A procedure using sodium dithionite diluted with KCl and small amounts of base has been developed for anaerobic reductive titrations of proteins in conjunction with EPR and light spectroscopy. The reductant is kept dry in a glass capsule until deoxygenation of the protein solution is completed. When 1:100 and 1:300 dilutions of dithionite were tested with solutions of FMN and cytochrome c, the data indicated titers of 88.6 ± 1.7 (S.D.) and 23.7 ± 1.2 (S.D.) nanoequivalents of electrons per mg, respectively. These figures represent recoveries of 92 and 81% of values calculated from the titer of the undiluted dithionite. The random errors associated with such titrations appear to be commensurate with those encountered with quantitative EPR spectroscopy. This procedure has been applied to small (<1 ml) aliquots of concentrated solutions of oxidative enzymes. © 1969 Academic Press, Inc.
