The reversibility of the isomerization of the delta8 to delta7 bond in cholesterol biosynthesis

Studies on the 14 alpha-demethylation mechanism in cholesterol biosynthesis

Identification of radioactive 5 alpha-cholest-8(14)-ene-3 beta,7 alpha-diol in extracts obtained from incubations of 3 beta-hydroxy-5 alpha-[7-3H]cholest-7-ene-14 alpha-carbaldehyde with rat liver microsomes is reported. Levels of this diol in incubations of the 14 alpha-[32-3H]carbaldehyde were measured by multiple selected ion monitoring and were found to be of the same order of those of [3H]formate released from the substrate during the removal of the C-32 atom. The results demonstrate that the diol does not originate from known intermediates of cholesterol biosynthesis, i.e. 5 alpha-cholesta-7,14-dien-3 beta-ol, 5 alpha-cholest-7-en-3 beta-ol and from 5 alpha-cholest-8(14)-en-3 beta-ol. Functionalization at position 7 in the metabolism of 3 beta-hydroxy-5 alpha-cholest-7-ene-14 alpha-carbaldehyde suggests the direct involvement of the double bond in the elimination of the 14 alpha-formyl group in the biosynthetic pathway from lanosterol to cholesterol. 5 alpha-Cholest-8(14)-en-3 beta-ol appears not to be involved in the metabolism of the 14 alpha-carbaldehyde.

Inversion of the unnatural cis C/D sterol ring junction of 5alpha, 14beta-cholest-7en-3beta-ol by rat-liver enzymes

Labelled 5alpha, 14beta-cholest-7-en-3beta-ol with the 'unnatural' cis C/D ring junction was synthesized. Incubations of this sterol with rat liver homogenate under aerobic conditions gave radioactive 5alpha-cholest-7-en-3beta-ol, 5alpha-cholest-8(14)-en-3beta-ol andcholesterol indicating the presence of enzymes in the rat liver capable of inverting the C/D cis to a trans configuration. No radioactivity was found associated with added 14beta-cholesterol showing the specificity of the 14alpha configuration for the enzymic conversion of a sterol molecule into cholesterol.

Enzymatic conversion of 5alpha-cholesta-7, 14-dien-3beta-ol to 5alpha-cholesta-8, 14-dien-3beta-ol

5alpha-Cholesta-7, 14-dien-3beta-ol, previously shown to be efficiently converted to cholesterol upon incubation with rat liver homogenate preparations under aerobic conditions, has been studied as to its possible conversion to 5alpha-cholesta-8, 14-dien-3beta-ol. Efficient conversion was observed upon incubation in the presence of washed mictosomes of rat liver under anaerobic conditions. This observation is of importance in consideration of possible metabolic pathways in the biosythesis of cholesterol.