Interpretation of the kinetics of consecutive enzyme-catalyzed reactions. Studies on the arginase-urease system.

Physiocochemical properties of beef liver arginase are reported, particular attention being given to its state of aggregation in the concentration range encountered in enzymic assays. It is shown that a species of molecular weight 114,000 is the operational kinetic unit. Evidence is also provided that arginase does not associate heterogeneously with urease, and therefore, in the absence of macromolecular interactions, the arginase-urease couple provides a suitable experimental system to test the applicability of theory previously developed to guide the interpretation of coupled assay results. Application of the theory led to values of the Michaelis constant and maximal velocity describing the first reaction in the sequence, catalyzed by arginase, which agreed within experimental error with the corresponding values obtained by studying the arginase-catalyzed reaction alone. Comment is also made on the product inhibition of arginase by ornithine, which must be considered in the comparison of experimental results describing the time course of a coupled assay with theoretical solutions obtained by numerical integration.