Ultrafiltration studies of steroid-protein binding.

Abstract

SUMMARY A method is described for determination of the extent of protein binding of steroids by ultrafiltration after addition of radioactively-labelled steroid tracers of high specific activity. Results obtained with specifically labelled 14C-steroids did not differ significantly from those with steroids randomly labelled with tritium. A number of steroids have been studied with this technique. The 'S' shaped curve obtained with some corticosteroids by plotting percentage ultrafilterable against steroid added to the plasma confirmed the presence of the corticosteroid-binding protein which has greater affinity for steroid than albumin. This method of plotting the ultrafiltration data afforded a method of assessing the amount of binding protein (or the number of sites). Cortisone, 17-hydroxy-11-deoxycorticosterone, cortisol, and Δ1 cortisol showed this type of curve. Binding of aldosterone, progesterone and 17-ketosteroids occurred to the same extent in plasma as in 5% albumin. The ultrafilterable fraction of these steroids in plasma was constant over a wide range of total steroid concentration. Binding of testosterone was greater with plasma than with albumin, and did not decrease upon addition of large amounts of testosterone.