Components of the heme biosynthetic pathway and mixed function oxidase activity in human fetal tissues

Prenatal induction of benzo(a)pyrene hydroxylases in mice

1. Benzo(a)pyrene hydroxylase (BPH) activity was measured in homogenates of fetal liver (day 18) or of whole-embryos of mice on day 9, 10 or 12 of gestation after maternal pretreatment with B(a)P on 3 consecutive days. A 3H-liberation assay with 3H-B(a)P labelled either generally or at the 6-position was used. The values obtained with the embryonic/fetal tissues were compared with those found in maternal liver. 2. Three oral doses of 17.5 mg B(a)P/kg body wt were found to just significantly induce BPH in maternal liver. An induction was observed after pretreatment with 24 mg B(a)P/kg body wt in 9-, 10- or 12-day-old whole-embryos, but the Vmax reached was only 10-20% (1% on day 9) of that of adult non-induced liver. The Km (6-hydroxylation) for all tissues tested were in the same range (600-900 nM). The induction was demonstrable in embryos at tissue levels about one order of magnitude lower than those required for induction in maternal liver. 3. Treatment with 25 mg B(a)P/kg body wt on 3 consecutive days was required to induce BPH in fetal liver on day 18 of gestation. The required B(a)P tissue concentrations were about one half of those necessary for induction in maternal liver. 4. Among a variety of other polycyclic hydrocarbons only chrysene showed an inducing potency similar to that of B(a)P in adult and fetal liver. For all compounds tested there was no correlation found in the inducing potency between adult and fetal liver (e.g. coronene).(ABSTRACT TRUNCATED AT 250 WORDS)

Immunochemical examinations of cytochrome P-450 in various tissues of human fetuses using antibodies to human fetal cytochrome P-450, P-450 HFLa

P-450 HFLa is a form of cytochrome P-450 purified from human fetal livers. The amounts of P-450 HFLa in several fetal tissues were determined immunochemically. Detectable amounts presented in livers, kidneys, adrenals, lungs and some other tissues of human fetuses. The amounts were the highest in livers. Activities of 7-ethoxycoumarin O-deethylase and benzo(a)pyrene hydroxylase in livers but not in adrenals were inhibited by the anti-P-450 HFLa antibodies, probably suggesting that distinct forms of cytochrome P-450 are responsible for the oxidations in livers and adrenals.

The Ah receptor: binding specificity only for foreign chemicals?

The murine Ah locus controls the induction of at least four drug-metabolizing enzymes: cytochromes P1-450, P2-450, and P3-450, and UDP-glucuronosyltransferase. The Ah gene codes for a cytosolic receptor. It is known that the induction response includes: (i) high-affinity binding of specific foreign chemicals to the Ah receptor; (ii) temperature-dependent translocation of the "activated" inducer-receptor complex into the nucleus; (iii) binding of the complex presumably to chromatin components; (iv) transcriptional activation of specific genes; (v) maximal increases in intranuclear high-molecular-weight precursor mRNA (pre-mRNA) that precede by several hours the maximal increases in cytoplasmic mRNA; (vi) translation of the mRNA principally on membrane-bound polysomes; and (vii) increases in the specific membrane-bound proteins (including architectural arrangement with other membrane-bound moieties) that reflect enhanced specific drug-metabolizing activities. It is not known how many of the other drug metabolism induction responses are also governed by receptors. The Ah locus studies have been chiefly unraveled in the mouse, due to several inbred strains having a receptor defect. In addition to "classical" pharmacologic methods (such as structure-activity studies) and standard biochemical techniques, the newer methods of recombinant DNA technology and somatic-cell genetics in culture are shown to be important in understanding the Ah receptor and its induction response. It is possible that this receptor is required for endogenous functions critical to life processes, as well as its function in the induction of drug metabolism by certain polycyclic aromatic compounds.

Development of basal and induced aryl hydrocarbon (benzo[a]pyrene) hydroxylase activity in the chicken embryo in ovo

The development of the hepatic microsomal mixed-function oxidase system was studied to determine the basal level of embryonic enzyme activity and the inducibility of this system throughout growth and differentiation. Chicken embryo livers were assayed for basal and inducible hepatic aryl hydrocarbon hydroxylase (AHHase; designated elsewhere as AHH) activity from the first appearance of the liver as a discrete organ at 5 days of incubation (DI) through day 10 after hatching. In addition, whole-embryo and viscera preparations were assayed at 3 and 4 DI. Basal AHHase activity was equal to or greater than adult levels from 3 DI through hatching in all preparations (approximately 0.3-0.5 nmol/min per mg). A 3-fold increase in basal activity above adult values occurred at hatching. The onset of inducibility in chicken embryo liver between 5 and 6 DI was concomitant with hepatocyte differentiation. A developmental profile of 24-hr 3,4,3', 4'-tetrachlorobiphenyl-induced AHHase activity showed 15- to 30-fold induction over controls from 7 DI through day 10 after hatching, with a maximum of 15 nmol/min per mg at 14 DI and day 1 after hatching, a specific activity greater than 50% greater than maximal induction in the adult. Embryonic AHHase activity was also induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin, 3-methylcholanthrene, beta-naphthoflavone, and sodium phenobarbital. Induction kinetics throughout embryonic development were similar to those reported for the adult chicken and other animals. These findings demonstrate development of a mixed-function oxidase system in very early embryogenesis and then in the liver as it differentiates. Liver AHHase activity is inducible throughout development and perinatally but such activity is under strict developmental regulation. The chicken embryo has adult levels of AHHase activity which would be sufficient to achieve metabolic activation of promutagens/carcinogens before and after hepatocyte differentiation.

