CHLOROQUINE AUGMENTS THE BINDING OF INSULIN TO ITS RECEPTOR

The effect of chloroquine on the interaction of insulin with its receptor has been investigated under both equilibrium and non-equilibrium conditions. Chloroquine was found to augment insulin binding in a pH-dependent manner between pH 6.0 and pH 8.5, with the maximum occurring at approximately pH 7.0. Analysis of the equilibrium binding data in terms of independent binding sites gave equivocal results but suggested an increase in the high-affinity component. Analysis using the negative co-operativity binding model of De Meyts, Bianco and Roth [J. Biol. Chem. (1976) 251, 1877-1888] suggested that the affinity at both high and low occupancy was increased equally. The kinetics of association of insulin with the plasma-membrane receptor indicated that, although the net rate of association increased in the presence of chloroquine, this was due to a reduction in the dissociation rate rather than an increase in the association rate. This was confirmed by direct measurement of the rates of dissociation. Dissociation was found to be distinctly biphasic, with fast and slow components. Curve fitting suggested that the decrease in dissociation rate in the presence of chloroquine was not due to a decrease in either of the two dissociation rate constants, but rather to an increase in the amount of insulin dissociating by the slow component. It was also found that the increase in dissociation rate in the presence of excess insulin, ascribed to negative co-operativity, could be accounted for by an increase in the amount of insulin dissociating by the faster pathway, rather than by an increase in the dissociation rate constant. Thus chloroquine appears to have the opposite effect to excess insulin, and evidence was found for the induction of positive co-operativity in the insulin-receptor interaction at high chloroquine concentrations. Evidence was also found for the presence of low-affinity chloroquine binding sites with binding parameters similar to the concentration dependence of the chloroquine-induced augmentation of insulin binding.