Molecular Weight and Quaternary Structure of Lactic Dehydrogenase

Abstract

Isoenzymes of lactic dehydrogenase from different sources have been reported to show identical molecular weights between 115,000 and 154,000. As compared to the protomer weight of 36,000 this corresponds to association numbers between 3 and 4 suggesting either dissociation‐association equilibria between the native enzyme and its protomers or systematic errors in the methods applied. To clarify this alternative the monodispersity of the homogeneous heart (H 4 ) and muscle (M 4 ) isoenzymes has been analysed comparing the mean values M̄ from sedimentation, light scattering and osmosis. Considering all correction terms in the evaluations, the results in the total range of enzymic activity clearly establish the monodisperse tetramer as the native unit giving M̄ s,D =M̄ w =M̄ n = 142,000 ± 3,600. No anomalous dependence of the molecular data on protein concentration and temperature have been observed on calculating this figure. Dissociation into subunits requires high net charge or high ionic strength ás well as high concentrations of guanidine. HCI, or urea at high ionic strength. Under all conditions dissociation is accompanied by conformational changes thus restricting a more detailed insight into the native quaternary structure in solution. Regarding the interprotomer binding sites variation of the medium proves Coulombic attractions to prevail in the stabilization of the tetrameric state.