Metabolism of benzo [a] pyrene by guinea pig adrenal and hepatic microsomes

Studies were carried out to compare the metabolism of benzo [a] pyrene (BP) by adrenal and hepatic microsomes obtained from adult male guinea pigs. Adrenal microsomes produced fluorescent metabolites (primarily phenols) approximately three to four times more rapidly than hepatic microsomes, but the differences in the rates were considerably smaller when total BP metabolism was assessed using an isotopic assay. The apparent discrepancy between the two assays is attributable to differences in the profiles of BP metabolites produced by adrenal and liver. Separation of metabolites by high pressure liquid chromatography revealed that adrenal microsomes converted BP to primarily a phenolic metabolite with a retention time identical to that of 3-hydroxy-BP. Liver microsomes, by contrast, produced approximately equal amounts of compounds co-chromatographing with 3-hydroxy-BP and BP-4,5-dihydrodiol. Small amounts of other metabolites were also produced by adrenal and hepatic microsomes. Liver microsomes catalyzed the conversion of BP to metabolites that became covalently bound to exogenous DNA. The amount of binding was dependent upon the duration of incubation and concentration of microsomal protein. Adrenal microsomes, by contrast, did not promote BP binding to DNA. Inhibition of microsomal epoxide hydratase activity with trichloropropene oxide (TCPO) blocked the formation of dihydrodiol metabolites of BP by adrenal and liver microsomes. In the presence of TCPO, liver microsomes produced large amounts of a BP metabolite co-chromatographing with BP-4,5-oxide. TCPO also increased the rate of production of DNA-binding metabolites by liver microsomes but had no effect on the formation of DNA-binding metabolites by adrenal microsomes. The results demonstrate major differences in the pathways of BP metabolism by guinea pig adrenal and hepatic microsomes. Although adrenal microsomes metabolize BP more rapidly than hepatic microsomes, far greater amounts of reactive metabolites are produced by the liver. Thus, adrenal metabolism of BP may be of little toxicological significance.

Drug metabolism by the human fetus

A review of the literature that pertains to drug biotransformation in human fetal tissues reveals that, in spite of several publications in this comparatively new area of research, only very limited definitive information is currently available. The large majority of the studies performed have dealt with the cytochrome P-450-dependent microsomal mono-oxygenase systems and for several of the common drug metabolising reactions, very little or no data are available at this time. Some of the more important data that have emerged include observations that important bioactivation reactions can be demonstrated in human fetal tissues obtained during the period of late embryogenesis (high susceptibility to chemical dysmorphogenesis) and that the human fetal adrenal gland possesses considerable capacity to catalyse several important oxidation-reduction reactions. From the data available to date, it would appear that, in most instances, the biotransformation of drugs in the human embryo and fetus would not affect maternal plasma concentrations significantly. From the viewpoint of parameters of the pharmacokinetics of parent drug (or other xenobiotic) substrates under steady-state conditions, human fetal drug metabolism probably is of little consequence in most cases, although exceptions may exist. Pharmacokinetic parameters observed after isolated exposure, however, are very likely to be affected, perhaps markedly, in some instances. The demonstrated capacity of human prenatal tissues and cells to generate reactive intermediary metabolites, including those that produce mutations, has attracted the greatest attention recently. This capacity may be associated with extremely important adverse reactions to drugs and other environmental chemicals. Such adverse responses include transplacental mutagenesis, carcinogenesis, dysmorphogenesis, and perhaps several other undesirable effects. Although far from conclusive, the data tend to suggest that humans and subhuman primates may be more vulnerable than the smaller common experimental animals to the toxic effects of foreign organic chemicals during prenatal life. These factors should be weighed whenever exposure of pregnant women to such agents (e.g. via drug administration) is contemplated.

The in vitro oxidative metabolism of benzo(a)pyrene in human liver measured by different assays

The specific metabolism of benzo(a)pyrene (BP) in adult human liver samples was studied by using triated BP and thin-layer chromatographic separation of metabolites and by using the conventional fluorometric assay. The incubation in vitro of BP with human liver homogenates yielded the usual pattern of metabolites, including several dihydrodiols, phenols and quinones, as well as products bound convalently to protein. Interindividual variation in the production of different metabolites were very large, several ten-fold. The over-all metabolism was inhibited by aminopyrine and SKF 525A but not by 7,8-benzoflavone. The formation of dihydrodiols was inhibited by several epoxide hydratase inhibitors with consequent accumulation of phenols and an unidentified metabolite (possibly BP 4,5-oxide). Radiometric measurement of phenols, total dihydrodiols and individual dihydrodiols correlated well with the fluorometric assay of BP hydroxylase activity.