L’hypoxanthine-guanine phosphoribosyltransférase des pousses de Topinambour Helianthus tuberosus L.

Abstract

Extracts from Jerusalem artichoke shoots exhibited hypoxanthine-guaninephosphoribosyltransferase activity. Only partial purification by DEAE-Sephacel chromatography was performed considering the instability of the enzyme. The HGPR Transferase showed an absolute requirement for divalent cations: the greatest activity was found with Mn++. The optimal pH for the enzyme activity was 7,5 to 8,5, in contrast to the optimal pH of the APR Transferase of the same species which was 5,5 to 6,5. The Km values for hypoxanthine, guanine and PRPP, at the optimal pH, were 6.1 x 10-6 M, 5.5 x 10-6 M and 8.8 x 10-5 M respectively, but the rate of phosphoribosylation of hypoxanthine was greater than that of guanine at the tested concentration. Competitive inhibition of the phosphoribosylation of hypoxanthine by guanine and inversely are consistent with the fact that only one enzyme catalyses the reaction. The product inhibition by IMP and GMP were competitive towards PRPP (Ki = 5.7 x 10-5 M and 6.3 x 10-6 M respectively) and uncompetitive towards hypoxanthine (Ki = 4.1 x 10-4 M and 5.3 x 10-5 M respectively). GMP was much more inhibitory than IMP; GDP and GTP were also inhibitory. The properties of HGPR Transferase of Helianthus tuberosus are discussed in relation to its physiological role.