The varying tissue distribution of L-glutamic acid labelled at three different sites

Methods using 5 and 6 enzymes were devised to synthesize L-[11C]glutamic acid (GA), labeled on the carboxyl group of either the alpha-(AGA) or gamma-(GGA) carbon atom. The distribution of the 11C-labeled AGA or GGA in rabbits was compared with that of L-[13N]glutamic acid. The results show that AGA was rapidly decarboxylated with loss of the 11C-label, and that GGA was also decarboxylated, but to a lesser degree. Thus, the radiolabel distribution may not reflect the distribution of the original compound. The results also demonstrate that positron labeled pharamaceuticals may be rapidly synthesized via complex enzymatic pathways.