Phylogenetics and Biosynthesis
The sterols of living systems are all derived by the aerobic cyclization of squalene through the intermediacy of epoxysqualene as discussed in depth elsewhere (Nes, 1977; Nes and McKean, 1977). However, epoxysqualene is not always cyclized to the same substance. Two major products (lanosterol and cycloartenol) are formed, and they are never known to be produced by cyclization in the same organism, whether the latter is unicellular or highly differentiated. This bifurcation in the steroid pathway is one of the most interesting phylogenetic markers we have, because it has no apparent influence on the structure of the functional steroid at the end of the pathway. There are two principal differences in naturally occurring, functional sterols, viz., the structure of the side chain and the double-bond character in the nucleus. Neither of these is influenced by whether lanosterol or cycloartenol is the precursor. Thus, it was shown in our laboratory (Russell et al., 1967; Raab et al.,1968; Gibbons et al., 1971) that either lanosterol or cycloartenol will yield 24-alkylsterols (sitosterol, etc.) in higher plants. Similarly, while lanosterol leads to cholesterol in animals (Tchen and Bloch, 1955), pollinastanol (14α-methyl-9,19-cyclo-5α-cholestanol, a metabolite of cycloartenol in which the Δ24 bond has been reduced and the two methyl groups at C-24 removed) has been converted to cholesterol in higher plants (Devys et al., 1969), and cycloartenol is present in red algae in which the dominant sterol is cholesterol (Ferezou et al., 1974).
KeywordsBile Acid Green Alga Cholic Acid Chain Elongation Sterol Biosynthesis
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