A difference in the incorporation of 14C into hippurate glycine from DL [2 14C]glutamate and DL [5 14C]glutamate in guinea pigs

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.