[Interaction of lactate dehydrogenase and the sarcoplasmic reticulum membrane]

The binding of lactate dehydrogenase (LDH) to sarcoplasmic reticulum membranes results in a 60-70% decrease of the enzyme specific activity. This binding occurs both in high (Kd = 1 microM) and low affinity sites. Addition of NADH or NAD+ and a increase of ionic strength lead to the solubilization of the bound enzyme. A similar effect is observed after addition of the fluorescent probes--anilinonaphthalene sulfonate (ANS) and auramine O (A0). The effect of ANS consists predominantly in its binding to the membrane, while that of A0 is due to the probe interaction with the enzyme. At low concentrations of toluidinylnaphthalene sulfonate (TNS) under conditions of predominant binding of the probe to the membrane, the LDH binding to microsomes is enhanced. A rise in the TNS concentration leads to the formation of the probe-LDH complex which interaction with membrane is hampered. The sites of the probes binding to the protein are located outside the enzyme active center but are, nevertheless, sensitive to it states. It is assumed that these sites of the LDH molecule are involved in its interaction with the membrane. The decline of activity of the bound enzyme is interpreted in terms of alterations of the physico-chemical properties of the medium during the enzyme transition from the solution to the perimembrane space.