Affinity cleavage at the divalent metal site of porcine NAD-specific isocitrate dehydrogenase

Abstract

A divalent metal ion, such as Mn²⁺, is required for the catalytic reaction and allosteric regulation of pig heart NAD-dependent isocitrate dehydrogenase. The enzyme is irreversibly inactivated and cleaved by Fe²⁺ in the presence of O₂ and ascorbate at pH 7.0. Mn²⁺ prevents both inactivation and cleavage. Nucleotide ligands, such as NAD, NADPH, and ADP, neither prevent nor promote inactivation or cleavage of the enzyme by Fe²⁺. The NAD-specific isocitrate dehydrogenase is composed of three distinct subunits in the ratio 2α:1β:1γ. The results indicate that the oxidative inactivation and cleavage are specific and involve the 40 kDa α subunit of the enzyme. A pair of major peptides is generated during Fe²⁺ inactivation: 29.5 + 10.5 kDa, as determined by SDS-PAGE. Amino-terminal sequencing reveals that these peptides arise by cleavage of the Val262–His263 bond of the α subunit. No fragments are produced when enzyme is incubated with Fe²⁺ and ascorbate under denaturing conditions in the presence of 6 M urea, indicating that the native structure is required for the specific cleavage. These results suggest that His263 of the α subunit may be a ligand of the divalent metal ion needed for the reaction catalyzed by isocitrate dehydrogenase. Isocitrate enhances the inactivation of enzyme caused by Fe²⁺ in the presence of oxygen, but prevents the cleavage, suggesting that inactivation occurs by a different mechanism when metal ion is bound to the enzyme in the presence of isocitrate: oxidation of cysteine may be responsible for the rapid inactivation in this case. Affinity cleavage caused by Fe²⁺ implicates α as the catalytic subunit of the multisubunit porcine NAD-dependent isocitrate dehydrogenase.