Akiho Yokota

Different Metabolic Fate of two Carbons of Glycolate in Euglena gracilis Z

Properties of glycolate metabolism and photorespiration of Euglena are quite different from those of higher C3 plants (Yokota, Kitaoka (1982), Kitaoka et al. (1983), Yokota et al. (1983b)). Glycolate synthesized in chloroplasts is oxidized to glyoxylate by glycolate dehydrogenase in mitochondria (Yokota et al. (1978a)). Euglena mitochondria also contain glutamate:glyoxylate aminotransferase. However, this aminotransferase does not have a high ability enough to guarantee transamination of all the glyoxylate produced. Consequently, up to 75% of glyoxylate formed in mitochondria stays as glyoxylate (Yokota et al. (1983b)). Continued flow of carbon through the glycolate pathway therefore requires that glyoxylate must be metabolized outside mitochondria. Euglena chloroplasts contain a high activity of Mn2+-dependent NADPH oxidase and form H2O2 (Yokota et al. (1983b)). The rate constant of the reaction of glyoxylate with H2O2 (2.27–19.98 M−1 s−1) and the concentrations of H2O2 in Euglena chloroplasts and of intracellular glyoxylate support the occurrence of the decarboxylation of glyoxylate by reacting with H2O2 (Yokota et al. (1983c)). However, there has been no information on the participation of the decarboxylation in the glycolate metabolism of E. gracilis and on the metabolic fate of hydroxymethyl carbon of glycolate after the decarboxylation. In the present study, we report the properties of glycolate decarboxylation and the subsequent metabolism of both glycolate carbons in E. gracilis.

The in vivo Functioning Forms of Ribulose 1,5-Bisphosphate Carboxylase/Oxygenase in Plants

Ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO) changes its functioning form depending on the concentration of ribulose bisphos-phate (RuBP) in the in vitro assay (Fig. 1) [1,2]. The specific activity of the carboxylase reaction is very different among the three forms. Binding of RuBP to the regulatory sites of RuBisCO has been inferred to cause the change of the form.

Properties of Selenium-Induced Glutathione Peroxidase in Low-CO2-Grown Chlamydomonas reinhardtii

Organisms have catalase, l-ascorbate peroxidase (AsAPOD), and glutathione peroxidase (GSHPOD) capable of scavenging H2O2, one of the toxic forms of O2. Catalase with a low affinity for H2O2 is found only in peroxisomes and cannot decompose the lipid hydroperoxides. Thus, AsA- and GSHPODs detoxify such active oxygens in energy-generating organelles. While both peroxidases have the same function in protection of the cell from oxidative damage, the distribution of the peroxidases is apparently distinct: AsAPOD occurs only in plant tissues, protozooa, and algae, while GSHPOD is only present in animal tissues (1).

Model for the Relationship between Photosynthetic CO2 Assimilation and Glycolate Synthesis in Chlamydomonas reinhardtii

Ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO) acts at the branching point of photosynthetic carbon reduction and oxidation cycles (1). The properties of the enzyme and the CO2 and O2 concentrations around the enzyme decide the metabolic fate of photosynthetically fixed carbon, namely whether the carbon is directed to sugar synthesis or to the glycol-ate pathway to be released as CO2 (2).