Two different forms of glucose 6-phosphate dehydrogenase (EC 1.1.1.49) have been purified from etiolated and green leaves, respectively, of 6-day maize ( Glucose 6-phosphate dehydrogenase of dark-grown maize leaves isoelectric point (pI) 43 is replaced by a form with pI 4.9 during greening.The isozymes show some differences in their kinetic properties, K. of NADP+ being 2.5-fold higher for pI 43 form. Free ATP (K. = 0.64 millimolar) and ADP (K,,= 1.13 millimolar) act as competitive inhibitors with respect to NADPF in pI 4.3 isozyme, and both behave as less effective inhibitors with pI 4.9 isozyme. Magnesium ions abolish the inhibition.Glucose 6-P dehydrogenase in photosynthetic cells of most higher plants occurs as two distinct isozymes, localized in cytoplasm and chloroplasts, respectively, and with comparable activities (1,3,20). The plastid isozyme seems to resemble the cytoplasmic form in several general properties, including mol wt, kinetic properties, and control by certain effectors (1,11,14,20). A single isozyme has, however, been reported for C4 species of the 'malate formers' group (10). It will be shown in this paper that the isozyme of etiolated maize leaves is replaced by a different form during greening, in a way reminiscent of the substitution pattern of isozymes of NADP-dependent malic enzyme (18). The two glucose 6-P dehydrogenase isozymes show minor but distinctive kinetic differences. Subunit structure and association behavior of the purified enzyme are also described, and their resemblances to enzyme from animals and fungi are apparent. MATERIALS AND METHODSMaize plants (Zea mays L. cv Fronica) were germinated as described previously (18). Leaves and coleoptiles of6-d seedlings,