Antioxidants inhibit the rat liver microsomal mixed-function-oxidase-catalysed hydroxylation of benzo[a]pyrene. These antioxidants also decrease the formation of mutagenic products from benzo[a]pyrene as judged by the Ames bacterial-mutagenicity assay [B.N. Ames, J. McCann & E. Yamazaki (1975) Mutat. Res. 31, 347-364]. It is suggested that antioxidants exert their protective effect against cancer by inhibiting the formation of carcinogenic metabolites.
1. [3α- 3 H]Cholesta-7,9-dien-3β-ol is converted in high yield into cholesterol by a 10000 g av. supernatant fraction of rat liver homogenate. 2. Incubation of cholesta-7,9-dien-3β-ol with [4- 3 H]NADPH and rat liver microsomal fractions under anaerobic conditions resulted in 3 H being incorporated into the 14α-position of cholest-7-en-3β-ol. 3. Under anaerobic conditions in the absence of NADPH cholesta-7,9-dien-3β-ol was isomerized into cholesta-8,14-dien-3β-ol by rat liver microsomal fractions.
The synthesis of [7α- 3 H]lanosterol is described. It is shown that in the conversion of [7α- 3 H,26,27- 14 C 2 ]lanosterol into cholesterol by a rat liver system, it is the 7β-hydrogen atom that is predominantly removed. On the other hand, the conversion of doubly labelled lanosterol into ergosterol by whole yeast cells results in the loss of the 7α-hydrogen atom. These results therefore suggest that the C-7 hydrogen atoms with opposite stereochemistry are labilized by the rat liver and the yeast Δ 8 –Δ 7 steroid isomerases.
Cholesterol is biosynthesized from squalene in the presence of tritiated water. Chemical degradation reveals that a considerable percentage of the total radioactivity is present at C-15. This result confirms the previous observations on the involvement of a C-15 hydrogen atom in cholesterol biosynthesis.
The incubation of Δ 7 -cholestenol with a 10000 g supernatant or 105000 g microsomes in the presence of tritiated water is studied. The reisolated Δ 7 -cholestenol contained up to 0·67g.atom of tritium/mole. This result can best be explained by assuming the reversibility of the reaction Δ 8 -cholestenol ⇌ Δ 7 -cholestenol.
The biosynthesis of cholesterol from squalene and tritiated water is described. Degradation of the cholesterol indicated that C-15 may be involved in cholesterol biosynthesis. In accordance with this view it is shown that in the conversion of [2 RS − 3 H 2 ]mevalonic acid into cholesterol one of the hydrogen atoms at C-15 is removed. A mechanism for the removal of the 14α-methyl group in steroid biosynthesis that involves the labilization of a C-15 hydrogen atom is outlined. In accordance with the requirement of this scheme it is shown that 4,4′-dimethyl-cholesta-8,14-dien-3β-ol is converted into cholesterol.