Regulation by amino acids of photorespiratory ammonia and glycolate release from ankistrodesmus in the presence of methionine sulfoximine

Methionine sulfoximine induced release of ammonia from illuminated cells of Ankistrodesmus braunii (Naegeli) Brunnth, in normal air, but less in air enriched to 3% CO₂. In normal air, methionine sulfoximine also induced glycolate release. Addition of either glutamate, glycine, or serine suppressed glycolate release, whereas glutamate and glycine at the same time stimulated ammonia release. The results indicate that inhibition of glutamine synthetase and thereby inhibition of photorespiratory nitrogen cycling restricts the sink capacity for glycolate in the photorespiratory carbon cycle. An external supply of glutamate, glycine, or serine seems to stimulate glyoxylate transamination and thus partly restores the sink capacity. Calculations of total glycolate formation rates in air from glycolate and ammonia release rates in the presence of methionine sulfoximine and glutamate revealed values of approximately 20 micromoles glycolate per milligram chlorophyll per hour on the average. Similar calculations led to an estimated rate of photorespiratory ammonia release in air, in the absence of methionine sulfoximine, of about 10 micromoles per milligram chlorophyll per hour on the average, a value comparable to the primary nitrogen assimilation rate of 8 micromoles per milligram chlorophyll per hour.