Gastric proteases of the Greenland cod Gadus ogac. II. Structural properties

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Abstract
Three gastric proteases were isolated from the stomach mucosa of the Greenland cod (Gadus ogac). The cod proteases were all less stable to heating and protese I retained less activity at 5.degree. C when the pH was greater than 5 in comparison with porcine pepsin. The activities of cod proteases 1 and 2, with hemoglobin as the substrate, were doubled in the presence of 25 mM NaCl, while cod protease 3 and porcine pepsin were not stimulated by the salt. The cod proteases did not cross-react with antibodies raised against porcine pepsin. However, some cross-reactivity was noted with antibodies raised against proteases from psychrotrophic pseudomonads. The molecular weights of all the cod proteases were in the range of 36 000-38000. The amino acid compositions of the cod proteases as compared by the Metzger difference index differed from the mammalian gastric proteases by about the same extent that pepsin, gastricisin, and chymosin differ from each other. Of the cod enzymes, protease I differed from mammalian gastric proteases, while cod proteases 3 was more like chymosin with respect to amino acid composition. Cod protease I had the lowest hydrophobicity index and chymosin had the highest. The hydrophobicity indices of cod proteases 2 and 3 were intermediate between that of porcine pepsin and bovine chymosin. It is suggested that the Greenland cod proteases represent less differentiated forms of gastric proteases than the mammalian pepsins, gastricsins, and chymosins.