Slow structural changes shown by the 3-nitrotyrosine-237 residue in pig heart [Tyr(3NO2)237] lactate dehydrogenase.

1. The pKa of the phenolic hydroxy group of the Tyr(3NO2)-237 residue in pig heart [Tyr(3NO2)237]lactate dehydrogenase is 7.2 in the apoenzyme, 7.4 in the enzyme-NADH complex and 7.8 in the enzyme-NADH-oxamate complex. The alkaline shift from apoenzyme to ternary complex is ascribed to the approach of the Glu-107 residue during the movement of the polypeptide loop residues 98-110. 2. The affinities of the nitrated enzyme for NADH and for oxamate (in the presence of NADH) are slightly less than those of the native enzyme. The turnover number for the nitrated enzyme in the pyruvate-to-lactate direction is about 0.75 of the value for the native enzyme. 3. Temperature-jump relaxation experiments of the enzyme saturated with NADH but fractionally saturated with oxamate are interpreted to show that the pKa of the nitrotyrosine residue responds to a protein rearrangement after oxamate binds to the binary enzyme-NADH complex. 4. Transient-kinetic experiments show the environment of the Tyr(3NO2)-237 residue in the enzyme-NADH-pyruvate complex of the steady state to be similar to that in the enzyme-NADH-oxamate inhibitor complex.