Uracil phosphoribosyltransferase from Acholeplasma laidlawii: Partial purification and kinetic properties

Uracil phosphoribosyltransferase was purified 34-fold from sonicated extracts of Acholeplasma laidlawii by ammonium sulfate precipitation, binding to DEAE-Sephadex, Sephadex G-200 chromatography, and hydroxylapatite chromatography. The molecular weight of the enzyme by gel filtration was approximately 80,000. The pH optimum for phosphoribosylation was around 7.5, and the optimum MgCl₂ concentration was 5 mM. Initial velocity studies were conducted over a wide range of both uracil and 5-phosphoribosyl-1-pyrophosphate (P-Rib-PP) concentrations, and various equations for biomolecular reaction mechanisms were fitted to the data by nonlinear regression. When the equation for an ordered sequential mechanism was fitted to the data, the K(ia) thus obtained was not statistically different from zero. This is interpreted as evidence for a nonsequential ('ping-pong') reaction. Graphic analysis of the data by the Hanes-Woolf linear transform supported this conclusion. The enzyme has a high affinity for uracil (K(mUra) = 4.2 μM; K(mP.Rib.PP) = 66 μM), which provides supporting evidence that this activity is responsible for the incorporation of uracil and uridine into nucleotides.