Purification and Properties of Hypoxanthine Phosphoribosyltransferase from Streptomyces cyanogenus

Abstract

Hypoxanthine phosphoribosyltransferase (EC 2.4.2.8) of a strain of Streptomyces cyanogenus was purified 1,900-fold to an apparent homogenity from cell-free extracts. The enzyme had a molecular weight of 150,000 and consisted of eight identical subunits with a molecular weight of 18,000. The isoelectric point was at pH 4.4. The enzyme required Mg²⁺ or Ma²⁺ for activity and had a pH optimum at 8.5. Hypoxanthine and guanine were good substrates for the enzyme. Xanthine was a very poor substrate and adenine was not a substrate. Apparent Km values of the enzyme for hypoxanthine, guanine and 5-phosphoribose-1-pyro-phosphate were 1.6 × 10⁻⁸, 2.7 × 10⁻⁶ and 6.3 × 10⁻⁵ m, respectively. All purine nucleotides tested inhibited the activity significantly, apparently by competing with 5-phosphoribose-1-pyrophosphate.