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Nicotinamide Mononucleotide Pyrophosphorylase Activity in Animal Tissues

From the ¹ From the Department of Biochemistry, University of Miami, School of Medicine, Miami 36, Florida

An enzyme which catalyzes the formation of nicotinamide mononucleotide in the presence of nicotinamide, 5-phosphoribosylpyrophosphate, adenosine triphosphate, and Mg⁺⁺ 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 K_m values for nicotinamide, PP-ribose-P, and ATP are 2.62 µm, 3.57 x 10^-5 m, and 3.83 x 10^-4 m, respectively. The V_max was calculated to be 5.4 mµmoles per mg of protein per hour.

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