Stereochemical studies on the metabolism of sterols by Saccharomyces cerevisiae strain GL7



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Texas Tech University


The relationship between sterol structure and function was established using a sterol auxotroph Saccharomyces cerevisiae strain GL7. The 21 sterol substrates incubated with GL7 were obtained through natural product isolation and organic synthesis. The major source of sterol substrates was Cereus giganteus (cactus) pollen. From the pollen was isolated 27 sterols. The sterols were characterized in stereochemical detail by chromatographic (TLC, GLC, and HPLC) and spectral (MS, and 1H-NMR) methods. Several novel sterols were identified from cactus: desmosterol, 24(28)-methylenezymosterol, and 9(ll), 24(25)-dehydropollinastanol. 24(28)-Methylene cholesterol was found to be the dominant sterol in the plant. It was used as the major sterol substrate as well as the substrate for the preparation of several synthetic sterols. A key sterol, [24-2H]-24â-methyl cholesterol prepared for the metabolism studies was synthesized by stereoselective isotopic labeling of the sterol side chain. The Ä22-desaturase enzyme from GL7 was found to be stereoselective and regiospecific. For sterol transformation by the Ä22 desaturase, we found that the sterol side chain must be arranged in the staggered conformation at the time of catalysis. In this conformation the 22-Pro-S (bottom) and 23-Pro-S (top) hydrogen atoms may be removed by a syn mechanism to form the trans-22,23-bond; 24p alkyl sterols induce greater Ä22-desaturation than 24a alkyl sterols. The literature reports that other ergosterol-synthesizing fungi Aspergillus fumigatus and Blackeslea trispora introduced hydrogen at C22 and C23 by a 5>7z-mechanism. We interpret the results to imply that sterol specificity of the sterol Ä22-desaturase enzyme in more advancedascomycetous fungi is similar. In contrast to the sterol specificity observed in protein binding, a lack of sterol specificity was found in sterol-controlled growth of GL7.