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Enzymatic Formation of cis-Homoaconitic Acid, an Intermediate in Lysine Biosynthesis in Yeast

From the ¹ From the Department of Microbiology, Research Laboratories, Albert Einstein Medical Center, Philadelphia, Pennsylvania 19141

The enzymatic dehydration of homoisocitric acid (-hydroxy-ß-carboxyadipic acid) to the ,ß-unsaturated tricarboxylic acid, homoaconitic acid, has been shown spectrophotometrically by increase in absorbance at 240 mµ. The product was isolated by ether extraction and column chromatography and was identified by comparing its melting point, infrared spectrum, and R_f values on paper chromatograms with synthetic cis-homoaconitic acid. The enzyme product was also characterized by catalytic reduction to the saturated derivative, 1,2,4-butanetricarboxylic acid (homotricarballylic acid).

The cis structure of the enzymatically formed homoaconitic acid was established by its quantitative conversion into the anhydride under mild conditions and its transformation into the more stable trans isomer on heating in acidic solution. In addition, as expected for a cis isomer, the enzyme product and the corresponding synthetic isomer had a lower melting point, lower R_f values in paper chromatograms, and a lower absorptivity in ultraviolet light when compared with the trans isomer.

The cis isomer of synthetic homoaconitic acid reacted in the presence of yeast extract to yield homoisocitrate. The trans isomer was unreactive. The enzyme, which has been named cis-homoaconitase, was separated from cis-aconitase by ammonium sulfate precipitation, has an optimum pH of 8.6 in phosphate buffer, and is inhibited by p-chloromercuribenzoate, ,'-dipyridyl, and o-phenanthroline.

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