The Location of Glutamine Synthetase in Leaf Cells and its Role in the Reassimilation of Ammonia Released in Photorespiration

Abstract

The localization of glutamine synthetase within the cells of barley and pea leaves has been reinvestigated using either a mechanical technique or rupturing of isolated protoplasts to release the cellular organelles, and both differential and density gradient centrifugation to separate them. In no case could we find evidence for any significant association between glutamine synthetase and the mitochondria; our results suggest that the enzyme is present in the chloroplast and the cytoplasm of both species. Experiments with isolated mitochondria from spinach also failed to provide any suggestion that these organelles might contain glutamine synthetase. Thus there is no evidence to support the hypothesis, published by others, that mitochondria reassimilate ammonia, released from glycine oxidation, by means of their own glutamine synthetase. Further experiments were carried out to see if glutamate dehydrogenase present in the mitochondria could reassimilate ammonia under conditions in which the electron transport chain to oxygen was blocked. Although there was some evidence for a small amount of assimilatory glutamate dehydrogenase activity under these conditions it was dependent on adding relatively high concentrations of ammonia and was insufficient to sustain the rate of recycling of NAD required for glycine oxidation. The results were thus considered to be compatible with the operation of the photorespiratory nitrogen cycle as previously published.