Effects of Methionine Sulfoximine and Glycine on Nitrogen Metabolism of Maize Leaves in the Light

Abstract

Detached maize leaves with their cut bases in water or in solutions containing 15 mM [¹⁴C, ¹⁵N]glycine, 15 mM [¹⁴C]glutamate, 5 mM methionine sulfoximine (an inhibitor of glutamine synthetase) or the appropriate amino acid plus inhibitor, were incubated for up to 135 min in the light. The concentrations and the ¹⁵N content of ammonia and of amino acids involved in photorespiratory nitrogen cycling were determined. Incubation with methionine sulfoximine or glycine increased the ammonia content significantly, whereas glutamate showed no effect. The nitrogen of glycine was metabolized into serine and ammonia. Ammonia was first recycled into glutamine, and then into glutamate. The glycine carbon skeleton served as a precursor for serine. Based on the data for ammonia accumulation the minimum rate of photorespiratory CO₂ evolution in maize leaves was estimated to be about 1% of the rate of CO₂ assimilation.