Synthetic decarboxylated S-adenosyl-L-methionine (dcAdoMet), a mixture of the absolute configuration of S and R at the sulfonium center, was evaluated as a substrate for the measurement of spermidine synthase activity. The diastereomers were separated by HPLC with an isocratic elution, and the constant for racemization at the sulfur was determined to be 2.4x10(-6) s(-1) at 37 degrees C and pH 1.5 for the first-eluted biologically active isomer (S-dcAdoMet) and 2.0x10(-6) s(-1) for the second-eluted biologically inactive isomer (R-dcAdoMet). The peak area ratio of S-dcAdoMet to R-dcAdoMet of 48 to 52 in HPLC supported the different racemization constants. Similar substrate activity of dcAdoMet to that of S-dcAdoMet was demonstrated by enzymatic spermidine synthesis. It was shown from the result that the racemized [methyl-(14)C]dcAdoMet prepared in this report was useful for measuring spermidine synthase activity.