Trimeric Purine Nucleoside Phosphorylase from Chicken Liver Having a Proteolytic Nick on Each Subunit and Its Kinetic Properties

Abstract Chicken liver purine nucleoside phosphorylase, a trimer of molecular weight 90,000, is assumed to contain subunits of two different molecular weights (Murakami, K., and Tsushima, K. (1976) Biochim. Biophys. Acta 453, 205-210). The enzyme was purified by a new method including affinity chromatography. The purified enzyme had a molecular weight of 90,000, as before, but showed only a single band, of molecular weight 24,000, on sodium dodecyl sulfate gel electrophoresis. This band corresponded to the small subunit observed in enzyme purified by the conventional method. For enzyme purified by the new method, a quaternary structure consisting of three identical subunits, each of which had a single proteolytic nick at a definite peptide bond close to the COOH-terminal, was proposed. A Lineweaver-Burk plot with inosine as substrate was linear for this enzyme preparation (Km = 0.04 mM) in contrast to concave downward curvature with two apparent Km values of 0.04 and 0.2 mM for the enzyme purified by the conventional method.