Microcalorimetric Studies on the Product Inhibition of Hydrolysis of L-arginine Catalyzed by Bovine Liver Arginase

Abstract

A new thermokinetic reduced extent method for the product inhibition of single substrate enzyme-catalyzed reactions is proposed and compared with the traditional initial rate method in this paper. The arginase-catalyzed hydrolysis of L-arginine to L-ornithine and urea was studied at 37°C in 40 mM sodium barbiturate-HCl buffer solution (pH=9.4). Michaelis constant (K m ) for arginine and maximum velocity (V m ) of the reaction were determined by initial method and thermokinetic method. The activation of exogenous manganese to this reaction was also studied. The product inhibition constant (K p ), which cannot be obtained directly from the initial rate method, was determined by thermokinetic without adding L-ornithine to the reaction system. When the concentration of Mn 2+ in cell is 0.1 mM, the enzyme gets its full activity. Incubation arginase with appropriate concentration of Mn 2+ resulted in increased V max and a higher sensitivity of the enzyme to product with no change in the K m for arginine. We suggest that the exogenous manganese ions in solution have just recovered the activity of arginase, which was lost in dissolving and dilution, but no effect on the mechanism of the reaction.