Abstract. Leaves of 10 plant species, 7 with photorespiration (spinach, sunflower, tobacco, pea, wheat, bean, and Swiss chard) and 3 without photorespiration (corn, sugarcane, and pigweed), were surveyed for peroxisomes. The distribution pattern for glycolate oxidase, glyoxylate reductase, catalase, and part of the malate dehydrogenase indicated that these enzymes exist together in this organelle. The peroxisomes were isolated at the interface between layers of 1.8 to 2.3 M sucrose by isopycnic nonlinear sucrose density gradient centrifugation or in 1.95 M sucrose on a linear gradient. Chloroplasts, located by chlorophyll, and mitochondria by cytochrome c oxidase, were in 1.3 to 1.8 M sucrose.In leaf homogenates from the first 7 species with photorespiration, glycolate oxidase activity ran,ged from 0.5 to 1.5 ,rmoles X min-' X g71 wet weight or a specific activity of 0.02 to 0.05 Atmole X min-' X mgV protein. Glyoxylate reductase activity was oomparable with glycolate oxidase. Catalase activity in the homogenates ranged from 4000 to 12,000 /moles X min-' X g-' wet weight or 90 to 300 ,umoles X min'1 X mg7 protein. Specific activities of malate dehydrogenase and cytochrome oxidase are also reported. In contrast, homogenates of corn and sugarcane leaves, without photorespiration, had 2 to 5 t% as much glycolate oxidase, glyoxylate reductase, and catalase activity. These amounts of activity, though lower than in plants with photorespiration, are, nevertheless, substantial. Peroxisomes were detected in leaf homogenates of all plants tested; however, significant yields were obtained only from the first 5 species mentioned above. From spinach and sunflower leaves, a maximum of about 50 %o of the marker enzyme activities was found to be in these microbodies after homogenization. The specific activity for peroxisomal glycolate oxidase and glyoxylate reductase was about 1 gmole X min-1 X mg-' protein; for catalase, 8000 tmoles X min-' X mg-1 protein,and for malate dehydrogenase, 40 ,mol.es X min-' X mg-' protein. Only small to trace amounts of marker enzymes for leaf peroxisomes were recovered on the sucrose gradients from the last 5 species of plants. Bean leaves, with photorespiration, had large amounts of these enzymes (0.57 umole of glycolate oxidase X min-' X g-1 tissue) in the soluble fraction, but only traces of activity in the peroxisomal fraction. Low peroxisome recovery from certain plants was attributed to particle fragility or loss of protein as well as to small numbers of particles in such plants as corn and sugarcane.Homogenates of pigweed leaves (no photorespiration) contained from one-third to one-half the activity of the glycolate pathway enzymes as found in comparable preparations from spinach leaves which exhibit photorespiration. However, only traces of peroxisomal enzymes were separated by sucrose gradient centrifugation of particles from pigweed. Data from pigweed on the absence of photorespiration yet abundance of enzymes associated with glycolate metabolism is inconsistent with current hypotheses ...
Abstract. Chloroplasts, mitochondria, and peroxisomes from leaves were separated by isopycnic sucrcse density gradient centrifugation. The peroxisomes converted glycolate-14C or glyoxylate-14C to glycine, and contained a glutamate: glyoxylate aminotransferase as indicated by an investigation of substrate specificity. The pH optimum for the aminotransferase was between 7.0 and 7.5, and the Km for L-glutamate was 3.6 mN' and for glyoxylate, 4.4 mM. The reactioni of glutamate plus glyoxylate was not reversible. The isolated peroxisomes did not convert glycine to glyoxylate nor glycine to serine. (19,20). Thus, the peroxisome should be associated with photoresp)iration since this process is thought to be the result of the rapid metabolismni of glycolate as it is formlled during photosynthesis (8,14,28 2 On leave from the Central Research Institute, Japan Monopoly Corporation, 1-28-3, Nishishinagawa, Shinagawa-ku, Tokyo, Japan. 242 Materials and MethodsPlanit lIatterial. The leaves tise(I in these experimenlts wN-ere similar to those employed durinig a survey of plants for peroxisomes (20). In most cases, the peroxisomlle preparations were nmade from spinach leaves which wA-ere obtained from local grocery stores.Swiss chard, tobacco, bean. sunflower. pigweed, and corn were groxvn in the greenhouise. Fresh sugarcane leaves were shipped in ice by air express from Florida.Labeled Sz'bstratcs. For experimentet iinvolviin1g '4CO, release, the substrates wvere useld without carrier. In the trainsamninase assay, the labeled substrates were diluted 1000-fold witlh unlabeled carrier so that the specific radioactivity of the labeled sulbstrate ; were the following: 2.45 >Y 10 cpnm/niole for glycolate-1-14C, 1.45 X 10_ cpni/pmole for glyoxvlate-1,2-14C. 1.54 X 101 cpm/tmiole fol-glycine-2-'IC. 3.51 X 10n cp)m/mole for DL-glutamnate-2-14C, 3.78 X 10-2 cpm/pmole for aspartate-3-1 4C, and 6.6.3 X 1(); cpnm/imole for wt_-seriine-1 14C.
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