Subcellular localization and preliminary characterization of islet hormone-degrading activities in anglerfish islet tissue

The degradation of [¹²⁵I]iodoinsulin in anglerfish islet tissue was studied in a trichloroacetic acid solubilization assay system. The pH optima for insulin breakdown by acidic and neutral enzymes were determined in fish islets and compared with mammalian tissues (rat liver, pancreas, and islets of Langerhans). Two major insulinolytic activities of anglerfish islet tissue were partially characterized: 1) An acidic (pH 3.5) activity showing marked sensitivity to pepstatin, some sensitivity to antipain, leupeptin, phenylmethanesulfonyl fluoride (PMSF), and thiol proteinase inhibitors, but no inhibition by EDTA; and 2) a neutral (pH 7.3) activity showing marked sensitivity to thiol proteinase inhibitors, sensitivity to antipain and leupeptin, but no sensitivity to PMSF, EDTA, or pepstatin. Glucagonolytic activities were also observed at the same acidic and neutral pH optima. Following cell fractionation of anglerfish islet homogenates, acidic (pH 3.5) insulinolytic activities were distributed with the lysosome-rich, microsome, and cytosol fractions, whereas neutral (pH 7.3) activities were found chiefly in cytosol and microsomes. Little or no insulinolysis was observed in secretory granule fractions. The data suggest that insulin is degraded in islet tissue by at least two enzyme systems. Lysosomal insulinolysis was due principally to cathepsin D-like activity. Neutral insulinolysis, partially characterized in the cytosol fraction, was due to thiol proteinase activity. The activity profile indicates that islet tissue resembles other insulin-responsive tissues in its subcellular distribution of insulin-degrading activities. The cellular heterogeneity of islet tissue, and the presence of high concentrations of endogenous islet hormones require further efforts at purification before insulinolytic enzymes are fully characterized in this tissue.