Enzymatic Evidence for the Configuration of d-Isocitric Acid

Abstract

THE configuration of the α-carbon atom of the d-isocitric acid of the Krebs tricarboxylic acid cycle was designated L_S on the basis of an application of the Lovene displacement rule¹, but subsequent chemical^2–4 and X-ray⁵ studies indicated the D_S configuration. Other investigations have shown that the α- and β-asymmetric centres of d-isocitric acid possess opposite configurations^6,7. Although d-isocitric acid may be distinguished by the action of isocitric dehydrogenase from the three isomeric isocitric acids, none of the isomers of α-aminotricarballylic acid, the α-amino analogue of isocitric acid, has thus far been found to be attacked by enzymes. Greenstein and Winitz¹, who prepared both DL-α-aminotricarballylic and DL-allo-α-aminotricarballylic acids, were unable to find an enzyme capable of resolving these and therefore resorted to the use of brucine. They thus obtained the corresponding four optical isomers, which they converted to the respective isocitric acids by treatment with HNO₂.