چكيده لاتين :
Some kinetic properties of NAD+- and NADP+- dependent glucose 6-phosphate dehydrogenase (G6PD) purified from streptomyces aureofaciens were studied. Both NADH and NADPH inhibited the enzyme competitively and noncompetitively, with respect to the corresponding coenzymes and glucose 6-phosphate, respectively. ATP inhibited the NAD+ - linked reaction but not that of the NADP+- linked activity. The inhibition was competitive with respect to NAD+ and noncompetitive with respect to glucose 6-phosphate. Km values were 0.14 mM for NAD+ and 0.075 mM for NADP+. Similar Km values (0.75-0.79 mM) were obtaind for glucose 6-phosphate using either NAD+ or NADP+ as a coenzyme. The optimum pH was 6.6 for NAD+- and 7.4 for NADP+- dependent activity. Maximum protein fluorescence was increased by NAD+ (49%) and NADP+ (8%). Among bivalent cations studied, Cu2+ decreased NAD+- linked activity (40%), but increased the NADP+- linked reaction (10%). Ni2+ did not affect NAD+- linked, but stimulated NADP+- linked activity. Other cations such as Zn2+ and Mn2+ also differently affected the two reactions. The data suggested that binding of NAD+ and NADP+ produces a different conformational change in S. aureofaciens G6PD or an isomerisation process regulates coenzyme utilization.
