Purification and Properties of a Human Nicotinamide Ribonucleoside Kinase

Nicotinamide ribonucleoside kinase (NRK) phosphorylates at least two nucleoside analogs of potential clinical interest, tiazofurin and 3-deazaguanosine. In this study NRK has been purified to near homogeneity from human placenta. The purification procedure consists of several chromatographic steps including salt precipitation, DE-52 chromatography, sucrose density gradient fractionation, hydroxylapatite chromatography, and anion exchange FPLC. The final enzyme preparation is homogeneous as judged by a single silver-stainable band on both nondenaturing and denaturing polyacrylamide gels. The molecular weight of the enzyme, as estimated by sodium dodecyl sulfate–polyacrylamide gel electrophoresis and gel filtration on Superdex 75 HR 10/30, is approximately 29 and 32 kDa, respectively. The isoelectric pH for NRK is 5.6. The reaction requires ATP. The pH optimum is in the region 6.5–9.0. NRK in the purified preparations, with added bovine serum albumin, was stable for days at 4°C and for months at −70°C. The enzyme is very unstable at low protein concentration. NRK phosphorylated several substrates including nicotinamide ribonucleoside, guanosine, tiazofurin, and 3-deazaguanosine with apparentKmvalues of 9.6, 115, 90, and 16.5 μM, respectively.