Biosynthetic Studies of Marine Lipids. 33.1 Biosynthesis of Dinosterol, Peridinosterol, and Gorgosterol: Unusual Patterns of Bioalkylation in Dinoflagellate Sterols

Abstract

The biosynthesis of the 23-methylated dinoflagellate sterols gorgosterol (1-N), dinosterol (2-K), and peridinosterol (3-K) was demonstrated experimentally using cell-free extracts of the dinoflagellates Cryptothecodinium cohnii, Peridinium foliaceum, and the cultured zooxanthella symbiont of Cassiopea xamachana. In assays of sterol methyltransferases using [3H]-S-adenosylmethionine, radiochemioal conversions were demonstrated by reverse phase HPLC of the Δ24 sterol side chain (5) to the 24-methylene side chain (4); of the sequence 24(R)-4α-methylergost-22-enol (9-K) to dinosterol (2-K) to 4α-methylgorgostanol (1-K); and, in P. foliaceum, of 24-(R)-4α-methylergost-22-enol (9-K) to peridinosterol (3-K). Methylation of the 24(R)-methyl Δ22 side chain (9) in P. foliaceum is believed to involve more than one SAM-sterol methyltransferase based on changes in the ratios of the products (2 and 3) with changes in the conditions of the assay. Furthermore, deuterium substitution of the sterol substrate (9-K) at C-23 did not significantly alter the product ratio. A hypothesis is put forward that the attenuation of gorgosterol (1-N) production in aposymbiotic zooxanthellae is linked to an increase in dimethylpropiothetin biosynthesis via a decrease in S-adenosylmethionine concentration. © 1991, American Chemical Society. All rights reserved.