Protein Sulfhydryl Groups.

Abstract

The importance of the sulfhydryl (SH) groups of proteins in cellular respiration has been clearly recognized (deRey Pailhade, 1 Heffter 2 ) and the appearance of these groups in some proteins as the result of denaturation has been demonstrated (Arnold 3 ). For the study of both cellular respiration and protein chemistry, therefore, a means of measuring protein sulfhydryl groups is desirable. Investigation of them has depended mainly on the color reaction they give with nitroprusside. For this test the groups must be in the reduced (SH) and not in the oxidized (S-S) form. Heffter2 supposed that since many denatured proteins, the denatured serum proteins for example, did not give a color with nitroprusside, they contained disulfide (S-S) rather than sulfhydryl (SH) groups. He was indeed able to reduce the disulfide groups of denatured serum proteins with sodium sulfite so that they then gave a color reaction with nitroprusside. Protein disulfide groups have also been reduced by means of zinc and hydrochloric acid (Arnold 3 ), by soluble sulfhydryl compounds (Hopkins 4 ), and by cyanide (Walker 5 ). On the other hand, protein sulfhydryl groups can be oxidized by sulfur (deRey Pailhade, 1 Heffter 2 ), by disulfides (Hopkins4), and by other reagents. Since disulfide groups, as well as sulfhydryl, exist in proteins, and since these groups are interconvertible by oxidation and reduction, information will be incomplete until both the S-S and SH forms are measured.

First Method. We find that the color intensities given with the Folin-Marenzi 6 uric acid reagent by equivalent quantities of cysteine and cystine are precisely in the ratio of 2:1. This is contrary to what has been assumed (Folin and Looney 7 ). If equal quantities of a denatured protein are completely reduced and completely oxidized and then hydrolyzed, the color intensities with the Folin-Marenzi reagent of the 2 hydrolysates are also in the ratio of 2:1.