Studies on glucosaminidase. 5. Kidney N-acetyl-β-glucosaminidase and N-acetyl-β-galactosaminidase

Publisher Website
Google Scholar
Abstract
Two N-acetylhexosaminidase activities of extracts of rat kidney have been measured with the substrates p-nitrophenyl N-acetylglucos-aminide and p-nitrophenyl N-acetylgalactosaminide. No separation of the activities was achieved by fractionation with ammonium sulphate or with calcium phosphate gel or by partial heat inactivation. The Ki values of 6 competing substances, N-acetylglucosamine, 2-acetamido-2-deoxygluconolactone, phenyl N-acetylglucosaminide and the corresponding galactosamine derivatives were not the same when measured against p-nitrophenyl N-acetylglucosaminide and N-acetylgalactosaminide respectively. There was no evidence of partially competitive inhibition. Two schemes are suggested for the enzyme mechanism. The first is of 2 independent sites for the 2 activities on the same enzyme protein, together with inhibition of N-acetylglucosaminidase by N-acetylgalactosaminides and inhibition of N-acetylgalactosamini-dase by N-acetylglucosaminides. The 2d is of one site with competition, together with a direct displacement mechanism of one substrate by another with special assumptions for the velocity constants for the displacement. Further experimental evidence of a relationship between Michaelis and inhibitor constants of 2 competing substrates supports the 2d scheme.