Peroxisomes and reactive oxygen species, a lasting challenge

Oxidases generating and enzymes scavenging H2O2 predestine peroxisomes (PO) to a pivotal organelle in oxygen metabolism. Catalase, the classical marker enzyme of PO, exhibits both catalatic and peroxidatic activity. The latter is responsible for the staining with 3,3'-diamino-benzidine, which greatly facilitated the visualization of the organelle and promoted further studies on PO. d-Amino acid oxidase catalyzes with strict stereospecificity the oxidative deamination of d-amino acids. The oxidase is significantly more active in the kidney than in liver and more in periportal than pericentral rat hepatocytes. Peroxisomes in these tissues differ in their enzyme activity and protein concentration not only in adjacent cells but even within the same one. Moreover, the enzyme appears preferentially concentrated in the central region of the peroxisomal matrix compartment. Urate oxidase, a cuproprotein catalyzing the oxidation of urate to allantoin, is confined to the peroxisomal core, yet is lacking in human PO. Recent experiments revealed that cores in rat hepatocytes appear in close association with the peroxisomal membrane releasing H2O2 generated by urate oxidase to the surrounding cytoplasma. Xanthine oxidase is exclusively located to cores, oxidizes xanthine thereby generating H2O2 and O-2 (-) radicals. The latter are converted to O-2 and H2O2 by CuZn superoxide dismutase, which has been shown recently to be a bona fide peroxisomal protein.