Effect of Phospholipases and Lysophosphatides on Partially Purified Steroid Hormone Receptors

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Abstract
Treatment with phospholipase A2 of crude or partially purified preparations of the glucocorticoid receptor of rat liver resulted in an inactivation of the receptor, which could not be attributed to contaminating proteases. Similar enzymatic treatment of the progesterone receptor of rabbit uterus did not affect its steroid-binding activity. At various stages during purification the preparations of glucocorticoid receptor contained 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. Mixing experiments showed that in the presence of the glucocorticoid receptor, phospholipase A2 also inactivated the progesterone receptor. Both partially purified receptors were inactivated by similar concentrations of added lysophosphatides but were not affected by incubation with phospholipase C, which did not produce ionic detergents. The effects of phospholipase A2 and of added lysophosphatides could be overcome by an excess of bovine serum albumin, indicating that free lysophosphatides were involved in receptor inactivation, possibly due to their strong detergent properties.