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Acetyl Coenzyme A-Glutamate Acetyltransferase and N²-Acetylornithine-Glutamate Acetyltransferase of Chlorella

¹ United States Plant, Soil and Nutrition Laboratory, Agricultural Research Service, United States Department of Agriculture, Ithaca, New York 14853

The enzymic formation of acetylglutamate has been studied in Chlorella vulgaris extracts. Acetyl CoA and N²-acetyl-L-ornithine served as substrates for glutamate acetylation whereas acetylphosphate, N⁵-acetyl-L-ornithine, and N²-acetyl-2,4-diamino butyrate were ineffective. Acetyl CoA-glutamate transacetylase and acetylornithine-glutamate transacetylase activities have been purified over 180-fold with no indication of any separation of activities. The acetyl CoA activity was more labile than acetylornithine activity so that preparations having acetylornithine-glutamate transacetylase activity but no acetyl CoA-glutamate transacetylase activity were obtained. The two acetylating activities appear to be properties of one enzyme with one portion more easily denatured.

Both acetylating activities had pH optima between 8 and 8.5. The Km value for glutamate was 3 mM for both activities. The Km values were 0.2 mM for acetylornithine and 3.2 mM for acetyl CoA. Arginine inhibited acetyl CoA-glutamate transacetylase (Ki = 0.94 mM) and acetylglutamate phosphokinase (Ki = 0.5 mM) but had no effect on acetylornithine-glutamate transacetylase. The lack of an inhibitory effect of proline on any of the three enzymic activities indicates that acetylglutamate is not a normal intermediate in proline biosynthesis. Growth of Chlorella with arginine as a nitrogen source had no effect on enzyme levels, showing that end-product repression is not a control factor in arginine biosynthesis in Chlorella. In Chlorella, arginine controls its own biosynthesis by inhibiting acetylglutamate phosphokinase and controls the level of acetylated intermediates by inhibiting acetyl CoA-glutamate transacetylase.