Membranes of the adrenal medulla: A comparison of membranes of chromaffin granules with those of the endoplasmic reticulum

The protein and lipid composition of the membrane of the chromaffin granule has been compared with that of an endoplasmic reticulum fraction isolated from bovine adrenal medulla. The endoplasmic reticulum was purified by centrifugation of a microsomal fraction in sucrose and Ficoll density gradients. Only insignificant amounts of chromomembrin B, dopamine β-hydroxylase and cytochrome b₅₅₉, which are characteristic protein components of the chromaffin granule membrane, were present in the endoplasmic reticulum fraction; whereas glucose-6-phosphatase and galactosyltransferase, marker enzymes of membranes of endoplasmic reticulum and Golgi complex respectively, were absent from the membranes of chromaffin granules. The lipid composition of the two types of membranes varies in the relative amounts of lysolecithin, lecithin and sphingomyelin and in the molar ratio of cholesterol to phospholipid. Lysolecithin, which is a major phospholipid of the chromaffin granule membrane, is virtually absent from membranes of the endoplasmic reticulum. Different protein patterns were obtained when these two membrane fractions were solubilized with detergent and subjected to polyacrylamide gel electrophoresis.It is concluded that membranes of the endoplasmic reticulum and those of the chromaffin granules are basically different and therefore that the transport of secretory proteins from their site of synthesis to their site of storage occurs, in the chromaffin cell, by a mechanism that does not involve a mixing of the participating membranes.