Side chain hydroxylation of cholesterol, campesterol and beta-sitosterol in rat liver mitochondria

The extent of the side chain hydroxylation of cholesterol, campesterol (24 alpha-methylcholesterol), and beta-sitosterol (24 alpha-ethylcholesterol) in rat liver mitochondria has been compared. Two beta-sitosterol metabolites, tentatively identified by liquid chromatography, thin-layer chromatography, gas-liquid chromatography, combined with radioactivity detection, and gas-liquid chromatography-mass spectrometry as the 26- and 29-hydroxy derivatives, were formed in the proportion 1:1. The sum of 26-hydroxy- and 29-hydroxy-beta-sitosterol obtained amounted only to about one-fourth of the yield of 26-hydroxycholesterol. Campersterol appeared to give rise only to 26-hydroxycampesterol (tentatively identified), which was formed in similar yields as 26-hydroxycholesterol (0.2-0.4%). The formation of 29-hydroxy-beta-sitosterol but not of 28-hydroxycampesterol indicates that the omega-hydroxylation of the steroid side chain is dependent on the length of the side chain. Cholesterol gave rise to identifiable amounts of a 25-hydroxy derivative but the formation of 25-hydroxy derivatives of beta-sitosterol and campesterol could not be established with certainty. 24-hydroxycholesterol was also found to be formed in the mitochondrial system. The ratio between the yields of 26- and 25-hydroxychoelsterol ranged between 2 and 3, and that between 26- and 24-hydroxycholesterol was about 10.