Effect of phospholipases and lysophosphatides on partially purified steroid hormone receptors

Treatment with phospholipase A2 of crude or partially purified preparations of the glucocorticoid receptor of rat liver results in an inactivation of the receptor, which cannot be attributed to contaminating proteases. Similar enzymatic treatment of the progesterone receptor of rabbit uterus does not affect its steroid-binding activity. At various stages during purification the preparations of glucocorticoid receptor contain 10 to 50-fold higher concentrations of lipid phosphate than the corresponding preparations of progesterone receptor, suggesting that the effect of phospholipase A2 on the hepatic receptor could be mediated by lysophosphatides produced during hydrolysis of endogeneous phospholipids. In fact, mixing experiments show that in the presence of the glucocorticoid receptor, phospholipase A2 also inactivated the progesterone receptor. Both partially purified receptors are inactivated by similar concentrations of added lysophosphatides but are not affected by incubation with phospholipase C, which does not produce ionic detergents. In addition, the effects of phospholipase A2 and of added lysophosphatides can be overcome by an excess of bovine serum albumin, indicating that free lysophosphatides are involved in receptor inactivation, possibly due to their strong detergent properties.