An enzyme that performs the conversion of anglerfish prosomatostatin-II (pro-SS-II) to anglerfish SS-28 has been identified using an improved two-dimensional electrophoresis procedure. The enzyme is a single chain 39 kDa polypeptide with its isoelectric point at pH 5.9. The converting enzyme has an acidic pH optimum, consistent with the lowered pH of the intracellular site of propeptide conversion. Secretory granule extracts were examined to determine the inhibitor sensitivity and pH optimum of the conversion of anglerfish pro-SS-II to anglerfish SS-28 in this organelle. Production of anglerfish SS-28 by secretory granules was maximal at pH 4.2 and was completely inhibited by the addition of pepstatin. Since pepstatin is a specific inhibitor of aspartyl proteases, these results indicate that the purified enzyme is a member of this enzyme family. This conclusion was supported by the data from partial amino acid sequence analysis. Because these results are consistent with the role of the purified enzyme in the in vivo production of anglerfish SS-28, the identified aspartyl protease has been termed the anglerfish SS-28-generating propeptide-converting enzyme.
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