Studies on New Enzyme System for the Isoprenoid Biosynthesis in Bacteria

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The cells of Fravobacte rium okeanokoites were disrupted and the cell extracts were subjected to a chromatography on DEAE-Toyopearl 650S ion exchange column. Two peaks of the enzyme activity for ^<14>C-incorporation from[ ^<14>C]farnesyl diphosphate into non-polar products were observed. The purified enzymes obtained after rechromatography were designated Enzyme I and 11. The non-polar product formed in the reaction of Enzyme I with[ ^<14>C]farnesyl diphosphate in the presence of NADPH was squalene. Km values of the squalene synthesizing enzyme for farnesyl diphosphate and NADPH were 0.4 muM and 10 muM, respectively.In contrast, the radioactive non-polar products formed from[ ^<14>C]farnesyl diphosphate in the presence of NADPH by the catalysis of Enzyme II coincided with dehydrosqualene but not with squalene. Removal of NADPH from tlie complete assay mixture did not affect the radioactivity of the non-polar products and the main product was dehydrosqualene. The geometries of double bonds of dehydrosqualene obtained in the reaction catalyze by Enzyme 11 was 12-cis-dehydrosqualene. The Km value of the dehydrosqualene synthesizing enzyme for farnesyl diphosphate was 0.04 muM.These results indicate that Enzyme I is squalene synthase and Enzyme 11 is dehydrosqualene synthase. Both enzymes seem to catalyze the condensation of 2 moles of farnesyl diphosphate to yield presqualene diphosphate as an intermediate. Then from presqualene diphosphate the former yields squalene in the presence of NADPH and the latter produces dehydrosqualene in the presence and absence of NADPH.Dehydrosqualene synthase is a new enzyme and squalene synthase is the first enzyme obtained and partially purified from bacteria.