STABILIZING EFFECT OF SUCROSE AGAINST IRREVERSIBLE DENATURATION OF RABBIT MUSCLE LACTATE-DEHYDROGENASE

Measurements of the kinetics of activity loss by rabbit muscle lactate dehydrogenase in acetate-chloride buffer, pH 5.0, I 0.20, have shown that the enzyme exhibits greater stability against irreversible inacti vation when the buffer is supplemented with sucrose (0.1 M-0.5 M). On the basis of sedimentation equilibrium distributions obtained for enzy me in the absence and presence of sucrose (0.5 M), the lactate dehydro genase is essentially dimeric in both environments. The observed stabi lization of enzyme activity has therefore been considered in terms of the space-filling effects of sucrose on an isomerization equilibrium b etween native and unfolded forms of dimeric lactate dehydrogenase, whi ch precedes irreversible inactivation of the unfolded isomer. Interpre tation of the kinetic results on that basis has led to the conclusion that the initial stage of enzyme unfolding entails a minor change in v olume and/or asymmetry of the lactate dehydrogenase that gives rise to a 4% increase in the second virial coefficient describing excluded vo lume interactions between dimeric enzyme and sucrose.