Endopeptidase of the brush border membrane of rat enterocyte. Separation from aminopeptidase and partial characterization.

The brush border of the enterocytes of the rat was isolated by the method of differential centrifugation with CaCl2 according to Schmitz. This material was solubilized with papain, trypsin and Triton X-100. The greatest amount of membrane enzymes was released to the supernatant (105 000 X g) with the use of Triton X-100. The tritonized supernatant was treated in the next step by papain, bromelain, ficin and trypsin (individually or in combinations). After simultaneous proteolysis with papain and bromelain a partial separation of the aminopeptidase from the endopeptidase by Sephadex G-200 chromatography was observed. These two enzyme activities were distinctly separated by isoelectric focusing at pH 4--6. Two enzymatically active bands (RF 0.13 and 0.24) in the aminopeptidase fraction and one single active band (RF 0.16) in the endopeptidase fraction using polyacrylamide gel electrophoresis were found. Co-migrating proteins to all of these activities were detected. Endopeptidase activity splits 3-carboxypropionyltrialanin-4-nitroanilide (SucAla3NAp) in the position P2-P1. Liberated aminoacyl-NAP may be further split to generate chromogenic 4-nitroaniline through aminopeptidase activity. Endopeptidase of the brush border of the rat enterocytes is characterized by the following properties: 1) molecular mass 130000 +/- 15 000 dalton; 2) Km value (substrate: SucAla3NAp) 1.1 X 10(-3) M; 3) pI 5.23; 4) ph optimum 8.5; 5) 50% activity remains after 15 min of preincubation at 50 degrees C; 6) activity is strongly inhibited by EDTA, p-chloromercuribenzoate, Mn2 and Co2.