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Biomedical subjects

P C Macdonald

Publications and source records attributed to P C Macdonald.

5 recordsLinked to original sources
PubMed1997

Human saturated steroid 6alpha-hydroxylase.

This study was conducted to evaluate further the reaction catalyzed by the saturated steroid 6alpha-hydroxylase of extrahepatic human tissues. Progesterone and 5alpha-dihydroprogesterone (5alpha-DHP) are plasma-borne precursors of 5alpha-pregnan-3alpha-ol-20-one, an anxiolytic/anesthetic steroid, and 5alpha-pregnan-3beta-ol-20-one in extrahepatic human tissues. These two steroids are metabolized further by a saturated steroid 6alpha-hydroxylase enzyme(s) that is distinct from the cytochrome P450 6alpha-hydroxylase that catalyzes the 6alpha-hydroxylation of delta4-3-ketosteroids such as progesterone, cortisol, and testosterone. Products of this saturated steroid 6alpha-hydroxylase, viz. 3beta/alpha,6alpha-dihydroxy-5alpha-pregnan-20-ones, are major radiolabeled urinary metabolites (excreted as glucuronosides) of i.v. administered tritium-labeled 5alpha-DHP in women and men. T47-D human breast cancer cells, which are rich in saturated steroid 6alpha-hydroxylase activity, were used as the enzyme source in this study. The greatest total and the highest specific activity of saturated steroid 6alpha-hydroxylase were localized in microsome-enriched preparations; enzyme activity was linear with incubation time up to 30 min and with microsome-enriched tissue protein concentrations between 0.05-0.5 mg/mL incubation mixture. The velocity of the reaction was similar in incubations in which the pH was varied from 6.0-8.0, and NADH and NADPH were equally effective in supporting the 6alpha-hydroxylation of 5alpha-pregnan-3beta-ol-20-one and 5alpha-pregnan-3alpha-ol-20-one. The more efficient substrates for this enzyme were 5alpha-pregnan-3beta-ol-20-one and 5alpha-pregnan-3alpha-ol-20-one, and the apparent Km (approximately 3.5 micromol/L) and maximum velocity (approximately 150 pmol/min x mg microsome-enriched protein) for these two substrates were indistinguishable. 5alpha-Androstane-3beta,17beta-diol was less efficiently 6alpha-hydroxylated, and 5alpha-androstane-3alpha,17beta-diol was an inefficient substrate. The addition of a variety of inhibitors of cytochrome P450 monooxygenases to the incubation mixtures did not diminish significantly the 6alpha-hydroxylation of 5alpha-pregnan-3beta-ol-20-one, findings consistent with those of other investigators who suggested that human saturated steroid 6alpha-hydroxylase (of human prostate) is not a cytochrome P450.

Breast Neoplasms↗
PubMed1978

Initiation of human parturition. IX. Progesterone metabolism by placentas of early and late human gestation.

In in vitro studies it was found that, in the presence of added NADPH, the human placenta metabolizes progesterone to 20alpha-hydroxy-4-pregnen-3-one at increasing rates as pregnancy advances. A fivefold increase in 20alpha-hydroxysteroid oxidoreductase activity is observed in term placentas compared with the enzymic activity of placentas of 12 to 20 weeks' gestation (368 +/- 53 pmoles/mg protein/hr vs 71 +/- 8 pmoles/mg protein/hr). The 5alpha-reductase and 3beta-hydroxysteroid oxidoreductase activities were similar in placentas obtained from early and late human pregnancies.

20-Hydroxysteroid Dehydrogenases↗
PubMed1977

Initiation of human parturition. VI. Identification and quantification of progesterone metabolites produced by the components of human fetal membranes.

The fetal membranes play a central role in the initiation of human parturition, a role that may be subserved in part by quantitative changes in progesterone metabolism. In order to identify and to quantify the metabolites of progesterone produced by the components of the human fetal membranes, amnion and chorion laeve tissues and homogenates were incubated with [3H]progesterone in the presence of added NADPH. The radioactive metabolites, 20alpha-hydroxy-4-pregnen-3-one, 5alpha-pregnane-3,20-dione and 3beta-hydroxy-5alpha-pregnan-20-one, were isolated and identified by derivative formation, a combination of chromatographic techniques, and by crystallization to constant specific activity after addition of authentic standards. A simplified technique of metabolite quantification was developed that involves one thin layer chromatographic procedure and liquid scintillation assay of radioactivity. The accuracy of this method was established by comparison with data obtained using classical techniques. This study demonstrates the presence of human fetal membranes 5alpha-reductase, 20alpha-hydroxysteroid oxidoreductase, and 3beta-hydroxy-steroid oxidoreductase, and a qualitative difference between amnion and chorion laeve.

20-alpha-Dihydroprogesterone↗
PubMed1975

Initiation of human parturition. III. Fetal membrane content of prostaglandin E2 and F2alpha precursor.

Unesterified arachidonic acid is the obligatory precursor of the prostaglandins (PG), PGF2alpha and PGE2. In order to ascertain whether or not the human fetal membranes could represent a storage site for prostaglandin(s) precursor, the fatty acid content of human fetal membranes was measured. Approximately 20% of the fatty acids found in fetal membranes obtained from near-term, non-laboring women was arachidonic acid, whereas only 0.4% of the fatty acids of the parietal peritoneum of the mother is arachidonic acid. A small but significant decrease in the arachidonic acid concentration was found in the fetal membranes obtained from laboring women compared to that found prior to labor. On the other hand, the concentration of palmitic acid was increased in membranes obtained during labor while no significant changes in concentration in the remaining fatty acids were observed in membranes from laboring compared to non-laboring near-term gravidas. The significance of these observations in relation to the availability of prostaglandin precursor and the initiation of human parturition is considered.

Arachidonic Acids↗