Stereochemical importance of fucosterol epoxide in the conversion of sitosterol into cholesterol in the silkworm Bombyx mori.

Studies on the stereochemistry of 24,28-epoxy-24-ethylcholesterol, a key intermediate in sitosterol to cholesterol conversion in insects, were undertaken. In nutritional experiments, the 24R,28S and 24S,28R stereoisomers (isofucosterol epoxides) were unable to support growth and development of silkworm larvae. Incubation of 3 alpha-3H-labeled (24R,28R)- and (24S,28S)-epoxide (fucosterol epoxides) with a cell-free preparation from silkworm guts resulted in effective conversion into desmosterol and cholesterol; (24R,28R)-epoxide was the slightly better substrate. Fucosterol incubation yielded (24R,28R)-epoxide and the 24S,28S isomer in approximately equal amounts, while slightly preferential formation of the 24R,28R isomer was observed in in vivo experiments. These results indicate that both the formation of the epoxide from fucosterol and its conversion to desmosterol proceed with a low degree of stereospecificity.