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Sugar metabolism pathways such as photosynthesis produce dicarbonyls, e.g. methylglyoxal (MG), which can cause cellular damage. The glyoxalase (GLX) system comprises two enzymes GLX1 and GLX2, and detoxifies MG; however, this system is poorly understood in the chloroplast, compared with the cytosol. In the present study, we determined GLX1 and GLX2 activities in spinach chloroplasts, which constituted 40% and 10%, respectively, of the total leaf glyoxalase activity. In Arabidopsis thaliana, five GFP-fusion GLXs were present in the chloroplasts. Under high CO2 concentrations, where increased photosynthesis promotes the MG production, GLX1 and GLX2 activities in A. thaliana increased and the expression of AtGLX1-2 and AtGLX2-5 was enhanced. On the basis of these findings and the phylogeny of GLX in oxygenic phototrophs, we propose that the GLX system scavenges MG produced in chloroplasts during photosynthesis.

Sugar metabolism pathways such as photosynthesis produce dicarbonyls, e.g. methylglyoxal (MG), which can cause cellular damage. The glyoxalase (GLX) system comprises two enzymes GLX1 and GLX2, and detoxifies MG; however, this system is poorly understood in the chloroplast, compared with the cytosol. In the present study, we determined GLX1 and GLX2 activities in spinach chloroplasts, which constituted 40% and 10%, respectively, of the total leaf glyoxalase activity. In Arabidopsis thaliana, five GFP-fusion GLXs were present in the chloroplasts. Under high CO

To elucidate the scavenging systems of sugar- and lipid-derived reactive carbonyls (RCs) in the cyanobacterium Synechocystis sp. PCC 6803 (S. 6803), we selected proteins from S. 6803 based on amino-acid (AA) sequence similarities with proteins from Arabidopsis thaliana, and characterized the properties of the GST-fusion proteins expressed. Slr0942 catalyzed the aldo-keto reductase (AKR) reaction scavenging mainly sugar-derived RCs, methylglyoxal (MG). Slr1192 is the medium-chain dehydrogenase/redutase (MDR). It catalyzed the AKR reaction scavenging several lipid-derived RCs, acrolein, propionaldehyde, and crotonaldehyde. Slr0315 is a short-chain dehydrogenase/redutase (SDR), and it catalyzed only the reduction of MG in the AKR reaction. Slr0381 catalyzed the conversion of hemithioacetal to S-lactoylglutahione (SLG) in the glyoxalase (GLX) 1 reaction. Sll1019 catalyzed the conversion of SLG to glutathione and lactate in the GLX2 reaction. GLX1 and GLX2 compose the glyoxalase system, which scavenges MG. These enzymes contribute to scavenging sugar- and lipid-derived RCs as scavenging systems.

In Arabidopsis thaliana, the aldo-keto reductase (AKR) family includes four enzymes (The AKR4C subfamily: AKR4C8, AKR4C9, AKR4C10, and AKR4C11). AKR4C8 and AKR4C9 might detoxify sugar-derived reactive carbonyls (RCs). We analyzed AKR4C10 and AKR4C11, and compared the enzymatic functions of the four enzymes. Modeling of protein structures based on the known structure of AKR4C9 found an (α/β)8-barrel motif in all four enzymes. Loop structures (A, B, and C) which determine substrate specificity, differed among the four. Both AKR4C10 and AKR4C11 reduced methylglyoxal. AKR4C10 reduced triose phosphates, dihydroxyacetone phosphate (DHAP), and glyceraldehydes 3-phosphate (GAP), the most efficiently of all the AKR4Cs. Acrolein, a lipid-derived RC, inactivated the four enzymes to different degrees. Expression of the AKR4C genes was induced under high-[CO2] and high light, when photosynthesis was enhanced and photosynthates accumulated in the cells. These results suggest that the AKR4C subfamily contributes to the detoxification of sugar-derived RCs in plants.