Summary
Estrogens in high doses have been shown to inhibit, in vitro, the thrombin-neutralizing
action of antithrombin III (AT III). In this study we investigate the effect of estrogens
on AT III in greater detail. To increase the sensitivity of measurement of AT III
activity in the absence of heparin, we have developed an assay system utilizing human
platelets, AT III and thrombin. The two proteins derived from human plasma were prepared
in high purity. Platelet aggregation was induced by approximately 0.02 NIH U of thrombin.
AT III was added in amounts that suppressed 95% of the aggregation-inducing effect
of thrombin. Estrogens blocked the thrombin-neutralizing effect of AT III in dose-dependent
manner. This effect was shown to be specific for AT III. Neither aggregability of
platelets nor aggregating effect of thrombin were affected by the steroid hormone.
Evidence for binding of estrogen to AT III was obtained from changes in intrinsic
fluorescence of AT III. Activity of AT III was also reduced in increasing order of
effectiveness by cholesterol, cortisone, testosterone and progesterone. Our studies
suggest a direct effect of estrogens and other steroids on AT III, altering its specific
neutralization of thrombin.
Key words
Antithrombin III - Estrogen - Platelet aggregation - Thrombin - Crossed immunoelectrophoresis
