The effect of nitrate and nitrite on oxygen evolution and carbon-dioxide assimilation and the reduction of nitrate and nitrite by intact chloroplasts

Summary Intact chloroplasts isolated from spinach reduced NO₃^- and NO₂^- in the light without the addition of either co-factors or added enzymes. The maximum rate observed, however, for the reduction of NO₃^- was approximately 3Moles hr^-1 mg^-1 (chlorophyll) and for NO₂^- 6 Moles hr^-1 mg^-1 (chlorophyll). These rates were consistent with the enzyme content of whole chloroplasts, but much lower than those found in whole leaf extracts.The addition of both NO₃^- and NO₂^- in low concentrations resulted in transient increases in both O₂ evolution and CO₂ fixation. The increases in oxygen evolution were not consistent in amount and bore no relation to the amount of substrate reduced. Similar transients were observed in a number of experiments when NaCl or NH₄Cl were added.The addition of NO₂^- at concentrations of 10^-4 M and above resulted in marked inhibition of both O₂ evolution and CO₂ fixation. NO₂^- appears to inhibit by blocking the reduction of NADP. NO₃^- at similar concentrations had no such effect.An increase in the soluble amino nitrogen content of the chloroplasts was observed when NO₃^- or NO₂^- was reduced. There was, however, no increase in the incorporation of ¹⁴C from ¹⁴CO₂ into amino acids under these conditions. Even with the addition of ammonia the amount of ¹⁴C incorporated into the amino acids was not changed from less than 5% of the total ¹⁴C fixed. We conclude that while intact chloroplasts do have the ability to reduce both NO₃^- and NO₂^- at low rates, they do not synthesize appreciable amounts of amino acid directly, and this fact must be considered when formulating any pathways for nitrogen metabolism during photosynthesis.Supported in part by the National Research Council of Canada.