Nitrite photoreduction in vivo is inhibited by oxygen

It was hypothesized previously that an O(2) inhibition of NO(2) (-) photoreduction would reflect a competition between O(2) and NO(2) (-) 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% O(2)/80% N(2) (250 micromolar O(2) in aqueous solution) or, alternatively, streams of 100% N(2). Bicarbonate plus CO(2) and NO(2) (-) were supplied to reaction mixtures at levels just sufficient to promote maximal assimilations of CO(2) and NO(2) (-). In chloroplast isolates, there was a 9 to 30% O(2) inhibition of NO(2) (-) reduction while there were high rates of CO(2) fixation. In spinach and soybean (Glycine max) leaf cell isolates, NO(2) (-) photoreduction rates were 10 to 55% inhibited by O(2) at near ambient levels. It is possible that O(2) may compete, albeit weakly, with NO(2) (-) (nitrite reductase) for equivalents derived from reduced ferredoxin. Also, O(2) may oxidize sulfhydryl groups on nitrite reductase which are involved in substrate binding and/or activation.