Cholesterol side-chain cleavage activity in human placenta and bovine adrenals: an one-step method for separation of pregnenolone formed in vitro

Synthesis of new potent and selective aromatase inhibitors based on long-chained diarylalkylimidazole and diarylalkyltriazole molecule skeletons

A series of long-chained diarylalkylimidazoles and diarylalkyltriazoles were synthesized and evaluated for the inhibitory potency for aromatase (estrogen synthetase) activity in human placental microsomes. The relative specificity of inhibition was evaluated by measuring the inhibition of cholesterol side-chain cleavage enzyme (desmolase) in human placental mitochondria and the inhibition of 7-ethoxycoumarin O-deethylase (a typical drug-metabolizing enzyme activity) in rat liver microsomes. The structural requirements including substituent effects for the strongest potency and for the highest specificity were delineated. alpha,omega-Diarylalkyltriazoles and imidazoles were the most interesting molecules, in which the geometric and optical isomerism displayed remarkable selectivity for aromatase inhibition.

Expression of xenobiotic-metabolizing cytochrome P450 forms in human full-term placenta

The expression of individual xenobiotic-metabolizing cytochrome P450 (CYP) genes in human placenta was studied at the mRNA level by reverse transcriptase-polymerase chain reaction (RT-PCR). mRNAs of CYP1A1, CYP2E1, CYP2F1, CYP3A3/4, CYP3A5, and CYP4B1 were detected by RT-PCR, and CYP1A2, CYP2A6/7, CYP2B6/7, CYp2C8-19, CYP2D6, and CYp3A7 were not detected. Several enzyme activity assays and immunoblasts were used to further characterize expression of forms producing detectable mRNA transcripts. The catalytic activities of 7-ethoxycoumarin O-deethylase (ECOD), 7-ethoxyresorufin O-deethylase (EROD) and aryl hydrocarbon hydroxylase (AHH) were substantially increased in response to maternal cigarette smoking, and paralleled the amount of CYP1A1 mRNA and protein. Aromatase activities were slightly lower in placentas exposed to cigarette smoke compared with nonexposed placentas. These data show that several xenobiotic-metabolizing CYP genes are expressed in human placenta at a low level. The significant of such low-level expression is unknown, but it may have local physiological or toxic consequences.

The expression and environmental regulation of P450 enzymes in human placenta

The human placenta oxidizes several xenobiotics, although the spectrum of substrates and metabolic activities when compared with the liver appears somewhat restricted. Maternal cigarette smoking or PCB exposure increases the expression of CYP1A1. This induced activity is able to catalyze the activation of benzo(a)pyrene (B(a)P) into DNA-bound products, both in vitro and in vivo. Studies on adult human liver bolster the concept that CYP1A1 and -1A2 are differentially expressed in hepatic and extrahepatic tissues. Studies with cDNA probe or enzyme specific antibodies and substrates for CYP2A, -2B, -2C, -2D, and -2E gene products have yielded negative results. There are only minimal activities that can be found in substantial quantities in placentas without maternal smoking; one example is 7-ethoxycoumarin O-deethylase (ECOD). Aromatase and cholesterol side-chain cleaving P450 mRNAs, proteins, and activities are measurable in human placentas and do not seem to be affected by maternal cigarette smoking.

Xenobiotic and steroid-metabolizing monooxygenases catalysed by cytochrome P450 and glutathione S-transferase conjugations in the human placenta and their relationships to maternal cigarette smoking

Placentae from both smoking and non-smoking mothers were studied with respect to P450-dependent xenobiotic and steroid-metabolizing reactions, GSHt activity with different substrates and umbilical blood cotinine levels. Catalytic activities were determined in both freshly prepared homogenate subfractions and subfractions prepared after freezing the tissue sample. The results showed three correlation clusters: (i) AHH and ECDE (P less than 0.001), (ii) ECDE and ERDE (P less than 0.05), and (iii) Arom and CSCC reactions correlated with each other both in the mitochondria and microsomes. Among xenobiotic and steroid metabolizing activities, only CSCC and AHH showed a significant negative correlation. Our results agree with the earlier studies (Gottlieb and Manchester, 1986) reporting that xenobiotic metabolising MO and GSHt reactions did not show any statistically significant correlations, reflecting the fact that maternal cigarette smoking does not affect GSHt activities. However, in placentae from smoking mothers a statistically significant (P less than 0.01) positive correlation between GSHt and Arom activity was found. No plausible biological explanation is available for this finding. Among xenobiotic-metabolizing activities only ERDE correlated with plasma cotinine levels, suggesting that it is most closely related to the extent of maternal cigarette smoking. The present findings also suggest that cigarette smoke induced ERDE activity is a distinct one compared with the other xenobiotic-metabolizing P450s. Negative correlations between xenobiotic-metabolizing MO activities and CSCC lend some support to an earlier suggestion (Juchau et al, 1972) that cigarette smoking affects endogenous steroid-metabolizing CSCC reaction catalyzed by P450. Whether this finding is a true biological phenomenon and what is the mechanism behind it remain to be elucidated.

