In vitro yeast (Saccharomyces cerevisiae) presqualene and squalene synthesis related to substrate and cofactor availability.

Squalene synthase catalyses the synthesis of squalene from trans-farnesyl diphosphate in 2 separate steps requiring NAD(P)H. The kinetics of this enzyme in different fractions extracted from a wild-type Saccharomyces cerevisiae were studied. Although this protein is known to be a membrane-bound enzyme, we have found a cytosolic squalene synthase activity besides the microsomal enzyme. A spectrophotometric enzyme assay, not involving isotopic labelling, was established. The relative synthesis of presqualene and squalene was evaluated by using different substrate and cofactor concentrations during the incubation. The involvement of a single catalytic site promoting the 2 reactions of squalene synthesis is suggested.