Table VII.

Reactions of the SOD-Asc-GSH cycle and kinetic parameters used to model “standard” conditions

EnzymeSubstratesProductsReductant OxidantChloroplast
ReductantOxidantkcatKmKmConcentrationRelative occurrence
s−1 μM μM %
SODO Embedded Image + 2 H+ +O2 .− H2O2 +O2 106 4-a 3504-a  504-a 804-b
APX2 ASC +H2O2 2 MDA + 2 H2O2904-c 3004-c 304-c  704-d 794-e
MDARNAD(P)H + H+ +2 MDA2 ASC +NAD(P)+ 434-f 304-f 0.94-f  144-g 514-h
DAR2 GSH +DHAASC +GSSG1424-i 2,5004-i  704-i 1.74-j 284-k
GRNADPH + H+ +GSSG2GSH +NADP+ 5954-l 34-l 2004-l 1.74-g 674-k

The reaction velocities were calculated according to Equation 2, except for SOD. Because of the almost diffusion-controlled reaction rate, the velocity of the SOD reaction was calculated according to Equation 1using its pseudo-first order rate constant k(SOD) = 2 × 109 M−1 s−1 (Asada and Takahashi, 1987). To avoid overestimation of MDAR, its catalytic activity with NADPH, which is lower than that determined with NADH, has been used. Relative occurrence refers to the relative activity of an antioxidant enzyme in chloroplasts as compared with its activity in whole-leaf extracts (100%).