Peroxisomes as a source of superoxide and hydrogen peroxide in stressed plants.

Superoxide dismutases (SODs) and superoxide radicals in peroxisomes Peroxisomes are subcellular respiratory organelles that contain catalase and H202-producing flavin oxidases as basic enzymatic constituents [1,2]. In recent years, it has become increasingly clear that peroxisomes carry out essential functions in almost all eukaryotic cells [3-51, and an important characteristic of peroxisomes is that they have an essentially oxidative type of metabolism. SODs (EC 1.15.1.1) are a family of metalloenzymes that catalyse the disproportionation of superoxide (O;.) radicals, and they play an important role in protecting cells against the toxic effects of superoxide radicals produced in different cellular loci [6,7]. The presence of SODs in peroxisomes was first demonstrated in plant tissues [8-lo]. These metalloenzymes were found in peroxisomes from pea leaves [8,lO], watermelon cotyledons [9], carnation petals [ 113 , cucumber, cotton and sunflower cotyledons [ 121 and castor bean endosperm [13]. Very recently, the occurrence of SOD in peroxisomes from watermelon cotyledons has been confirmed by immunocytochemical methods (L. M. Sandalio, E. L6pez-Huertas and L. A. del Rio, unpublished work), and SOD has also been found in peroxisomes in human and animal cells. Human fibroblasts and hepatoma cells have been found to contain Cu-Zn-SOD in peroxisomes [ 14,151, and the occurrence of Cu-Zn-SOD has also been demonstrated in rat liver peroxisomes [16,17]. The type of SOD present in peroxisomes varies depending on the tissues origin. In peroxisomes from watermelon cotyledons, two SOD isoenzymes were detected: a Cu-Zn-SOD in the organelle matrix and a manganese-containing SOD bound to the external side of the peroxisomal membrane [18]. In peroxisomes from carnation flowers a Mn-SOD and an Fe-SOD have been found [l l] . Peroxisomes from pea leaves contain only a Mn-SOD, which is located in the peroxisomal matrix [8,10,18]. Recently, two peroxisomal SODs have been purified and charac-