Cyclized internal amide of l-glutamic acid found in vegetables, fruits, grasses, and molasses. Easily prepd from l-glutamic acid by autoclaving with an equal wt of water at 135-140°: Dearborn, Stekol, US 2528267; purification: Blish, US 2738353 (1950, 1956, both to International Minerals and Chem.). Review: Orlowski, Meister, in The Enzymes vol. 4, P. D. Boyer, Ed. (Academic Press, New York, 3rd ed., 1971) pp 123-151; C. Moret, M. Briley, Trends Pharmacol. Sci. 9, 278-279 (1988).