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

Am m onia 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 C 0 2 partial pressure (330 ppm). If these cells are analyzed at high C 0 2-concentration (3% C 0 2 in air) only little ammonia is excreted in the presence o f L-MSO. In the absence of L-MSO no ammonia is excreted under either condition. In agreement with this observation Chlorella cells grown under high C 0 2 partial pressure (3% C 0 2 in air) but tested under normal C 0 2 partial pressure excreted only very little ammonia. Under these conditions neither “High C 0 2-cells” nor “Low C 0 2-cells” exhibited any glycolate excretion. H owever, glycolate excretion was observed in the presence o f a-HPM S (a-hydroxy-2-pyridyl m ethanesulfonate) an inhibitor of glycolate dehydrogenase or INH (isonicotinyl hydrazide) an inhibitor o f the glycine-serine aminotransferase, irrespective of the presence or absence o f L-M SO. INH inhibited ammonia excretion. The above described high ammonia excretion in “Low C 0 2-cells” in the presence of L-MSO was suppressed or substantially reduced by 0.1 mM ethoxyzolam ide an inhibitor o f carbonic anhydrase which, however, at the same time caused a substantial excretion of glycolate into the medium. The same qualitative effect o f ethoxyzolam ide was observed in “High CÖ2-cells” (tested under normal C 0 2 partial pressure) although the amount o f 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 o f 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-l,5-bisphosphate carboxylase/oxygenase system is increased towards oxygen, which leads to a stimulation of the photorespiratory carbon cycle.