Role of thiol group(s) in the enzymatic activity of the membrane-bound delta5-3beta-hydroxysteroid dehydrogenase from bovine adrenal cortex microsomes

The role of the available sulphydryl group in liver butyryl-coenzyme A synthetase

Butyryl-CoA synthetase from acetone-dried ox liver mitochondria can be activated up to 80% by low concentrations of iodoacetate with the concomitant modification of one available enzyme thiol group. The relative molecular mass of the carboxymethylated enzyme was unchanged, and there were no major changes in the complex kinetic properties of the enzyme. Extraction experiments on liver mitochondria indicate that in vivo the enzyme is linked to a mitochondrial membrane by a bond cleavable by disulphide bond reducing agents.

Active site-directed irreversible inhibition of 3 beta-hydroxysteroid dehydrogenase from human placenta

To obtain a placental microsome preparation able to convert androstenedione and testosterone specifically to estrogens, 3 beta-hydroxysteroid dehydrogenase must be eliminated. After solubilisation by Triton X-100, the remaining 3 beta-hydroxysteroid dehydrogenase activity is inhibited by 11 alpha-bromoacetoxyprogesterone, an alkylating analogue of progesterone, which behaves as an irreversible active site-directed inhibitor. The enzyme is protected against inactivation and alkylation by steroid and coenzyme substrates. The inhibition is specific to 3 beta-hydroxysteroid dehydrogenase and the estrogen synthetase (aromatase) activity contained in the preparation is not affected by this inhibitor.