Symmetry of squalene epoxidation in vivo

Abstract

Radioactive mevalonic acid, in which the radioactivity was due solely to 5 S tritiated molecules, was injected intraperitoneally into living rats. After 45 min the animals were killed and radioactive cholesterol was isolated from the livers. This cholesterol was incubated with Myco­bacterium phlei in conditions that forced accumulation of androst-1, 4-dien-3, 17-dione. After removal of alkali-labile tritium from the 16 β position the dione (A) was incubated with Aspergillus tamarii to yieldandrost-1, 4-dien-11 β -ol-3, 17-dione, oxidized by chromic acid to androst-1, 4-dien-3, 11, 17-trione (B). When this was treated with alkali the recovered trione (C) was almost non-radioactive. The ratio of the specific radioactivities A : B : C was 1.00 : 0.51 : 0.004. It was concluded that with in experimental error the cholesterol was equally labelled with tritium at the 11 β and 12 α positions and therefore that both double bonds in the isotopically asymmetric intermediate squalene were epoxidized to an equal extent. It follows that squalene, when biosynthesized by intact ratliver in vivo , is not transferred in a spatially oriented manner from the enzyme that produces it to the enzyme that epoxidizes it.