Endocrinology, Vol 120, 457-468, Copyright © 1987 by Endocrine Society
Characterization of an islet carboxypeptidase B involved in prohormone processing
RB Mackin and BD Noe
An islet carboxypeptidase B-like enzyme (CP B) has been identified and
characterized in secretory granules of anglerfish islets. By employing
several different column chromatography methods (gel filtration, ion
exchange, and hydroxylapatite), it was determined that the islet secretory
granules contained only one detectable CP B. This enzyme is present in both
secretory granule- and microsome-enriched subcellular fractions and is
membrane associated at pH 5.2. The specific activity of the islet CP B was
approximately 4-fold higher in the secretory granule- and
microsome-enriched subcellular fractions than in the lysosome-enriched
fraction. It is a metallo-enzyme that is stimulated by Co++, and has a pH
optimum in the range of 5.2-6.2. The isoelectric point of the islet CP B is
at pH 4.9. The enzyme is a glycoprotein and has an approximate molecular
size of Mr 30,000 by gel filtration. The substrate analogs
guanidinoethylmercaptosuccinic acid, guanidinopropylsuccinic acid, and
aminopropylmercaptosuccinic acid competitively inhibited the islet CP B
with inhibition constant (Ki) values of 23, 21, and 230 nM, respectively.
In experiments employing purified prohormone substrates it was demonstrated
that the action of a CP B-like enzyme was required for the complete
processing of anglerfish proinsulin and prosomatostatin-II. These results
indicate that the anglerfish islet CP B is involved in prohormone
processing and has properties which are very similar to those of enkephalin
convertase.
