The stoichiometry of the conversion of cholesterol and hydroxycholesterols to pregnenolone (3beta-hydroxypregn-5-en-20-one) catalysed by adrenal cytochrome P-450.

The stoichiometry of the side-chain cleavage of cholesterol (cholest-5-en-3beta-ol), 20alpha-hydroxycholesterol (cholest-5-ene-3beta,20alpha-diol), and 20alpha,22-dihydroxycholesterol (cholest-5-ene-3beta,20alpha,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(+) + 3O(2) --> Pregnenolone (3beta-hydroxypregn-5-ene-20-one) + isocapraldehyde + 3 TPN(+) 4H(2)OStoichiometry of 20 alpha-hydroxycholesterol is 2TPNH and 2O(2) per mole of cleavage and with 20alpha, 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 --> 20alpha-OH-Cholesterol --> 20alpha, 22-di-OH-Cholesterol --> pregnenoloneDiscordant observations are reviewed and it is concluded that insufficient data are available to sustain objections to this pathway.