Effect of ammonia, nitrite, glutamate, and inhibitors of N metabolism on photosynthetic carbon fixation in isolated spinach leaf cells

Abstract

Photosynthetic ¹⁴CO₂ fixation in spinach leaf cells was stimulated by ammonia. CO₂ fixation in the presence of NH₃ was inhibited by glutamate, NO₂⁻, methionine sulfoximine, azaserine, or aminooxyacetate. In all instances, though, the decrease in ¹⁴CO₂ fixation was largely due to decreased ¹⁴C in the neutral fraction rather than to decreased ¹⁴C in the basic or acidic fractions which are those fractions specifically increased by ammonia addition. Ammonia appears to stimulate "dark" ¹⁴CO₂ fixation in addition to photosynthetic ¹⁴CO₂ fixation. The treatments mentioned above did not abolish this "dark" ¹⁴CO₂ fixation but rather slightly inhibited photosynthetic ¹⁴CO₂ fixation so that total ¹⁴CO₂ fixation was reduced. It would seem that the mechanism by which nitrogen assimilation stimulates ¹⁴CO₂ incorporation into the basic fraction is still unresolved.