Evidence for cooperative effects in human liver arginase.

The reaction kinetics of human liver arginase (L-arginine amidinohydrolase, EC 3.5.3.1) in terms of arginine concentration is strikingly altered by varying the pH. Lowering the pH from the optimum (9.5) toward a more physiological value (7.5) there is a transition from hyperbolic to sigmoidal kinetics. The cooperative effects are observed in the presence and absence of the product ornithine. Dimers of arginase exhibit typical Michaelis-Menten kinetics even in the presence of ornithine. Dimer-dimer interactions are suggested to explain the kinetic properties of arginase at pH 7.5.