The Effect of Nitrogen Starvation on the Activity of Nitrate Reductase and other Enzymes in Chlorella

Abstract

Chlorella vulgaris grown with nitrate as nitrogen source had high nitrate reductase activity. During nitrogen starvation the rate of nitrate assimilation increased but the specific activity of nitrate reductase in extracts decreased sharply; a similar decrease occurred after transfer to an ammonium medium. Organisms grown with ammonium as nitrogen source did not assimilate nitrate but acquired the ability to do so after nitrogen starvation; nitrate reductase activity in these organisms was initially very low but increased during nitrogen starvation and then decreased. After 18 hr of nitrogen starvation ammonium-grown and nitrate-grown organisms assimilated nitrate rapidly at a rate some ten times greater than the activity of nitrate reductase in cell-free extracts would allow. The specific activity of glutamic dehydrogenase was about 40% higher in extracts from ammonium-grown organisms than in extracts from nitrate-grown organisms and its activity increased by 40-100% during nitrogen starvation. The activity of malic dehydrogenase was unaffected by 18 hr of nitrogen starvation. The activity of isocitrate lyase decreased by about one-quarter when acetate was present but decreased much more markedly when the organisms were nitrogen-starved in the presence of glucose.