A dehydrogenase-mediated recycling system of NADPH in plant peroxisomes

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Abstract
The presence of the two NADP-dependent dehydrogenases of the pentose phosphate pathway has been investigated in plant peroxisomes from pea (Pisum sativum L.) leaves. Both enzymes, glucose-6-phosphate dehydrogenase (G6PDH; EC 1.1.1.49) and 6-phosphogluconate dehydrogenase (6PGDH; EC 1.1.1.44), were present in the matrix of leaf peroxisomes, and their kinetic properties were studied. G6PDH and 6PGDH showed a typical Michaelis-Menten kinetic saturation curve, and had specific activities of 12.4 and 29.6 mU/mg protein, respectively. The Km values of G6PDH and 6PGDH for glucose 6-phosphate and for 6-phosphogluconate were 107.3 and 10.2 μM, respectively. Dithiothreitol did not inhibit G6PDH activity. By isoelectric focusing of peroxisomal matrices, the G6PDH activity was resolved into three isoforms with isoelectric points of 5.55, 5.30 and 4.85. The isoelectric point of peroxisomal 6PGDH was 5.10. Immunoblot analyses of peroxisomal matrix with an antibody against yeast G6PDH revealed a single cross-reactive band of 56 kDa. Post-embedment, EM immunogold labelling of G6PDH confirmed that this enzyme was localized in the peroxisomal matrices, the thylakoid membrane and matrix of chloroplasts, and the cytosol. The presence of the two oxidative enzymes of the pentose phosphate pathway in plant peroxisomes implies that these organelles have the capacity to reduce NADP+ to NADPH for its re-utilization in the peroxisomal metabolism. NADPH is particularly required for the ascorbate-glutathione cycle, which has been recently demonstrated in plant peroxisomes [Jiménez, Hernández, del Río and Sevilla (1997) Plant Physiol. 114, 275-284] and represents an important antioxidant protection system against H2O2 generated in peroxisomes.