The Stoichiometry of the Conversion of Cholesterol and Hydroxycholesterols to Pregnenolone (3β -Hydroxypregn-5-en-20-one) Catalysed by Adrenal Cytochrome P-450

• Published in: Proceedings of the National Academy of Sciences - PNAS 1974-04, Vol.71 (4), p.1441-1445
• Main Authors: Shikita, Mikio, Hall, Peter F.
• Format: Article
• Language: English
• Subjects: Adrenal Glands - cytology; Adrenal Glands - enzymology; Animals; Biological Sciences: Biochemistry; Carbon Isotopes; Catalysis; Cattle; Cholesterol - metabolism; Cholesterols; Cytochrome P-450 Enzyme System - metabolism; Cytochromes; Electrons; Enzymes; Ferredoxins - metabolism; Hydrogen-Ion Concentration; Hydroxylation; Mitochondria; Mitochondria - enzymology; Models, Chemical; Molecules; NADH, NADPH Oxidoreductases - metabolism; Oxidation; Oxygen; Oxygen Consumption; Pregnenolone - biosynthesis; Stoichiometry
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.71.4.1441

The stoichiometry of the side-chain cleavage of cholesterol (cholest-5-en-3β -ol), 20α -hydroxycholesterol (cholest-5-ene-3β ,20α -diol), and 20α ,22-dihydroxycholesterol (cholest-5-ene-3β ,20α ,22-triol) has been examined with respect to TPNH, oxygen and H+. Side-chain cleavage of cholesterol can be described by the following equation: Cholesterol + 3TPNH + 3H++ 3O2→ Pregnenolone (3β -hydroxypregn-5-ene-20-one) + isocapraldehyde + 3 TPN+4H2O Stoichiometry of 20 α -hydroxycholesterol is 2TPNH and 2O2per mole of cleavage and with 20α , 22-dihydroxycholesterol the values are 1:1:1. In addition 1 mole of H+is consumed per mole of TPNH oxidized in each of these reactions. These observations are in keeping with a mechanism previously proposed in the literature, namely: Cholesterol → 20α -OH-Cholesterol → 20α , 22-di-OH-Cholesterol → pregnenolone Discordant observations are reviewed and it is concluded that insufficient data are available to sustain objections to this pathway.