[Alkali treatment of proteins. I. Behavior of sulfhydryl and disulfide groups in alkali-treated beta-lactoglobulin and alpha-lactalbumin].

The alkali treatment of beta-lactoglobulin and alpha-lactalbumin results in the splitting of disulphide bonds in the protein molecules. When a 0.8-10(4) M beta-lactoglobulin solution in a 0.022 N sodium hydroxide solution (pH = 12) is heated at 90 degrees C for 30 min, 0-4 M disulphide groups, 2.2 M sulfhydryl groups and 1.8 M sulphide ions/M dimeric beta-lactoglobulin are detectable of the total of 4 M disulphide groups and 2 M sulfhydryl groups/M dimeric beta-lactoglobulin. The sulphide ions can be determined directly in the form of hydrogen sulphide or by calculating the difference between the values from the amperometric-argentometric titration and those from the method of ELLMAN (reaction with DTNB). The disulphide groups are determined with the aid of DTNB after reduction with sodium borohydride. The sulfhydryl groups obtained by reduction with sodium borohydride re-oxidize, the reaction velocity being of the second order. If the disulphide groups are reduced with sodium borohydride, the argentometric-amperometric determination of the sulfhydryl groups by means of the platinum rotating-disk electrode is disturbed by the presence of boric acid.