STUDIES ON MECHANISM OF RAT LIVER NICOTINAMIDE MONONUCLEOTIDE PYROPHOSPHORYLASE

Preparations of rat liver have been shown to possess a nicotinamide mononucleotide: pyrophosphate phosphoribosyl transferase (EC 2.4.2.12) which specifically requiresadenosine triphosphate, as well as 5-phosphoribosyl 1-pyrophosphate. These same preparations will synthesize nicotinamide mononucleotide in the absence of adenosine triphosphate if the concentration of both 5-phosphoribosyl 1-pyrophosphate and magnesium is greatly increased. Similar heat labilities, identical gel electrophoretic patterns, and molecular weights indicate that nicotinamide mononucleotide synthesis stimulated by adenosine triphosphate or high levels of 5-phosphoribosyl 1-pyrophosphate and magnesium is probably catalyzed by the same enzyme. These data, together with an analysis of the kinetics of cotinamide mononucleotide formation in the presence and in the absence of adenosine triphosphate, lead to the suggestion that adenosine triphosphate is a positive allosteric effector of nicotinamide mononucleotide pyrophosphorylase. Analysis of the product inhibition data for nicotinamide mononucleotide synthesis in the presence of adenosine triphosphate and initial velocity patterns of the reaction in the absence of adenosine triphosphate indicate that the mechanism for the enzymatic formation of nicotinamide mononucleotide is either an ordered sequence beginning with the binding of 5-phosphoribosyl 1-pyrophosphate and culminating in the release of nicotinamide mononucleotide, or an Iso-Theorell-Chance mechanism, in which case the order of substrate binding cannot be predicted from the data. © 1969, American Chemical Society. All rights reserved.