Glutamic acid and glutamine are present in unusually large concentrations in brain. The only known enzymatic mechanism which has been reported for the synthesis of glutamine is that catalysed by glutamine synthetase (E.C.18.104.22.168). These amino acids provide a very useful probe for studying precursor-product relationships in brain tissue in vivo. It is evident that the non-steady state situation presented by injection of a single dose of inhibitor followed by a tracer dose of a labelled precursor at some later time-a practice which has been used extensively in previous attempts to clarify the effects of metabolic inhibitors-makes it rather difficult to interpret the results. In vitro preparations are an attractive way to avoid some of these problems. In addition, there have been cases where a labelled precursor injected intravenously or intraperitoneally, for example alanine, was first metabolized to glucose with scrambling of the label before entering the brain (Koeppe and Hahn, 1962)
Clarke, Donald Dudley PhD and Berl, Soll, "Alteration in the expression of compartmentation: in vitro studies / D. D. Clarke and S. Berl" (1972). Chemistry Faculty Publications. 30.