The cytosolic and chloroplastic isoforms of glucose-6-phosphate dehydrogenase (G6PDH) were separated and purified from barley leaves (Hordeum vulgare L.). In etiolated leaves, only the cytosolic isoform was expressed. The molecular mass of the cytosolic enzyme, G6PDH1, was 112±8 kDa and that of the chloroplast enzyme, G6PDH2, was 136±7 kDa. The Km values for glucose-6-phosphate and NADP were 0.133 and 0.041 mM for G6PDH1, and 0.275 and 0.062 mM for G6PDH2, respectively. The pH optimum was 8.2 for G6PDH1 and 7.8 for G6PDH2. The enzyme is absolutely specific for NADP. NADPH is a competitive inhibitor of the G6PDH1 in respect to glucose-6-phosphate (G6P) and NADP (Ki = 0.050 and 0.025 mM, respectively). NADPH is a competitive inhibitor of the G6PDH2 in respect to NADP (Ki = 0.010 mM), but a non-competitive inhibitor in respect to the G6P. ADP, AMP, UTP, NAD, and NADH had no effect on the activity of G6PDH. ATP inhibited the G6PDH2 activity. and A. Semenihina ... [et al.].