose tissue, or bile. In the antibiotic-pretreated rats dosed with m-DCB, metabolite concentrations in the blood and the three tissues markedly decreased. These findings suggest that the formation of methylsulfonyl metabolites from m-DCB depends largely upon the metabolism of some precursor(s) excreted in the bile by intestinal microflora. The increasing effects of m-DCB administration on the activities of aminopyrine and aniline metabolizing enzymes and the contents of cytochromes P-450 and b 5 in hepatic microsomes were scarcely observed in the bile duct-cannulated and antibiotic-pretreated rats, in which the drug-metabolizing enzymes were able to be induced by phenobarbital treatment. On the other hand, in rats administered 2,4- or 3,5-dichlorophenyl methyl sulfone hepatic distribution of each methyl sulfone was similar to that in intact rats, and the degree of increase of the above four parameters was nearly the same as that in the intact rats. These findings provide evidence that the induction of drug-metabolizing enzymes by m-DCB is not due to the action of m-DCB but is due to its methylsulfonyl metabolites.

Drug interactions with oral contraceptive preparations.

Oral contraceptives (OCs) should be used with additional caution in combination with other drugs in certain disease states. This discussion focuses on only those interactions which are due to identifiable pharmacokinetic processes. Pharmacological interactions between OCs and other compounds may be of 2 kinds: drugs may impair the effectiveness of the OCs to cause breakthrough bleeding or to allow pregnancy to occur; and OCs may interfere with the metabolism of other componds. In general interactions of the 1st kind are due to interference with the absorption metabolism or excretion of estrogens and interactions of the 2nd kind are due to competition for metabolic pathways. It is difficult to obtain estimates of the frequency of interactions but it seems likely that as low-dose preparations become more widely used interactions of the 1st type will become more common and those of the 2nd type less common. Although it is probably only those women with low plasma hormone concentrations who are at risk that other drugs may interfere with contraceptive efficacy and women with high plasma concentrations of estrogen who are at risk of side-effects and that OCs may interfere with other drugs there is no way of detecting women in either category. Consequently it is safer for the present to assuume that all women who take OCs might be at risk of some form of interaction. Significant drug interactions in which contraceptive efficacy is reduced are more likely to occur in the early part of the cycle when circulating hormone concentrations are of critical importance. It is well known that infective diarrhea can induce OC failure by increasing gastrointestinal motility and reducing hormone absorption. Thus any drug which reduces gut transit time and causes diarrhea is potentially likely to reduce circulating concentrations of OCs. The sulphation of ethinl-estradiol in the gastrointestinal wall has been shown to be a site of interaction for the enhancement of the activity of OCs. Ascorbic acid also undergoes sulphate conjugation in the gut wall and acts as a competitive inhibitor for sulphation of estrogens. The most clinically significant group of interactions occurs with other drugs that are metabolized by the same hepatic microsomal pathways as is estrogen. Most of the anticonvulsant drugs are inducers of microsomal enzymes. Rifampicin is known to be a potent inducer of hepatic microsomal enzymes and has been shown to increase the rate of metabolism of both estrogeens and progestogens. A recent report has suggested that the antifugal drug griseofulvin may cause a clinically important interaction with OCs. There are numerous well documented clinical reports of women who have taken OCs without missing a dose who have become pregnant while taking a variety of antibiotic agents such as ampicllin and tetracycline. Estrogens are inhibitors of hepatic microsomal enzymes. In this way they may show the metabolsim of other drugs thus increasing their plasma and tissue concentrations and increasing the risk of side-effects. These interactions might be expected to become less common as the concentration of estrogen in OCS decreases.

