Site of Action of Inhibitors of Carbon Dioxide Assimilation by Whole Lettuce Chloroplasts

Abstract

The sites of action of several compounds, reported to inhibit CO₂ fixation by chloroplast preparations were located by developing assays in lettuce chloroplasts to test their effect on partial reactions of the carbon cycle and on carbonic anhydrase. The results indicated that: d, l-glyceral-dehyde and 5′-AMP inhibit phosphoribulose kinase or isomerase. 3-Phosphoglyceric acid and 6-phosphogluconate inhibit ribulose diphosphate carboxylase. Azide, Mg²⁺, and nitrite inhibit the activity of carbonic anhydrase of lettuce chloroplasts and light-dependent CO₂ fixation by intact chloroplasts with similar sensitivities. None of these inhibited CO₂ fixation in ruptured chloroplasts. It is suggested that the inhibition by azide, nitrite, and magnesium ions of CO₂ fixation by intact chloroplasts is due to their inhibition of the activity of carbonic anhydrase.