Document Type: Regular Paper
Department of Clinical Biochemistry, Isfahan University of Medical Sciences, Isfahan, I. R. of Iran
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.