What is the physiological origin of free D-amino acids in mammals?

A previously unknown metabolite of vitamin B6, tentatively identified as adenosine-N6-diethylthioether-N1-pyridoximine-5'-phosphate, accounts for up to 30% of the total intracellular vitamin B6 observed in tumor cells cultured in the presence of radiolabeled pyridoxine. When various animal and human tumor cells were incubated with radiolabeled pyridoxine, the formation of this metabolite was greatest in rapidly growing cells that were the least differentiated. If analytical methods are verified, the presence of the compound in serum may provide an indicator of in vivo tumor growth.