Abstract :
Conversion of 5-cholestene-3β,7α-diol (7α-hydroxycholesterol) into 7α-hydroxy-4-cholesten-3-one was studied with microsomes from different pig tissues and with liver subcellular fractions. Dehydrogenase/isomerase activity was efficient in microsomes from liver, ovary and lung, but less efficient in microsomes from adrenal gland and kidney. Microsomes from these tissues, with the exception of lung, were also active in dehydrogenation/isomerization of dehydroepiandrosterone and pregnenolone. Inhibition studies were carried out with trilostane, a competitive inhibitor of 3β-hydroxysteroid dehydrogenases active in steroid hormone biosynthesis (C19/C21-dehydrogenases), and a monoclonal antibody raised against a purified hepatic 3β-hydroxy-Δ5-C27-steroid dehydrogenase. The results showed that the C27-dehydrogenase activity in the tissues was not dependent on the C19/C21 dehydrogenases, but was dependent on the 3β-hydroxy-Δ5-C27-steroid dehydrogenase. Liver mitochondria, cytosol and peroxisomes lacked dehydrogenase/isomerase activity towards 7α-hydroxycholesterol when microsomal contamination was taken into account. Immunoblotting experiments with monoclonal antibodies raised against the 3β-hydroxy-Δ5-C27-steroid dehydrogenase showed immunoreactivity only with protein in liver microsomes. Immunohistochemical studies showed localization of the 3β-hydroxy-Δ5-C27-steroid dehydrogenase in the bile duct epithelium. It is concluded that 7α-hydroxycholesterol is converted into 7α-hydroxy-4-cholesten-3-one by the microsomal 3β-hydroxy-Δ5-C27-steroid dehydrogenase in liver and extrahepatic tissues.
