Leaf peroxisomes

Fedtke, C. (1983). Leaf peroxisomes deaminate as-triazinone herbicides Method of detoxification by tolerant plants. Naturwissenschaften, 70 199-200. 

Yamaguchi, K., Hori, H., and Nishimura, M., 1995, A novel isoenzyme of ascorbate peroxidase localized on glyoxysomal and leaf peroxisomal membranes in pumpkin. Plant Cell Physiol. 36 1157nll62. 

The enzymes utilizing serine as amino donor and glyoxylate as amino acceptor are considered together here. Much attention has been paid to these aminotransferases, with the role of these enzymes in photorespiration being a particular focus of interest (see Keys, this volume. Chapter 9). Initial studies on impure extracts indicated the presence of glyoxylate aminotransferases which could use glutamate, aspartate, alanine, and serine as amino donors. The best donor to glyoxylate varied from one tissue to another (Cossins and Sinha, 1965). Leaf peroxisomes were subsequently shown to... 

Density gradient fractionation techniques some time ago demonstrated the presence of glutamateiglyoxylate aminotransferase in leaf peroxisomes (Kisaki and Tolbert, 1%9). Similar methods subsequently confirmed the presence of peroxisome-localized serinerpyruvate and aspartate aminotransferases in spinach leaves (Yamazaki and Tolbert, 1970). The same study demonstrated aspartate aminotransferase in both chloroplasts and mitochondria. A microbody location for aspartate aminotransferase was also observed by Cooper and Beevers (1969), who purified castor bean glyoxy-somes on density gradients. 

Yamazaki,R.K., Tolbert, N.E. Malate dehydrogenase in leaf peroxisomes. Biochem. Bio-phys. Acta 178,11-20(1969)... 

For Method see (10,13). By using hydroxypyruvate reductase as marker enzyme, the activity obtained with the isolated peroxisomes was related to the chlorophyll content in a spinach leaf. 

In the leaf cells of C3 plants there are at least five main compartments separated by membranes through which protons cannot freely pass. These are the thylakoid space and the stroma of the chloroplasts, the stroma of mitochondria and peroxisomes and the cytosol. As a result, achieving the correct pH in the separate compartments must be exerted metabolically, by the diffusion of gases such as carbon dioxide or by special pumps transporting species such as the bicarbonate ion. In the steady state, it is necessary that biochemical reactions producing alkalinity or acidity be compensated by some means, if pH is not to change. 

Fig. 3. Primary carbon metabolism in a photosynthetic C3 leaf. An abbreviated depiction of foliar C02 uptake, chloroplastic light-reactions, chloroplastic carbon fixation (Calvin cycle), chloroplastic starch synthesis, cytosolic sucrose synthesis, cytosolic glycolysis, mitochondrial citric acid cycle, and mitochondrial electron transport. The photorespiration cycle spans reactions localized in the chloroplast, the peroxisome, and the mitochondria. Stacked green ovals (chloroplast) represent thylakoid membranes. Dashed arrows near figure top represent the C02 diffusion path from the atmosphere (Ca), into the leaf intercellular airspace (Ci), and into the stroma of the chloroplast (Cc).SoHd black arrows represent biochemical reactions. Enzyme names and some substrates and biochemical steps have been omitted for simplicity. The dotted line in the mitochondria represents the electron transport pathway. Energy equivalent intermediates (e.g., ADP, UTP, inorganic phosphate Pi) and reducing equivalents (e.g., NADPH, FADH2, NADH) are labeled in red. Membrane transporters Aqp (CO2 conducting aquaporins) and TPT (triose phosphate transporter) are labeled in italics. Mitochondrial irmer-membrane electron transport and proton transport proteins are labeled in small case italics.

Enzyme activities of nonreversible NADP-glyceraldehyde 3-phosphate dehydrogenase and NADP-isocitrate dehydrogenase as marker for the cytosol, hydroxypyruvate reductase for peroxisomes and cytochrom c reductase for mitochondria were 0.3 to 2% compared to those of the total leaf extract. 

During the operation of glycolate oxidase in normal photorespiration, hydrogen peroxide is formed and is removed by peroxisome-based catalase. The potential toxic action of hydrogen peroxide has already been discussed, and the herbicidal action of aminotriazole is assumed to be, in part, due to catalase inhibition.Mutants of barley that are catalase deficient are rapidly bleached in normal air, in which photorespiration may operate, but will grow normally under C02-enriched air. " It is of interest that in this barley mutant and also in aminotriazole-treated plants, there was an enhancement in the level of leaf glutathione, probably due to increased pressure upon radical-scavenging systems. 

In contrast, the glyoxysomes from germinating seeds are capable of the complete breakdown of fatty acids to acetyl-CoA. They also integrate this metabolism with the operation of the glyoxylate cycle which allows plants (in contrast to animals) to synthesize sugars from acetyl-CoA. Leaf tissues also contain peroxisomes and recent work indicates that )8-oxidation in leaves is confined to peroxisomes with no detectable activity in mitochondria. 

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