Bitten Stripp

Studies on the mechanism of the lung toxicity of paraquat: Comparison of tissue distribution and some biochemical parameters in rats and rabbits

Abstract After iv injection of [14C]paraquat (20 mg/kg) tissue localization was preferential in lungs of rats as well as rabbits although the latter did not show any histopathologic or biochemical signs of lung damage. No preferential subcellular localization of [14C]paraquat was found in lungs of either species, but all subcellular levels decreased more rapidly in the rabbit than in the rat. [14C]Paraquat was not covalently bound to tissue macromolecules. In vitro measurements of lipid peroxidation, H2O2 formation and lung lysosomal stability failed to account adequately for the lung damage in the rat.

Enhanced hepatotoxicity of carbon tetrachloride, thioacetamide, and dimethylnitrosamine by pretreatment of rats with ethanol and some comparisons with potentiation by isopropanol

Abstract Elevated plasma glutamic-pyruvic transaminase (GPT) activity induced by carbon tetrachloride (CCl 4 ), thioacetamide, or dimethylnitrosamine in male rats was increased by pretreatment with four doses (each 5 ml/kg) of ethanol orally 48, 42, 24, and 18 hr before the hepatotoxic agent. The potentiated hepatotoxicity of CCl 4 was confirmed by histologic evaluation. During the pretreatment, blood ethanol concentrations fluctuated between 0 and 300 mg/100 ml, but were less than 5 mg/100 ml when a hepatotoxic agent was injected ip. Pretreatment with ethanol did not affect the hepatic concentrations of CCl 4 or its metabolite, chloroform (CHCl 3 ), at 1 hr after administration of CCl 4 . The CCl 4 -induced diene conjugation tended to increase after ethanol pretreatment and was significantly potentiated by pretreatment with isopropanol or pyrazole and a single dose of ethanol. In rats pretreated with ethanol, covalent binding of 14 CCl 4 to liver protein and lipid in vivo was significantly greater at 6 and 24 hr, but not during the first 3 hr, than in control rats. The in vitro binding of 14 CCl 4 , 14 CHCl 3 , and 14 CBrCl 3 to hepatic microsomal protein was increased by ethanol pretreatment. Ethanol pretreatment also doubled the in vitro rate of demethylation of dimethyl-nitrosamine by liver microsomes, but did not affect the amount of microsomal protein and cytochrome P-450, NADPH c reductase activity nor the rate of N-demethylation of ethylmorphine. The similarities in microsomal effects of pretreatment with isopropanol and pretreatment with four doses of ethanol suggest that similar mechanisms are involved in their potentiation of CCl 4 -induced hepatotoxicity. The potentiation by pretreatment with ethanol, but not with isopropanol, of the hepatotoxicity of thioacetamide and dimethylnitrosamine suggests that ethanol pretreatment also activates some additional mechanisms.

Enhanced Hepatic Microsomal Activity by Pretreatment of Rats with Acetone or Isopropanol

Summary Rat liver microsomes isolated from rats pretreated orally with acetone or isopropanol possessed an enhanced capacity to bind 14CCl4 and 14CHCl3 covalently and to demethylate DMN. The N-demethylation of ethylmorphine was unchanged by this pretreatment. Since there was no increase in the level of microsomal protein, and microsomal cytochrome P-450 or the activity of NADPH-cytochrome c reductase, the mechanism of this enhanced metabolism remains unclear. The authors acknowledge the excellent technical assistance of E. M. Boykins, M. A. Williams and W. Saul.

Investigation of the effects of methyltestosterone, cortisone and spironolactone on the hepatic microsomal mixed function oxidase system in male and female rats

Abstract Acute (4 days) and chronic (4 weeks) administration of methyltestosterone, cortisone and spironolactone changes drug metabolism in hepatic microsomes of male and female rats by altering the degree of binding of drugs to the cytochrome P-450, the activity of NADPH-cytochrome c reductase, and the rate of reduction of the cytochrome P-450-drug complex. These steroids also alter the ratio of stoichiometric relationship between drug metabolism and CO-inhibitable NADPH-oxidation. This ratio is sex dependent, being approximately 1·0 in males and 0·2 in females. The effect of the three steroids on the ratio is also sex dependent. The steroids tend to increase the ratio in females and decrease it in males.

