The Function of Homoglutathione and Hydroxymethylglutathione for the Scavenging of Hydrogen Peroxide

The function of homoglutathione (γGluCysβAla, hGSH) and hydroxymethylglutathione (γGluCysSer, HMGSH) for the photoreduction of H2O2 was investigated. Five to ten percents of the total leaf tripeptide were found in the chloroplasts of the hGSH species Glycine max and Phaseolus coccineus and of Triticum aestivum, a species which has HMGSH in addition to GSH. The Km of bean glutathione reductase (GR) for oxidized hGSH and for GSSG were nearly the same, whereas GSSG was better substrate for wheat and spinach GR. All GR's showed a much lower affinity to oxidized HMGSH than to GSSG. Intact chloroplasts of bean, soybean and wheat showed a photoreduction of H2O2 only when they were isolated in the presence of ascorbate. The reduction rate for wheat chloroplasts was the same as for pea and spinach chloroplasts, whereas bean and soybean chloroplasts had a much lower rate at 25–100 μM H2O2. The activities of ascorbate peroxidase and monodehydroascorbate reductase in the chloroplasts of all five species were comparable. The chloroplasts of bean and soybean, however, had no or little dehydroascorbate reductase and GR activities, therefore, ascorbate regeneration by hGSH as a reductant will be of limited significance in both the plants under normal conditions.