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

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Abstract
A convenient method for the enzymic 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. The enzymically formed product, m.p. [melting point] 99-100.degree. C, .**GRAPHIC**. = -16.4 (water), is related to the known stereochemistry of the Senecio alkaloid jacobine and to a l-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. A 3-dimensional X-ray structure analysis was performed and the structure refined to an R value of 0.049. The asymmetric unit contains 3 independent threo dimethylmalic acid molecules. The anomalous dispersion effects of C and O were used to determine the absolute configuration. These measurements yielded a (2R,3S) configuration. (2R,3S)-2,3-dimethylmalate is probably the substrate of the lyase. Previously isolated racemic 2,3-dimethylmalic acids, m.p. 143.degree. C and m.p. 104-106.degree. C, may represent the erythro and threo pair, respectively.

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