LDL receptors in coated vesicles isolated from bovine adrenal cortex: binding sites unmasked by detergent treatment.

Coated vesicles isolated from bovine adrenal cortex contain specific binding sites that recognize 125I-labeled human low density lipoprotein (LDL). These sites share the properties of the functional LDL receptors previously demonstrated on the surface of adrenal cells and in unfractionated adrenal membranes. Approximately 90% of the LDL receptors of the isolated coated vesicles were initially masked. Binding of 125I-LDL increased 10 fold after the vesicles were disrupted with the detergent octylglucoside. The LDL receptors of intact coated vesicles were also shielded from destruction by pronase; proteolytic destruction occurred only after the vesicles had been disrupted with octylglucoside. The adrenal coated vesicles measured 60 nm in diameter, suggesting that they were derived from the Golgi apparatus. Like the previously studied coated vesicles from brain and other tissues, the coated vesicles from adrenal cortex contained clathrin as the major protein component. In contrast to the coated vesicles of adrenal cortex, however, the brain coated vesicles failed to reveal masked LDL receptor activity when treated with octylglucoside. The current data indicate that isolated coated vesicles from the adrenal cortex contain LDL receptors and that these receptors exist in a masked form, apparently because their binding sites face the interior of the vesicle.