Non‐specific Lanosterol and Hopanoid Biosynthesis by a Cell‐Free System from the Bacterium Methylococcus capsulatus

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Abstract
1 A cell-free system from the bacterium Methylocoecus capsulatus was incubated with [12-3H]squalene; diploptene and diplopterol, normally present in the bacterium, were labelled. 2 The same cell-free system was incubated with (RS)-2,3-epoxy-2,3-dihydro-[3-3H]squalene. Several radioactive 3-hydroxytriterpenes were purified. Lanosterol, which is normally present in this bacterium, was found labelled as well as 3-epilanosterol. In addition, radioactive 3α-hydroxy and 3β-hydroxydiploptene were formed. 3 These data may be explained by the coexistence of two cyclases in M. capsulatus: a squalene/hopane cyclase and a squalene epoxide/lanosterol cyclase. The squalene cyclase exhibits the same lack of substrate specificity as those of Acetobacter pasteurianum and Tetrahymena pyriformis, i. e. in addition to its normal substrate squalene, it can cyclize the two enantiomers of squalene epoxide into 3-hydroxyhopanoids. 4 The presence of a squalene epoxide/lanosterol cyclase activity, which was suspected in view of the unique 3β-hydroxy 4α-methyl steroids of M. capsulatus, was demonstrated by the labelling of lanosterol. More surprisingly 3-epilanosterol was also present and labelled. We showed that this does not derive from lanosterol by isomerization via a 3-oxo compound. Therefore the squalene epoxide cyclase of M. capsulatus, like the one of eukaryotes cyclizes the (3S) enantiomer of squalene epoxide into lanosterol. But it is definitely less substrate-specific as it can also cyclize the (3R) enantiomer into 3-epilanosterol.