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SUMMARY: Valine did not inhibit the production of diacetyl from pyruvate by extracts of Saccharomyces cerevisiae that contained coenzyme A (CoA) and the co-factors necessary for forming acetyl-CoA. However, it did inhibit the production of diacetyl from glucose by a growing culture, though Ca-pantothenate, a precursor of CoA, was supplied. α-Ketoisovaleric acid enhanced the production of diacetyl from pyruvate and acetyl-CoA by extracts if mercaptoethylamine was present. Results indicate that α-ketoisovaleric acid has two roles in the synthesis of diacetyl by S. cerevisiae, a synthesis that generally requires an acidic environment, and that each is inhibited by excess valine. In addition to serving as an intermediate in the synthesis of acetyl-CoA, α-ketoisovaleric acid, which was not formed from valine by transamination at pH below 6·0, enhanced the activity of diacetyl synthetase.
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