Development and maturation of drug-metabolizing enzymes

Developmental changes in hepatic activation of dietary mutagens by mice

Metabolic activation of the food mutagens 2-amino-3,4-dimethylimidazo[4,5-f]quinoline (MeIQ), 3-amino-1-methyl-5H-pyrido[4,3-b]indole (Trp-P-2) and aflatoxin B1 by female BALB/c mice of different ages (2-24 weeks) was investigated in vivo and in vitro using Salmonella typhimurium TA98 as the indicator organism. The in vivo activation of the three mutagens was investigated in 4- and 24-week-old mice using an intrasanguineous host-mediated assay. All three compounds showed reduced levels of activation with the older hosts. Hepatic S9 fractions from female mice of varying ages between 2 and 24 weeks were used in the in vitro mutagenicity assay. To achieve optimal activation to bacterial mutagens, 5% S9 was required for aflatoxin B1 and Trp-P-2 and 10% S9 for MeIQ; age of donor generally had little effect on the profile of these protein activation curves. Under these optimal conditions MeIQ and Trp-P-2 both exhibited, as before, age-dependent decreases in activation over a wide range of mutagen concentrations, however the in vitro activation of aflatoxin showed no consistent change with age. Spectrophotometric measurements of S9 cytochrome P-450 content showed a decrease in concentration with increasing age, but this was not sufficient to account for changes observed in hepatic mutagen activation. However, changes in the activities of certain cytochrome P-450 isoenzymes and cytosolic GSH-transferases, which in turn result in changes in the activation and detoxification capacity of the liver, would appear to explain age-dependent changes in the activity of mutagens in vivo.

Activities in chick embryos of 7-ethoxycoumarin O-deethylase and aryl hydrocarbon (benzo[a]pyrene) hydroxylase and their induction by 3,3',4,4'-tetrachlorobiphenyl in early embryos

Basal activities of 7-ethoxycoumarin O-deethylase and aryl hydrocarbon (benzo[a]pyrene) hydroxylase were determined in whole-liver homogenates from chick embryos of different ages, from newly hatched chicks and from chicks a few days old. The enzyme activities increased substantially in chick embryo livers between days five and 10, remained at a fairly constant level until day 19, and reached a peak in activity one day after hatching. The optimal pH value was lower than 7.0 for both enzyme activities. The total increase in activity from day five to one day after hatching was about 300-fold for 7-ethoxycoumarin O-deethylase and about 75-fold for aryl hydrocarbon (benzo[a]pyrene) hydroxylase. Treatment of eggs with 3,3',4,4'-tetrachlorobiphenyl resulted in increased metabolism of both substrates by five-day-old chick embryo livers. The increase in aryl hydrocarbon (benzo[a]pyrene) hydroxylase activity was 14-fold while that of 7-ethoxycoumarin O-deethylase was approximately double.

Regulation of brain and hepatic glutathione-S-transferase by sex hormones in rats

Our results indicate that sex hormones play an important role in the regulation of brain and hepatic GST protein during maturity. The conjugating factor GSH does not appear to be under the influence of sex hormones. These observations are of great significance in view of the possibility of continued exposure to neurotoxic chemicals like DDT and Kepone which can cause significant alterations in levels of sex hormones. A reduced GSH activity could lead to a retarded biotransformation of electrophiles and thus to an enhanced toxicity.

Developmental aspects of hepatic glutathione S-transferase activities in male rats

The postnatal development of glutathione S-transferase activities was investigated in post-microsomal supernatants of the livers of male rats. A period of up to the age of 200 days was covered, using styrene 7,8-oxide as the electrophilic substrate for the transferase. Activity in animals on the day of delivery was small but significantly above the level of spontaneous conjugation. During 5 weeks postpartum the specific activity increased regularly to about sixfold when expressed as per mg of cytosolic protein, thereafter the activity slightly decreased. During the 5th postnatal week a sudden decrease in activity was observed, but it returned to the original value within about 5 days. The effect of weaning on this phenomenon was studied by comparing weaned with non-weaned rats in an investigation of glutathione S-transferase maturation in the 4th and 5th week postpartum, using styrene 7,8-oxide and 1-chloro-2,4-dinitrobenzene as the electrophilic substrates. Weaned rats exhibited the decrease in activity only when styrene oxide was the substrate. It is concluded that glutathione S-transferase levels in rat liver are low at birth, and develop gradually during the first 5 postnatal weeks. Transferase activities towards styrene oxide and chlorodinitrobenzene appears to mature independently and are partly associated with weaning.

Brain distribution and kinetics of 11C-chlorpromazine in schizophrenics: positron emission tomography studies

The positron emitter 11C (20 minutes half-life) permits the labeling of chlorpromazine (CPZ) and the study of its distribution in humans by external counting. Trace amounts of 11C-CPZ were injected intravenously into 22 schizophrenic patients all untreated for several months with neuroleptics. The brain uptake was 6.04 +/- 1.6% of the injected dose 15 minutes after the injection, and it remained constant for 45 minutes. By positron emission tomography, the drug distribution was shown to be in the gray matter, and such structures as the cortex, caudate nucleus, thalamus and putamen could be identified. This new methodology will be helpful in studying specific receptors in humans in a noninvasive way.