Formation of Dehydrosqualene Catalyzed by Squalene Synthetase in Saccharomyces cerevisiae1

When microsomal fraction of Saccharomyces cerevisiae was incubated with farnesyl pyrophosphate or presqualene pyrophosphate in the presence of Mn 2+ , 12,13- cis -dehydrosqualene (DeH 2 Sq) 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 DeH 2 Sq- 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 DeH 2 Sq formation in yeast is catalyzed by squalene synthetase. Divalent cation was essential for this reaction and Mn 2+ was six times more effective than Mg 2+ . DeH 2 Sq 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 DeH 2 Sq and squalene formations are discussed.