Sulfite oxidase (EC 1.8.3.1) from the plant Arabidopsis thaliana is the smallest eukaryotic molybdenum enzyme consisting of a molybdenum cofactor-binding domain but lacking the heme domain that is known from vertebrate sulfite oxidase. While vertebrate sulfite oxidase is a mitochondrial enzyme with cytochrome c as the physiological electron acceptor, plant sulfite oxidase is localized in peroxisomes and does not react with cytochrome c. Here we describe results that identified oxygen as the terminal electron acceptor for plant sulfite oxidase and hydrogen peroxide as the product of this reaction in addition to sulfate. The latter finding might explain the peroxisomal localization of plant sulfite oxidase. 18 O labeling experiments and the use of catalase provided evidence that plant sulfite oxidase combines its catalytic reaction with a subsequent non-enzymatic step where its reaction product hydrogen peroxide oxidizes another molecule of sulfite. In vitro, for each catalytic cycle plant SO will bring about the oxidation of two molecules of sulfite by one molecule of oxygen. In the plant, sulfite oxidase could be responsible for removing sulfite as a toxic metabolite, which might represent a means to protect the cell against excess of sulfite derived from SO 2 gas in the atmosphere (acid rain) or during the decomposition of sulfur-containing amino acids. Finally we present a model for the metabolic interaction between sulfite and catalase in the peroxisome. Sulfite oxidases (SO)3 from vertebrates (published as EC 1.8.3.1) play an essential role in sulfur detoxification by catalyzing the reaction (1), which is the terminal step in the oxidative degradation of cysteine and methionine. Different electron acceptors were reported to interact with vertebrate SO including cytochrome c, ferricyanide, and oxygen (2-4). In mammals, SO is localized in the intermembrane space of mitochondria (5) where electrons derived from sulfite are passed to cytochrome c, the physiological electron acceptor. Vertebrate SO is a homodimeric protein with monomers subdivided into a Moco domain and a heme domain, as verified by the atomic structure of chicken SO (6).Recently we have described plant SO (7) from Arabidopsis thaliana, which is the fourth molybdenum enzyme present in plants in addition to nitrate reductase, xanthine dehydrogenase, and aldehyde oxidase. Cloning and characterization of plant SO was possible by using sequence homologies to the mammalian counterpart. However, in contrast to the animal enzyme plant SO lacks the heme domain, which is evident from the amino acid sequence, its enzymological and spectral properties (7), and the atomic structure (8). Also its subcellular localization differs from that of animals, in plants we showed SO to be localized in peroxisomes (9). SO is wide spread and highly conserved within the plant kingdom; the SO gene is present in higher and lower plants, and the protein encoded seems to be highly conserved because antibodies directed against Arabidopsis SO detect proteins of the correct size...

SUMMARYHeterotrophic plastids of seeds perform many biosynthetic reactions. Understanding their function in crop plants is crucial for seed production. Physiological functions depend on the uptake of precursors by a range of different metabolite translocators. The 2-oxoglutarate/malate translocator gene (PsOMT), which is highly expressed during pea (Pisum sativum) embryo maturation, has an important role during seed storage. PsOMT functions have been studied by antisense repression in maturing pea embryos, and were found to reduce mRNA levels and transport rates of 2-oxoglutarate and malate by 50-70%. Combined metabolite and transcript profiling revealed that OMT repression affects the conversion of carbohydrates from sucrose into amino acids and proteins, decreases seed weight and delays maturation. OMT-repressed pea embryos have increased levels of organic acids, ammonia, and higher ratios of Asn : Asp and Gln : Glu. Decreased levels of most other amino acids indicate the reduced usage of organic acids and ammonia for amino acid biosynthesis in plastids, possibly caused by substrate limitation of the plastidial glutamine synthetase/glutamine-2-oxoglutarate aminotransferase cycle. Expression of storage proteins is delayed, and mature seeds have reduced protein content. Downregulated gene expression of starch biosynthesis and plastidial glucose-6-phosphate transport in asOMT embryos reveals that decreased 2-oxoglutarate/malate transport capacity affects other pathways of central carbon metabolism. Gene expression analysis related to plastid physiology revealed that OMT repression delays differentiation of storage plastids, thereby maintaining gene expression associated with green chloroplasts. We conclude that OMT is important for protein-storing crop seeds, and is necessary for amino acid biosynthesis in pea seeds. In addition, carbon supply as mediated by OMT controls plastid differentiation during seed maturation.