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1) A convenient method for the enzymatic preparation of a chemically and optically pure isomer of 2,3-dimethylmalic acid in g-amounts is described. Propionate, pyruvate and partially purified 2,3-dimethylmalate lyase (from Clostridium barkeri) were applied. 2) The enzymically formed product, m.p. 99--100 degrees C, [alpha]D20 = -16.4 (water), is related to the known stereochemistry of the Senecio alkaloid jacobine and to a laevorotatory 2,3-dimethylmalic acid derived from jaconecic acid, a degradation product of the alkaloid. From this relationship it appears likely that the substrate of the lyase is a component of the threo racemate and is of (2R,3S) configuration. 3) A three-dimensional X-ray structure analysis was performed and the structure refined to an R value of 0.049. The asymmetric unit contains three independent threo dimethylmalic acid molecules. The anomalous dispersion effects of carbon and oxygen were used to determine the absolute configuration. These measurements yielded a (2R,3S) configuration. 4) We conclude from these results that (2R,3S)-2,3-dimethylmalate is the substrate of the lyase. The results also establish that previously isolated racemic 2,3-dimethylmalic acids, m.p. 143 degrees C and m.p. 104--106 degrees C, represent the erythro and threo pair, respectively.

Nicotinic Acid Metabolism. Enzymic Preparation and Absolute Configuration of the Substrate for 2,3-Dimethylmalate Lyase