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Ammonia was excreted at high rates in the presence of L-methionine sulfoximine (L-MSO) from Chlorella cells which have been grown and analyzed at normal CO 2  partial pressure (330 ppm ). If these cells are analyzed at high CO 2 -concentration (3% CO 2  in air) only little ammonia is excreted in the presence of L-MSO. In the absence of L-MSO no ammonia is excreted under either condition. In agreem ent with this observation Chlorella cells grown under high CO 2  partial pressure (3% CO 2  in air) but tested under normal CO 2  partial pressure excreted only very little ammonia. Under these conditions neither “High CO 2 -cells” nor “Low CO 2 -cells” exhibited any glycolate excretion. However, glycolate excretion was observed in the presence of a-HPMS (a-hydroxy-2-pyridyl methanesulfonate) an inhibitor of glycolate dehydrogenase or INH (isonicotinyl hydrazide) an inhibitor of the glycine-serine am inotransferase, irrespective of the presence or absence of L-M SO. INH inhibited ammonia excretion. The above described high ammonia excretion in “Low CO 2 -cells” in the presence of L-MSO was suppressed or substantially reduced by 0.1 mм ethoxyzolamide an inhibitor of carbonic anhydrase which, however, at the same time caused a substantial excretion of glycolate into the medium. The same qualitative effect of ethoxyzolam ide was observed in “High CO 2 -cells” (tested under normal CO 2  partial pressure) although the amount of glycolate excreted in this type of culture was very small. It was generally noted that glycolate excretion caused by ethoxyzolamide was stoichiometrically always more important than the rate of ammonia excretion which was inhibited. This shows that excretion and therefore most probably also the formation of glycolate are enhanced by ethoxyzolamide. The experiments seem to show that due to the inhibition of carbonic anhydrase the affinity of the ribulose-1,5-bisphosphate carboxylase/oxygenase system is increased towards oxygen, which leads to a stimulation of the photorespiratory carbon cycle.

Stimulation of Photorespiration by the Carbonic Anhydrase Inhibitor Ethoxyzolamide in Chlorella vulgaris