Sep 7, 1976

Influence of substrates and coenzymes on the role of manganous ion in reactions catalyzed by pig heart triphosphopyridine nucleotide-dependent isocitrate dehydrogenase

R S Ehrlich, Roberta F Colman


The interaction of manganous ions with pig heart triphosphopyridine nucleotide (TPN) specific isocitrate dehydrogenase has been studied by kinetic experiments and by direct ultrafiltration measurements of manganous ion binding. At low metal ion concentrations, a lag is observed in the time-dependent production of reduced triphosphopyridine nucleotide (TPNH) that can be eliminated by adding 20 muM TPNH to the initial reaction mixture. A plot of 1/upsilon vs. 1/ (Mn2+) obtained at relatively high TPNH concentrations (20 muM) is linear and yields of Km value of 2 muM for metal ion, which is comparable to the direct binding constant measured in the presence of isocitrate. A similar plot at low TPNH concentrations (2 muM) reveals a biphasic relationship: at high metal concentrations the points are collinear with those obtained at high levels of TPNH, but at low metal concentrations that line is characterized by a Km of 19 muM for Mn2+. A difference in the deuterium oxide solvent isotope effect on Vmax observed with 20 muM TPNH as compared with 2 muM TPNH suggests that at high TPNH concentrations or high manganous ion concentrations the rate-limiting step is the dehydrogenation of isocitrate, while at low manganous ion concentrations...Continue Reading

Mentioned in this Paper

Isocitrate Dehydrogenase-I
Family suidae
Enzyme Activation

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