the high 13c enrichment ( 99 % ) at the guanidino carbon of creatine gave rise to an adequate signal-to-noise ratio for both creatine and phosphocreatine so that a study of the time course of the creatine phosphokinase reaction could be performed and provided the means for a determination of the km ( 15.5 mm ) of the reaction .
C0010286
creatine
C0031634
phosphocreatine