Nicotinamide mononucleotide pyrophosphorylase activity in animal tissues.

An enzyme which catalyzes the formation of nicotinamide mononucleotide in the presence of nicotinamide, S-phosphoribosylpyrophosphate, adenosine triphosphate, and Mgff has been purified from rat liver. The reaction is highly specific for ATP and is not substituted for by a variety of nucleotides. Enzymic activity has been found in all of the tissues studied, and in the crude homogenate it is apparently in an inhibited state. The inhibitor is removed by protamine sulfate. A broad activity peak is observed between pH 7.5 and 9.5 with maximal activity around pH 8. The enzymic activity is stable at these pH values, but it is acid labile. The apparent Km values for nicotinamide, PP-ribose-P, and ATP are 2.62 pM, 3.57 X 1O-5 M, and 3.83 X 10m4 M, respectively. The Vmax was calculated to be 5.4 mpmoles per mg of protein per hour.