Identification of D-threo-alpha-methylisocitrate as stereochemically specific substrate for bovine heart aconitase and inhibitor of TPN-linked isocitrate dehydrogenase

Abstract

DL-threo-alpha-Methylisocitrate (3-hydroxy-1,2,3-butanetricarboxylate) is a substrate for bovine heart aconitase and an inhibitor of TPN-linked isocitrate dehydrogenase from liver and heart. The isomer of alpha-methylisocitrate formed from alpha-methyl-cis-aconitate (cis-2-butane-1,2,3-tricarboxylate) by aconitase inhibits TPN-linked isocitrate dehydrogenase and has been identified as D-threo-alpha-methylisocitrate (2S,3R)-3-hydroxy-1,2,3-butanetricarboxylate) by optical rotation and circular dichroism studies. Mitochondrial bovine heart aconitase catalyzes a reversible reaction between D-threo-alpha-methylisocitrate (Km, 0.2 mM) and alpha-methyl-cis-aconitate (Km, 0.05 mM) at pH 7.4. However, formation of methylcitrate (2-hydroxy-1,2,3-butanetricarboxylate) from these substrates or utilization of synthetic methylcitrate for formation of these products could not be demonstrated with bovine heart aconitase. DL-threo-alpha-Methylisocitrate is also a substrate for aconitase from rat liver cytosol (Km, 0.1 mM); Vmax with citrate is approximately 1.4 times that with DL-threo-alpha-methylisocitrate. The ratio of activities for these substrates observed with the bovine heart enzyme is about 5. Formation of alpha-methyl-cis-aconitate from synthetic methylcitrate could not be detected spectrophotometrically with the liver aconitase; if it occurs with either the liver or the heart enzyme, the rate would be less than 0.1% that obtained with DL-threo-alpha-methylisocitrate. A new synthesis of methylcitric acid in good yields from diethyl alpha-methyl-beta-ketoglutarate (diethyl 2-methyl-3-oxoglutarate) and cyanide has been described. NMR spectroscopy indicates that this synthetic methylcitric acid contains the two racemic pairs of diastereoisomers.