Pre- and postnatal enzyme capacity for drug metabolite production

Most lipid-soluble foreign compounds including drugs, insecticides and many environmental pollutants are metabolized in animals by cytochrome P-450 enzymes in the endoplasmic reticulum of liver. These enzymes are virtually absent in fetuses of laboratory animals, but their activities increase to adult levels within 3-8 weeks after birth. In human fetuses, the enzymes appear during the first half of pregnancy, and their activities during gestation reach about one third of those found in adults. The species differences in fetal activities apparently parallel the differences in the development of liver endoplasmic reticulum. In laboratory animals, the rough-surfaced reticulum does not develop until 4 days before birth adn the smooth-surfaced retuculum develops only after birth. In man, however, the rough-surfaced form appears at about 7 to 9 weeks of gestation, whereas the smooth-surfaced form appears at about the 3rd month of pregnancy. Despite the early development of these enzymes in humans, they probably play only a minor role in limiting the accumulation of most foreign compounds in human fetuses. Nevertheless, they may play an important role in drug-induced toxicities, particularly those that are mediated through the formation of chemically reactive metabolites.

Spironolactone and cytochrome P-450: Impairment of steroid hydroxylation in the adrenal cortex

Abstract The effect of spironolactone administration on the content of adrenal microsomal cytochrome P-450 and on the activity of adrenal 17α-hydroxylase was examined in male cortisol and corticosterone-producing animals. Decreases in the content of microsomal cytochrome P-450 and in the activity of the 17α-hydroxylase after spironolactone treatment occur only in those animals which predominantly produce cortisol rather than corticosterone and which have a high activity of adrenal steroid 17α-hydroxylase. The administration of spironolactone to cortisol-producing animals, namely the guinea pig and the dog, caused a 50 to 80% loss of microsomal cytochrome P-450 with a concomitant decrease in the activity of the microsomal 17α-hydroxylase. In contrast to its effect in cortisol-producing animals, the administration of spironolactone caused either an increase or slight alteration in the concentration of adrenal microsomal cytochrome P-450 in corticosterone-producing animals such as the rat and the rabbit.

Active products of fetal drug metabolism

Theoretical relationships between the activities of placental and fetal enzymes and their ability to perturb the steady‐state tissue levels of drugs have been calculated. Although the calculations suggest that the activities of the enzymes that metabolize desipramine and 3,4‐benzpyrene in human placentas and fetuses may be high enough to decrease the steady‐state concentrations of these substances, the rates of metabolism of most drugs are probably too slow to affect the fetal levels of most lipid‐soluble compounds. It is also probable that the rate of bromobenzene metabolism by cytochrome P‐450 enzymes in fetal liver is too slow to cause liver necrosis caused by this toxicant. Nevertheless, it is possible that enzymes that catalyze the reduction of nitro compounds, such as nitrofurazone, to hydroxylamino derivatives may mediate various drug toxicities in fetuses.

Effect of chronic treatment with phenobarbital or 3-methylcholanthrene on the male reproductive system in rats

Abstract Treatment of rats for 4 weeks with phenobarbital (PB) did not inhibit the growth of the seminal vesicles, nor did it affect the biosynthesis of testosterone by testis microsomes. Moreover, neither the concentration of cytochrome p-450 or the 17 α-hydroxylase activity in testis microsomes were affected. In contrast, treatment with 3-methylcholanthrene (3-MC) for 4 weeks markedly decreased the weights of the seminal vesicles. The decrease was probably related to an impairmant of testosterone formation in the gonads, since testosterone biosynthesis as well as the concentration of cytochrome p-450 and the activity of 17 α-hydroxylase in testis microsomes were significantly decreased in the 3-MC treated rats. No histopathological changes were seen in testes from any of the PB or 3-MC treated rats.

Effect of Methylcholanthrene or Phenobarbital Pretreatment of Rabbits and Rats on the Extinction Coefficient for Cytochrome P-450

Phenobarbital (PB) or methylcholanthrene (3-MC) pretreatment of rats or rabbits causes induction of cytochrome P-450. In rabbits neither pretreatmentchanges the extinction coefficient for cytochrome P-450-CO complex when this is estimated from the amount of totalheme, cytochrome b5 and absorbancy at 450 nm. In rats PB pretreatment does not change the extinction coefficient, while 3-MC causes an increase in the average extinction coefficient from 85 to 102 mM–1cm–1.

Extinction Coefficients for Cytochrome P-450 in Hepatic Microsomes from Male and Female Rats

There is a sex difference in the apparent extinction coefficients for the CO-cytochrome P-450 complex in hepatic microsomes from rats. The average coefficients obtained in the present study were 79.6