Glucose 6-Phosphate Dehydrogenase Isozymes of Maize Leaves

Abstract

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,

Lignification related enzymes in Picea abies suspension cultures

“…To determine the degree of contamination of the cell wall protein fractions with cytoplasmic proteins, glucose 6-phosphate dehydrogenase activity was assayed as described by Valenti et al (1984). e of NADPH as 6220 M ª1 cm ª1 was used.…”

Isoenzymes of Glucose 6-Phosphate Dehydrogenase from the Plant Fraction of Soybean Nodules

“…Negative cooperativity with glucose-6-P has been reported for G6PD from sweet potato (13), black gram (2), as well as for the enzyme from animal and microbial sources (13). Close similarities in the physical and kinetic properties of isoenzymes of G6PD, such as observed for G6PD I and II from the plant fraction of soybean nodules, have also been noted for the multiple forms of the enzyme in pea and maize leaves (17,22). G6PD I and II had regulatory properties which could be important in the fine control of the pentose phosphate pathway in the plant fraction ofsoybean nodules.…”

“…The concentrations of NADP+ and glucose-6-P at which soybean nodule G6PD I and II were half-maximally active were comparable to those for the enzyme from other plant sources. In general, G6PD from plants has been reported to have Michaelis-Menten kinetics when NADP+ is the varied substrate (5,15,22). However, the dimeric form of G6PD from human erythrocytes displayed sigmoidal kinetics with NADP+ at high concentrations of glucose-6-P (1).…”

“…G6PD from other plants and animals is also inactivated rapidly in the absence of NADP+ (5,13,18). NADP+ has been shown to promote aggregation of G6PD from sweet potato (16), maize leaves (22), and various animal and microbial sources (13): these enzymes are converted from dimers with apparent mol wt of 110,000 into tetramers. In contrast, the chloroplast and cytoplasmic forms of G6PD from pea leaves have an apparent mol wt of 240,000, which is not affected by NADP+ (5,18).…”

“…In plants, where G6PD occurs in the cytoplasmic and plastid compartments, the enzyme has been studied most extensively from leaves and regulation of its activity has been considered mainly in relation to photosynthetic metabolism (5,11,12,17,18,22). In comparison, there have been relatively few studies ofG6PD from nonphotosynthetic tissues (2, 15).…”