Comparison of the immunochemical properties of human placental and bovine adrenal cholesterol side-chain cleavage enzyme complex

The immunochemical relatedness between human and bovine proteins catalyzing the cholesterol side-chain cleavage reaction was investigated. In dot-immunobinding analysis, antibodies against bovine adrenocortical cytochrome P-450SCC, adrenodoxin, and adrenodoxin reductase recognized the corresponding proteins in a dose-dependent manner in mitochondrial preparations from human placenta. Limited proteolysis with trypsin cleaved bovine P-450SCC into fragments F1 and F2, which represent the NH2- and C-terminal parts of P-450SCC, respectively. Identical trypsin treatment yielded similar-size fragments from human placental P-450SCC. In Western immunoblots, anti-F1 and anti-F2 antibodies recognized the corresponding fragments in both trypsin-digested bovine and human P-450SCC. Antibodies against bovine P-450SCC, fragments F1 and F2, adrenodoxin and adrenodoxin reductase inhibited cholesterol side-chain cleavage activity in bovine adrenocortical mitochondria by 24-51%, but failed to affect the activity in human placental mitochondria. These data indicate that human and bovine P-450SCC share common antigenic determinants located outside the enzyme active site. The immunological similarity between bovine adrenodoxin and human ferredoxin allowed for a simple purification protocol of human placental P-450SCC by adrenodoxin affinity chromatography. The P-450SCC obtained by this method was electrophoretically homogeneous and showed characteristics typical to P-450SCC.

Inhibitor panel studies of human hepatic and placental cytochrome P-450-associated monooxygenase activities

1. A panel of nine inhibitors displaying some P-450 isozyme specificity was used to characterize aryl hydrocarbon hydroxylase (AHH) and 7-ethoxyresorufin 0-deethylase (ERDE) activities in human liver and placenta in vitro in comparison with liver enzymes from control, phenobarbital (PB) and 3-methylcholanthrene (MC) treated rats. 2. SKF 525A and cimetidine inhibited more potently hepatic AHH than the placental enzyme. 7,8-Benzoflavone inhibited more efficiently placental AHH than the hepatic enzyme, whereas ERDE was inhibited at the same level in both tissues. Quinine, quinidine, SKF 525A and metyrapone inhibited ERDE almost to the same extent in both tissues, but the variability was larger with the liver enzyme. Aminoglutethimide, debrisoquine or tetrahydrofuran did not inhibit AHH or ERDE significantly in either tissue. 3. When compared with inhibition profiles obtained with rat liver microsomes, the human hepatic and placental ERDE resembled most that of MC-treated rat liver enzyme. Inhibition profile of placental AHH activity was also similar, but the inhibition characteristics of hepatic AHH activity resembled more closely control or PB-induced rat liver. It also seems that isozymes for alcohol induction or debrisoquine hydroxylation do not contribute significantly to hepatic or placental AHH or ERDE. 4. The inhibitor panel selected on the basis of known pretreatment and isozyme specificity might be useful in the characterization of enzymes and metabolic biotransformations participating in the metabolism of new substrates.

Purification and immunological characterization of human placental mitochondrial cytochrome P-450

Human placental mitochondrial cytochrome P-450 was purified to electrophoretic homogeneity by hydrophobic, anion exchange and cation exchange column chromatography. The specific content of the purified protein was 15.7 nmol/mg protein and it showed a single band mol. wt 48,000 D in SDS-gel electrophoresis. When reconstituted with bovine adrenal adrenodoxin reductase and adrenodoxin it converted cholesterol to pregnenolone (cholesterol side-chain cleavage activity, CSCC) at the rate of 1 pmol/min/pmol P-450. Antibodies against the purified protein were raised in rabbits. Inhibition studies demonstrated 85% inhibition of placental CSCC activity at an antibody/protein ratio of 10:1. Placental microsomal aromatase activity was inhibited by 47% at the same antibody/protein ratio. The antibody inhibited bovine mitochondrial CSCC activity by 87% at the same antibody/protein ratio. Placental microsomal 7-ethoxycoumarin O-deethylase, aryl hydrocarbon hydroxylase and 7-ethoxyresorufin O-deethylase activities were not significantly inhibited by the antibody. The results indicate that the purified protein catalyzes cholesterol side-chain cleavage reaction, human placental microsomal aromatase and bovine adrenal mitochondrial P-450scc may share common antigenic determinants with placental P-450scc, but the placental microsomal xenobiotic-metabolizing cytochrome(s) is (are) distinctly different.

Freezing affects the activity and subcellular distribution profile of human placental xenobiotic- and steroid-metabolizing enzymes

Human placentae from both smoking and non-smoking mothers were examined with respect to the activity and apparent subcellular distribution of cytochrome P-450-linked xenobiotic-and steroid-metabolizing activities. Freezing of the placental sample in pieces before preparing mitochondria and microsomes led to decreases in some catalytic activities and changed the distribution profile of activities between mitochondria and microsomes. Cholesterol side-chain cleavage enzyme and aryl hydrocarbon hydroxylase were especially sensitive to the destructive effect of freezing. Activities in the microsomes were apparently more sensitive to freezing than were those in the mitochondrial fraction. In addition to the destruction of activities by freezing, cross-contamination of microsomes and mitochondria was postulated. Freezing tended to level off the differences between induced and uninduced material. In JEG cells, the specific activities of xenobiotic-metabolizing enzymes were higher in the mitochondrial fraction. In liver biopsy samples the expected, principally microsomal, localization of xenobiotic-metabolizing enzymes was observed. It is suggested that the placenta should be processed fresh at least as far as the postmitochondrial supernatant before freezing to avoid the destructive and cross-contaminating effects of freezing.