Evidence for the role of carboxyl groups in activity of endopolygalacturonase of Aspergillus niger. Chemical modification by carbodiimide reagent

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Abstract
Endopolygalacturonase (E.C. 3.2.1.15) of Aspergillus niger was modified with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide and glycine ethyl ester. The modification resulted in total irreversible inactivation of the enzyme and derivatization of carboxyl acid residues and tyrosine residues. The treatment of the modified enzyme with hydroxylamine led to a restoration of modified tyrosine residues but not to reactivation of the enzyme. The inactivation with carbodiimide was pH dependent, the rate of inactivation increased with decreasing pH. Tri(D-galactosiduronic acid), a competitive inhibitor, or crosslinked pectic acid protected the enzyme against the inactivation. In bioaffinity chromatography of partially inactivated endopolygalacturonase, all residual enzyme activity was retained on the adsorbent while all inactive fraction passed without retardation through the column. On the basis of these results, as well as proximity of the rate constants for enzyme inactivation and the carboxyl group modification it is suggested that the loss of endopolygalacturonase activity is due to the modification of carboxylic acid residues and that at least one is essential for enzyme activity.

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