Enkephalin convertase (EC; carboxypeptidase-E or -H) is a carboxypeptidase-B-like enzyme proposed to be involved in the synthesis of enkephalins and other neuropeptides. In this study we have characterized and localized EC in the rat heart and examined its correspondence with atrial natriuretic factor. Heart homogenates bind [3H]guanidinoethylmercaptosuccinic acid ([3H]GEMSA), a selective ligand for enkephalin convertase, with specificity and high affinity (Kd = 5–10 nM). The pharmacology of these sites matches that of convertase catalytic activity and of [3H]GEMSA binding in other tissues. The binding sites in the heart can be purified 4000-fold using paminobenzoylarginine affinity chromatography, and the purified sites have Co2+ stimulated carboxypeptidase activity identical to EC. By subcellular fractionation studies [3H]GEMSA-binding sites are localized to the granule fraction with ANF immunoreactivity. [3H]GEMSA autoradiography localizes EC in the heart to the left and right atria, with very low levels in mature ventricles. In neonatal rats, the enzyme is found in both atria and ventricles. The distribution, biochemical properties, and developmental course of EC suggest that it may be involved in the synthesis of atrial natriuretic factor.
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