The specific radioactivity of urinary hippurate glycine was determined after injecting guinea pigs with benzoate and either dl-[2-14C]glutamate or dl-[5-14C]glutamate. The isotope dilution factor for the formation of [14C]glycine was significantly greater (30%) with C-2 labelled glutamate. With either form of labelled glutamate the hippurate glycine was largely carboxyl-group labelled. The observations suggest a route for the incorporation of glutamate carbon into glycine that involves C-5 but not C-2. A hypothesis for glycine biosynthesis from l-glutamate is advanced, consistent with these findings, that includes conversion of l-glutamate to 4-hydroxy-2-oxoglutarate, the scission of the latter to glyoxylate and pyruvate, and the formation of glycine by transamination.

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