The redox levels and subcellular distribution of pyridine nucleotides in illuminated barley leaf protoplasts studied by rapid fractionation

The redox level and compartmentation of pyridine nucleotides was studied under photorespiratory and non‐photorespiratory conditions using rapid fractionation of barley (Hordeum vulgare L. cv. Gunilla, Svalöv) leaf protoplasts. From comparative measurements of the NADPH/NADP+ ratio and the ATP/ADP ratio one acidic and one alkaline extraction medium was chosen which quenched the metabolism very efficiently. The mitochondrial NADH/NAD+ was higher under photorespiratory conditions than under non‐photorespiratory conditions. Aminoacetonitrile, an inhibitor of the photorespiratory conversion of glycine to serine, lowered the mitochondrial NADH/NAD+ ratio. This supports the hypothesis that glycine oxidation is coupled to oxidative phosphorylation to provide ATP to the cytosol. The chloroplastic NADPH/NADP+ as well as the NADH/NAD+ ratios were quite stable in saturating and limiting CO2 as well as in the presence of aminoacetonitrile, although the triosephosphate/phosphoglycerate ratios changed. Thus, the redox level in the stroma seems to be tightly regulated.