Phosphate promotes glycation of antithrombin III which interferes with heparin binding

Nonenzymatic glycation of antithrombin III has been reported to cause the reduction of heparin-catalyzed thrombin-inhibiting activity in diabetes. The effect of in vitro nonenzymatic glycation of pure antithrombin III on heparin binding and heparin-potentiated activity under a variety of buffers and pH values was studied to further clarify the physiological significance of this reaction. The extent of glycation, measured by the fructosamine assay and [14C]glucose binding, was enhanced by the presence of phosphate ion (pH 7.45, 8.5 and 9.5) and increased linearly with increasing phosphate ion concentration from 0.01 to 0.2 M phosphate. Conversely, the heparin-catalyzed antithrombin activity decreased from 93.1% of controls for 0.01 M phosphate to 73.5% for 0.2 M phosphate as the extent of glycation increased. The increase in intrinsic fluorescence induced by binding of heparin to antithrombin III was also moderated by glycation of antithrombin III in a dose-dependent manner with a negative correlation coefficient of -0.94. Direct measurement of the heparin binding by affinity chromatography showed a decrease in the heparin-binding fraction which correlated with the degree of glycation and the decrease in heparin-catalyzed activity. These studies suggest that nonenzymatic glycation may be responsible for the reduction in antithrombin III activity observed in some diabetics.