We quantitated the contributions of the spectra of the protium-and deuterium-containing enzymes on the basis of the integrated original spectra, indicating that 80% of the active BSS molecules had reverted to the protium-state, while 20% still contained the deuterated glycyl radical (Fig. red curve). Since the buffer exchange procedures lasted approximately 1 h in either direction, this observation correlates well with the postulated faster rate of exchanging protium to a deuteron at the glycyl radical than that of the reverse process. In this respect, it is noteworthy that the EPR experiments on BSS or PFL were always performed in cell extracts that contained substrates or products, while EPR on ARNR was performed in the presence of formate but in the absence of nucleoside triphosphates .