Purification and SomeProperties of Nicotinate Phosphoribosyltransferase from Hog Liver

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Purification of nicotinate phosphoribosyltransferase (NPRTase, EC 2.4.2. 1 1) from hog liver and some of its properties were investigated. Purified enzyme was found to be homogeneous by the criterion of polyacrylamide gel disc electrophoresis and to have a molecular weight of about 120, 000. The subunit molecular weight was found to be 64, 000, using SDS polyacrylamide gel disc electrophoresis. The optimum pH and isoelectric point of the enzyme were 7.3-7.4 and 4.8, respectively. Mn²⁺, Co²⁺, and Mg²⁺were effective in meeting the NPRTaserequirement for a divalent cation. ATPhad a stimulative effect on the enzymeactivity in the presence of Mg²⁺, on the other hand, in the presence of Mn²⁺, ATPhad an inhibitory effect on the enzyme activity. Of the purine nucleotides examined, only ATPstimulated NPRTase activity, and GTPdid not. On the contrary, all other nucleotides inhibited the activity. Nicotinate mononucleotide, which is a reaction product, inhibited NPRTaseactivity. Amongthe nicotinic acid analogues tested, only pyrazine-2- carboxylic acid inhibited the activity. Nicotinamide, quinolinic acid, adenine, and hypoxanthine were not phosphoribosylated by NPRTase. Incubation of NPRTase with SH-modifying reagents caused loss of NPRTaseactivity.