As well as the known sporidesmolides I, II and III, produces sporidesmolides or similar compounds which contain residues of -isoleucine and α-hydroxyisocaproic acid. When -L-isoleucine was added to the growth medium, synthesis of sporidesmolides I and III was inhibited and synthesis of isoleucine-containing sporidesmolides, mainly Sporidesmolide II, was promoted. -L-isoleucine in the medium was poorly utilized, but its presence resulted in production of a very complex mixture of sporidesmolides containing a higher proportion of isoleucine residues than the control. Epimerization of -D-isoleucine occurred in the medium. Neither epimer was well utilized, and this isomer had little effect on sporidesmolide production or composition; it promoted some increase in the synthesis of isoleucine-containing sporidesmolides. -D-isoleucine had effects qualitatively similar to but less pronounced than those of -L-isoleucine, probably because of extracellular epimerization. The use of epimeric pairs of amino acids as tools for investigating biochemical processes involving inversion of configuration at an amino acid α-carbon atom is suggested.


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