Demonstration of altered antithrombin III activity due to nonenzymatic glycosylation at glucose concentration expected to be encountered in severely diabetic patients.

The effect of nonenzymatic glycosylation on the kinetics and structure-function relationships of antithrombin III were investigated at normal physiologic concentrations of antithrombin III and glucose, which are 5.2 microM and 5 mM, respectively. The results were compared with antithrombin III incubated at the glucose concentration expected to be found in severely diabetic patients (15 mM). Antithrombin III incubated at 5 mM lost 33% of the heparin cofactor activity after 7 days, whereas antithrombin III incubated at 15 mM lost 50% for the same period. Under both conditions, half of the heparin cofactor activity was lost after 15 days. When D-[U-14C]glucose was used as tracer, approximately 0.6 mol glucose/mol protein was incorporated after 10 days at both concentrations of glucose. A detailed evaluation of the kinetics of inhibition of thrombin by glycosylated antithrombin III revealed that the second-order rate constant is three times smaller than that of normal antithrombin III. On the basis of these data, it is concluded that glycosylated antithrombin III with 50% depressed heparin cofactor activity is three times weaker than normal antithrombin III as an inhibitor of thrombin. The implications of these observations with respect to the possible pathogenesis of thrombosis in diabetes are discussed.