Molecular Properties of Lactic Dehydrogenase under the Conditions of the Enzymatic Test

In order to investigate the correlation of enzymatic activity and subunit structure of lactic dehydrogenase from pig heart, the molecular weight, activity and conformation of the enzyme were determined under conditions of the enzymatic test, chosing optimum conditions of the medium regarding subunit dissociation or hybridization and biological function. At concentrations of 10mg/ml to 0.1 μg/ml no dissociation is to be observed. A population analysis on the basis of high‐speed equilibrium ultracentrifugation does not show significant fractions of other species besides the tetramer. At c < 0.1 μg/ml both activity transport and gelfiltration prove dissociation to the dimer to occur which is accompanied by deactivation. The first‐order rate constant of the dissociation reaction is of the order of 0.002 min−1. In the presence of the coenzyme, reassociation and reactivation take place, the binding of NADH leading to a tightening of the quaternary structure (Δscorr= 0.17 S). Nicotinamide which does not influence the activity of the native enzyme inhibits reassociation, while ATP does not show any appreciable effect on the dissociation reaction. The lack of enzymatic activity of the dimer in the presence of nicotinamide and the parallel occurrence of subunit dissociation and deactivation at c < 0.1 μg/ml confirm previous observations implying enzymatic activity to require the intact quaternary structure of the enzyme.