Substrate activity of structural analogs of isocitrate for isocitrate dehydrogenases from bovine heart

D-Garcinia acid (D-threo-1,2-dihydroxy-1,2,3-propanetricarboxylate), like D-isocitrate, has an alpha-DS-hydroxyl group and a beta-LS configuration of the second carboxyl group. The maximal velocity of pyridine nucleotide reduction with D-garcinia acid is 8 and 21% of D-threo-isocitrate with the DPN-linked and TPN-linked isocitrate dehydrogenase from bovine heart, respectively. The other stereoisomers of hydroxycitrate [L-garcinia acid, D- and L-hibiscus acid (D- and L-erythro-1,2-dihydroxy-1,2,3-propanetricarboxylate)] are inactive. DL-threo-Homoisocitrate (DL-threo-1-hydroxy-1,2,4-butanetricarboxylate) supports DPN+ reduction at 10-15% of the rate observed for isocitrate with the DPN-specific enzyme, but is not a substrate for TPN-linked isocitrate dehydrogenase. The values of apparent S0.5 for total isocitrate and total garcinia acid are similar with both enzymes; the apparent S0.5 of total homoisocitrate is two- to threefold higher than that of total isocitrate with the DPN-linked enzyme. Enzymatic oxidative decarboxylation of garcinia acid and homoisocitrate leads to formation of alpha-keto-beta-hydroxyglutarate and alpha-ketoadipate, respectively. DL-Methylmalate (DL-1-hydroxy-2-methylsuccinate) is inactive as a substrate for either dehydrogenase as are the newly synthesized compounds: DL-threo-gamma-isocitrate amide (DL-threo-1-hydroxy-3-carbamy01,2-propanedicarboxylate), beta-methyl-DL-isocitrate (DL-1-hydroxy-2-methyl-1,2,3-propanetricarboxylate), beta-methyl-DL-garcinia acid (DL-threo-1-hydroxyl-2-methoxy-1,2,3-propanetricarboxylate), DL-1-hydroxyl-1,2,2-ethanetricarboxylate, and DL-1,4-dihydroxy-1,2-butanedicarboxylate.