Abstract
It was hypothesized previously that an O2 inhibition of NO2− photoreduction would reflect a competition between O2 and NO2− for electrons from ferredoxin at the site of plastid nitrite reductase. In order to test this in vivo, intact spinach (Spinacia oleracea L.) leaf chloroplast and mesophyll cell isolates held in high light were aerated with streams of 20% O2/80% N2 (250 micromolar O2 in aqueous solution) or, alternatively, streams of 100% N2. Bicarbonate plus CO2 and NO2− were supplied to reaction mixtures at levels just sufficient to promote maximal assimilations of CO2 and NO2−. In chloroplast isolates, there was a 9 to 30% O2 inhibition of NO2− reduction while there were high rates of CO2 fixation. In spinach and soybean (Glycine max) leaf cell isolates, NO2− photoreduction rates were 10 to 55% inhibited by O2 at near ambient levels. It is possible that O2 may compete, albeit weakly, with NO2− (nitrite reductase) for equivalents derived from reduced ferredoxin. Also, O2 may oxidize sulfhydryl groups on nitrite reductase which are involved in substrate binding and/or activation.
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