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Whereas the optimum pH for digestion of wool by papain-bisulphite-urea is 7.0, a higher pH value is required for optimum digestion when bisulphite is replaced by sodium thioglycollate, cysteine hydrochloride, or potassium cyanide. Thioglycollate at pH 9.5 is superior to cysteine at the same pH or to bisulphite at pH 7.0 in promoting wool digestion by papain solutions containing urea. Cyanide is least effective. When wool is incubated in solutions of bisulphite, thioglycollate, cysteine, or cyanide in the absence of papain and urea, the extent of splitting of disulphide bonds runs parallel with the enzyme digestion observed in the presence of these reagents. The number of disulphide bonds split was measured by the iodosobenzoic acid method. Wool digestion in a solution containing only fully active papain and urea is less than one-tenth of that observed when sodium bisulphite is also present. The cystine-free protein, collagen, is digested most rapidly in papain-thioglycollate-urea at acid pH values. At all pH values the presence in the papain solution of both urea and a reagent rupturing disulphide bonds leads to greater wool digestion than either of these compounds used singly with papain. The combined action of sodium bisulphite and sodium thioglycollate in papain-urea solution increases wool digestion over a range of pH values.

Further Studies on the Digestion of Wool Keratin by Papain-Urea: the Effect of Adding Compounds that Rupture Disulphide Bonds in Alkaline Solution