Leucyl-β-naphthylamide-splitting enzymes in the mammalian endocrine pancreas

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Abstract
The proteolytic capacity of the endocrine pancreas in obese–hyperglycaemic mice was evaluated by using the chromogenic substrate l-leucyl-β-naphthylamide (LNA). The analytical sensitivity obtained with this substrate in photometric and fluorimetric assays permitted quantitative determinations of C–N-bond-splitting activity in both crude islet homogenates and electrophoretic fractions thereof. The following observations were made: (1) The rate of LNA cleavage was maximal at about pH7 in islets as well as in acinar tissue. An apparent Km of 4×10−5–6×10−5m was calculated for both the endocrine and exocrine pancreas. (2) The level of LNA-splitting enzyme activity was of the same magnitude in the islets as in the liver, and significantly higher in the islets than in the exocrine pancreas. Starving the animals for 7 days did not affect the enzyme activity levels. (3) Two distinct LNA-splitting enzymes could be separated from the pancreatic islets by means of disc electrophoresis, the most rapidly migrating band representing the highest activity. Though similar electrophoresis patterns were obtained with acinar tissue and liver, only one enzyme could be demonstrated in serum. The data suggest that the β-cells, in addition to being highly specialized for the production of a specific protein, contain a comparatively high capacity for protein catabolism.