Abstract
1. 1. The activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoA reductase) mevalonate: NADP oxidoreductase (CoA acylating; EC 1.1.1.34) in microsomes from early- and term-pregnancy placenta has been found to be 24 ± 2 and 6 ± 3 pmol/min per mg protein, respectively. 2. 2. Inactivation of the enzyme required the addition of ATP and Mg 2+ and was dependent on the time of preincubation. 3. 3. Reactivation of the enzyme was also dependent on the incubation time and prevented by the presence of fluoride—a phosphoprotein phosphatase inhibitor. These data suggest that (despite a low activity) placental HMG-CoA reductase is covalently modulated via the phosphorylation-dephosphorylation system. 4. 4. The conversion of [ 14C]acetate and [ 3H]mevalonate into digitonin precipitable placental sterols indicates that the lower reductase activity in term, than in early, placental microsomes is accompanied by a less active conversion of [ 14C]acetate in this tissue.
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