Formation of dehydrosqualene catalyzed by squalene synthetase in Saccharomyces cerevisiae.

When microsomal fraction of Saccharomyces cerevisiae was incubated with farnesyl pyrophosphate or presqualene pyrophosphate in the presence of Mn2+, 12,13-cis-dehydrosqualene (DeH2Sq) and some related compounds were found to be formed. Incubation in the presence of NADPH gave rise to only squalene. By heat treatment of the microsomal fraction, the DeH2Sq- and squalene-forming activities were inactivated at approximately the same rate. The elution patterns of both activities upon Sephacryl S-200 chromatography of the enzyme solubilized from the microsomal fraction with taurodeoxycholate coincided completely. These results indicate that DeH2Sq formation in yeast is catalyzed by squalene synthetase. Divalent cation was essential for this reaction and Mn2+ was six times more effective than Mg2+. DeH2Sq formation was also observed when microsomes of pig liver were used instead of yeast microsomal fraction, suggesting that this reaction is a ubiquitous one among the eucaryotes which are capable of synthesizing sterols. Based on these observations, the mechanisms of DeH2Sq and squalene formation are discussed.