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precursors, including lanosterol, into poriferasterol (III) by Ochromonas malhamensis has been demonstrated (Hall et al., 1969; Lenton et al., 1971). Since a cell-free extract of peas can alkylate lanosterol, cycloartenol or desmosterol but not 24,25-dihydrolanosterol (I) or cholesterol (11), it has been suggested that sterols without a A24-bond cannot be alkylated (Russell et al., 1967). However, there is an isolated report of the conversion of cholesterol into 22,23dihydrostigmasterol (presumably sitosterol) by the flowers of Wistaria sinensis (Badiello et al., 1967). Therefore the biosynthetic role of 24,25-dihydrolanosterol, which has been isolated from 0. malhamensis (G. H. Beastall, unpublished work), is uncertain. This sterol may undergo C-24 alkylation and further metabolism into poriferasterol, the major sterol in the organism (Gershengorn et al., 1968), or it may be a specific precursor of a C27 sterol such as cholesterol. Cholesterol cannot be detected in the free and esterified sterol fractions of 0. malhamensis, but low concentrations of the compound have been characterized by g.l.c.-mass spectrometry in the sterol glycoside ester and water-soluble sterol fractions of the organism (G. H. Beastall, unpublished work). The present communication describes the conversion of 24,25-dihydro[2-3H2]1anosterol (a) into both cholesterol (I) and poriferasterol (III) by 0. malhamensis. Since plant systems can alkylate exo-

Studies on phytosterol biosynthesis: metabolism of 24,25-dihydrolanosterol and cholesterol in Ochromonas malhamensis