1. Intact, unhydrolyzed proteins possess in addition to SH groups other reducing groups which can be oxidized by ferricyanide.
2. The activity of these reducing groups, like that of SH groups, is enhanced by denaturation of the protein and by increase of pH and temperature.
3. These groups differ from SH groups in the manner in which their activity is dependent on concentration of ferricyanide and time of contact with ferricyanide.
4. The activity of these groups is increased if protein SH groups are present.
5. The number and activity of these groups varies from protein to protein.
6. These groups are probably contained in the tyrosine and tryptophane components of proteins.
7. The significance of these reducing groups for an understanding of protein denaturation and the reducing properties of tissues is indicated.
