Effects of calcium and limited proteolysis on membrane-bound and releasable dopamine beta-hydroxylase in adrenomedullary catecholamine granules.

Bovine chromaffin granules were shown to contain two potent proteolytic systems resulting in limited proteolysis of granule proteins at pH 6.0 in the cold in the presence of inhibitors of serine and thiol proteases. Calcium, whether added during lysis or remaining bound to the lysate protein during dialysis in non-chelating solutions, enhanced recoveries of total immunoreactive dopamine beta-hydroxylase (14% of total protein) and soluble enzyme (9% of lysate protein) due to degradation of of chromogranins. A pepstatin A-blockable, catepsin D-like proteolytic system converting membrane-bound enzyme to its soluble counterpart at pH 6.0 was detected in the granule membrane fraction.