Synthesis, biologic and toxic properties of 2,3,7,8-TCDD metabolites

2-Hydroxy-3,7,8-trichlorodibenzo-p-dioxin has been identified as a major urinary metabolite of 2,3,7,8-TCDD in several laboratory animal studies. This compound was synthesized via coupling of 2,5-dichloro-4-nitroanisole with 4,5-dichlorocatechol in HMPA. The resultant 3-methoxy-3,7,8-trichlorodibenzo-p-dioxin product was demethylated in boron tribromide/methylene chloride to give 2-hydroxy-3,7,8-trichlorodibenzo-p-dioxin. The metabolite was administered at dose levels of 100, 1000 and 5000 ug/kg to immature male Wistar rats and the effects of this compound on body weight loss, thymic atrophy and hepatic microsomal monooxygenase enzyme induction were determined. Comparable dose-response studies for 2,3,7,8-TCDD were also carried out. At dose levels of 4–8 ug/kg 2,3,7,8-TCDD caused significant body weight loss and thymic atrophy in the Wistar rats whereas 2-hydroxy-3,7,8-trichlorodibenzo-p-dioxin did not elicit these toxic effects at a dose level of 5000 ug/kg. The ED 50 values for the induction of hepatic microsomal benzo[a]pyrene hydroxylase and ethoxyresorufin O-deethylase were 1.25 and 2.35 ug/kg for 2,3,7,8-TCDD and 2,820 and 3550 ug/kg for 2-hydroxy-3,7,8-trichlorodibenzo-p-dioxin respectively. These results demonstrated that 2-hydroxy-3,7,8-trichlorodibenzo-p-dioxin is at least 3 orders of magnitude less toxic than 2,3,7,8-TCDD and this is consistent with the lower receptor binding affinity of the metabolite coupled with the expected more rapid clearance of this compound.

Enzyme inducers improve insulin sensitivity in non-insulin-dependent diabetic subjects.

The reduction in blood glucose in non-insulin-dependent diabetes mellitus (NIDDM) brought about by the use of phenobarbital (PB), a hepatic microsomal enzyme inducer, suggests an improvement in insulin sensitivity. The effect of PB on insulin-mediated glucose metabolism was hence investigated using the euglycemic clamp technique in 10 women with NIDDM aged 56-75 yr. The addition of PB to sulfonylurea therapy, concurrently for 6 wk, reduced fasting blood glucose (BG, from 12.8 +/- 1.6 to 10.2 +/- 3.2 mmol/L, P less than 0.01) and immunoreactive insulin (IRI) levels (from 32.4 +/- 13.6 to 24.7 +/- 9.8 mU/L, P less than 0.01), whereas body weight remained unaltered. During the trial, there was a significant change in the glucose disposal rate (M, from 1.27 +/- 0.60 to 2.82 +/- 0.86 mg/kg/min, P less than 0.001), the metabolic clearance rate of glucose (from 0.89 +/- 0.41 to 2.24 +/- 1.27 ml/kg/min, P less than 0.01), the insulin sensitivity index (from 1.10 +/- 0.44 to 2.86 +/- 1.54 mg/kg/min: mU/L X 100, P less than 0.001), and the plasma antipyrine clearance rate (from 28.3 +/- 11.7 to 51.4 +/- 20.2 ml/min, P less than 0.001), an in vivo index of liver microsomal enzyme activity. The antipyrine clearance rate correlated with insulin-mediated glucose metabolism (r2 = 0.560, P less than 0.01). This correlation could be interpreted as indicating that, in NIDDM patients, peripheral glucose utilization and the liver microsomal enzyme system share common regulators. Our study suggests a new approach to the improvement of insulin sensitivity in NIDDM patients.

Genetic differences in oxygen toxicity are correlated with cytochrome P-450 inducibility.

Susceptibility to oxygen toxicity was studied in three inbred and two hybrid strains of mice. Because in vitro studies have shown that the cytochrome P-450 enzymes can produce oxygen radicals and H2O2, we tested the hypothesis that inducibility of these enzymes might play a role in oxygen toxicity. Mice responsive to hepatic microsomal enzyme induction by aromatic hydrocarbons [C3H/HeJ, C3H/HeN, C3H/HeJ X DBA/2J (designated C3D2F1/J), C3H/HeN X DBA/2J (designated C3D2F1/N)] were more sensitive to the toxic effects of 100% oxygen exposure than were genetically unresponsive mice (DBA/2J). DBA/2J mice survived significantly longer exposure periods with less lung damage. Lung and liver cytochrome P-450 levels increased 2-to 3-fold in C3H and F1 mice during 100% oxygen exposure (maximum levels at 72-96 hr) and subsequently fell prior to death. No increases were seen in cytochrome P-450 levels in DBA/2J mice. Metabolic pathways involving cytochrome P-450 enzymes may initiate or modulate oxidative damage due to oxygen radicals. The difference in responsiveness of mice to microsomal enzyme induction may imply genetic differences in susceptibility to oxidative stress, may help to explain species differences in susceptibility, and may have long-term implications in therapeutics and patient care if similar inherited differences exist in humans.