Membrane dopamine beta-hydroxylase: a precursor for the soluble enzyme in the bovine adrenal medulla

Searching for endogenous proteolytic activities converting the membrane form of dopamine beta-hydroxylase (dopamine beta-monooxygenase, DBH) into the soluble and releasable form, DBH was monitored enzymatically and immunologically in aqueous and detergent-solubilized extracts of the adrenomedullary fractions. Degradation of the soluble DBH and acidic chromogranins by activation of endogenous proteases occurred during lysis in H2O. Shifts in the hydrophobicity of the membrane DBH were also apparent. Loss in enzyme protein or activity was, on the other hand, not observed for buffer-dialysed CG (pH 5-6). Limited proteolysis within the membrane phase was, however, indicated by the shift towards dominance of the intermediate hydrophobic DBH in the buffer-dialysed CG. By two-dimensional, crossed immunoelectrophoresis with cationic detergent the microsomal DBH was immunologically identical to the granule-bound enzyme but differed from the latter in molecular heterogeneity and in susceptibility to proteolytic solubilization by endogenous protease activities. DBH in the membranes of the chromaffin granules was proteolytically solubilized at pH 6-8 and the soluble DBH further degraded at pH 5. The results indicate that a post-translational conversion of the amphiphilic DBH into the soluble form, initiated at the level of the microsomes, may continue within the light and the heavy granule fractions which contain several DBH-converting and degrading proteolytic activities with acid optima.