STUDIES IN VITRO By ANNEMARIE HERZFELD
نویسنده
چکیده
Two common ways of measuring the potential for glutamine synthesis in a tissue are the rates of formation of y-glutamylhydroxamate either by synthesis from glutamate (the glutamylhydroxamate synthetase reaction) or by transfer from glutamine (the glutamyltransferase reaction); it has not been established, however, that either reaction is a specific measure of glutamine synthetase. By differential extraction of glutamylhydroxamate synthetase and glutamyltransferase activities from water homogenates of several rat tissues I obtained an extract, rich in glutamylhydroxamate synthetase activity but nearly devoid of glutamyltransferase activity, and a fraction, solubilized by deoxycholate from the pellet, which contained virtually no glutamylhydroxamate synthetase activity but most of the original glutamyltransferase activity. Synthesis of glutamine, quantitatively similar to the y-glutamylhydroxamate formed by glutamylhydroxamate synthetase, is catalysed in the water extract but not in the particulate fraction. y-Glutamylhydroxamate formation by glutamylhydroxamate synthetase and glutamyltransferase shows discrepant substrate and metal specificities and can be differentially inhibited by L-methionine sulphoximine, phosphate and adenine nucleotides. The concordance between the formation of glutamine and y-glutamylhydroxamate by glutamylhydroxamate synthetase but not by glutamyltransferase and the different solubilities of the glutamylhydroxamate synthetase and glutamyltransferase enzyme activities demonstrate that these two activities are not inextricably associated; they therefore cannot be catalysed exclusively by the same protein.
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تاریخ انتشار 2005