Chromogranin A-processing proteinases in purified chromaffin granules: contaminants or endogenous enzymes?

It was the purpose of this study to define the chromogranin A-processing proteinases present in highly purified preparations of bovine chromaffin granules. The most active enzyme had a pH optimum of 5.0 and was inhibited by pepstatin. It could be identified immunologically as a cathepsin D-like enzyme and subcellular fractionation established its lysosomal origin. After removal of this enzyme the remaining activity at pH 5.0 was mainly due to a cathepsin B-like proteinase. The presence of this enzyme could also be attributed to lysosomal contamination. In the presence of calcium, a further proteolytic activity became apparent at pH 5.0. This enzyme which was inhibited by p-chloromercuriphenylsulfonic acid was localized in chromaffin granules. A trypsin-like peptidase, most active at pH 8.2, was enriched in a membrane wash of chromaffin granules. Subcellular fractionation indicated that this enzyme is preferentially bound to the membranes of very dense particles probably representing a subpopulation of chromaffin granules. This study establishes that the most active chromogranin A-degrading proteinases present in highly purified chromaffin granules are attributable to lysosomal contamination. Two enzymes with low activity (a Ca²⁺ activated proteinase and a trypsin-like enzyme) are, apparently, true constituents of chromaffin granules.