Differential distribution of carboxypeptidase-processing enzyme activity and immunoreactivity in membrane and soluble components of chromaffin granules.

A comparison of carboxypeptidase-processing enzyme activity and immunoreactivity showed that they were differentially distributed in soluble and membrane components of bovine adrenal medulla chromaffin granules. The majority of enzyme activity (80% of total activity in the granules) was present in the soluble fraction, but the number of enzyme molecules was distributed equally between the soluble and membrane fractions. When equivalent amounts of carboxypeptidase enzyme (ng of immunoreactivity) in each fraction were compared, the carboxypeptidase in the soluble component appeared to be five to six times more active than the membrane-bound form of the enzyme. The soluble and membrane components of the granules may represent populations of enzyme at different states of activation. This finding could have important implications for the regulation of the carboxypeptidase-processing enzyme.