Evidence for the presence of an enzyme capable of opening the cyclopropane ring of cycloeucalenol

Sterol biosynthesis de nova via cycloartenol by the soil amoeba Acanthamoeba polyphaga

“…The presence of a plant biosynthetic pathway for the sterol biosynthesis in this protozoon and particularly the identification of cycloartenol as-sterol precursor were fully unexpected. This implies the presence of at least three enzymic reactions that are typical of phytosterol biosynthesis: (i) the cyclization of squalene epoxide into cycloartenol (Goad, 1971;Heintz & Benveniste, 1970), (ii) the first methylation step occurring at the 4,4dimethylsterol level and revealed by the presence of 24-methylenecycloartanol (II) (Wojciechowski et al, 1973;Fonteneau et al, 1977), and (iii) finally the isomerization of cycloeucalenol to obtusifoliol by a cyclopropane isomerase revealed by the presence of these two 4a-methylsterols (Heintz et al, 1972;Heintz & Benveniste, 1974). From the point of view of sterol biosynthesis it is possible to divide all living eukaryotes into two categories: lanosterol is the sterol precursor in all non-photosynthetic phyla (animals and fungi), and cycloartenol in all photosynthetic phyla (plants and algae) (Goad, 1971, and references cited therein).…”

The Effect of Medium Modification and Selected Precursors on Sterol Production by Short-Term Callus Cultures of Euphorbia tirucalli

“…Lanosterol does not appear to be a direct intermediate in sterol biosynthesis (6, 16) and, therefore, may not be expected to be transformed in culture to other sterols. Triterpenes appear to be synthesized via several intermediates in a non-specific process imvolving cycloartenol as the first cvclized product (22,23,24). On the other hand, ["C]-lanosterol has been shown to be incorporated into a wide variety of sterols in Nicotiana tabacum cultures (25) and illustrates the capacity of triterpene-synthesizing enzyme systems to utilize these various metabolites.…”

Metabolism of cyclopropane fatty acids by Tetrahymena pyriformis

“…amensis to cleave the 9»19-cyclopropane ring of some phytosterols such as cycloartenol and 4-methylene cycloartenol to form poriferasterol. In plants, an enzyme from bramble tissues was reported (Heintz et al, 1972a) which can open the 9,19-cyclopropane ring of cycloeucalenol, converting it to obtusifoliol. 4,4-dimethyl sterols, cycloartenol sind 24-methylene cycloartenol, were found to be very poor substrates for that enzyme (Heintz et al, 1972b).…”