Abstract
Abstract Nicotinate phosphoribosyltransferase from human erythrocytes has been purified approximately 30,000-fold. The purified enzyme migrates as a single protein band on disc electrophoresis at pH 7.9 and 9.5. The protein forms an inactive precipitate in salt concentrations below 50 mm. Addition of 25% glycerol protects against this precipitation. ATP stimulates the enzymatic reaction but is not an obligatory substrate. When present, ATP is hydrolyzed to ADP and Pi. This hydrolysis is stoichiometric with nicotinate mononucleotide formation. In the presence of saturating concentrations of ATP, the apparent Km values for nicotinate and 5-phosphoribosyl 1-pyrophosphate are decreased from 24 to 0.5 µm and 100 to 5 µm, respectively. The enzyme is not an ATPase in the absence of any of the other substrates. Its molecular weight is 86,000 by gel filtration in the absence or presence of 1.0 mm MgATP.
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