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The use of the reagent tributyl phosphine (Bu3P) to reduce disulfides (Ruegg, U.T., and Rudinger, J., Methods Enzymol. 47, 111-116, 1977) and of 4-(aminosulfonyl)-7-fluoro-2,1,3-benzoxadiazole (ABD-F) to block free sulphydryl groups (Toyo'oka, T., and Imai, K. Anal. Chem. 56, 2461-2464, 1984)