Activated oxygen‐mediated metabolic functions of leaf peroxisomes

Peroxisomes are subcellular organelles with an essentially oxidative type of metabolism. The presence in these organelles of superoxide dismutases and the generation of superoxide radicals (O2•−) was first demonstrated in plant tissues and in recent years different experimental evidence has suggested the existence of cellular functions related to activated oxygen species. Some of these functions are analyzed in this work. In purified intact peroxisomes from pea (Pisum sativum L.) leaves, xanthine oxidase and urate oxidase were found to be present. The occurrence and the level of the metabolites xanthine, hypoxanthine, uric acid, and allantoin were studied in extracts of pea leaf peroxisomes by HPLC. Xanthine, uric acid, and allantoin were detected in peroxisomes. These results suggest a cellular role for leaf peroxisomes in the catabolism of purines. In peroxisomal membranes, 3 polypeptides (PMPs) with molecular masses of 18, 29 and 32 kDa, respectively, have been shown to generate superoxide radicals. These PMPs were purified from pea leaf peroxisomal membranes and characterized. While the 18‐ and 32‐kDa PMPs use NADH as electron donor for O2•− production, the 29‐kDa PMP was clearly dependent on NADPH. Very recently, the occurrence in pea leaf peroxisomes of all the enzymes of the ascorbate‐glutathione cycle has been demonstrated. NADPH is required for the glutathione reductase activity of the cycle and this implies the reduction of NADP+ to NADPH. This recycling function could be carried out by the NADP‐dependent glucose‐6‐phosphate dehydrogenase (G6PDH), 6‐phosphogluconate dehydrogenase (6PGDH), and isocitrate dehydrogenase (ICDH). These 3 dehydrogenases have been demonstrated to be present in the matrix of pea leaf peroxisomes. The catabolism of purines, the superoxide‐generating PMPs, the ascorbate‐glutathione cycle, and the dehydrogenase‐mediated recycling of NADPH, are activated oxygen roles of leaf peroxisomes that add to other functions previously known for peroxisomes from eukaryotic cells.