The nature of “masked” and “free” sulfhydryl groups in proteins

The relative amounts of readily titrable (‘free’) SH groups, versus that of SH groups titrable readily only following denaturation (‘masked’) has been shown by Szent-Gyorgyi to vary significantly from normal to cancerous organ tissue. It is important therefore to inquire into the causes of the two forms of protein-borne SH. Of the four suggested mechanisms for the “masking” of protein SH groups, (1) sequestration in hydrophobic regions-whether between chain folds or between agglomerated subunits, (2) local steric hindrance, (3) cyclic hydrogen-bonding to local peptide amino acid residues, or (4) covalent bonding as in thiazolines; the first mechanism, that of sequestration in hydrophobic regions appears from present evidence to be the most likely cause. Various spectroscopic, reaction rate, and entropy arguments are presented and compared leading to this conclusion.