Parameters affecting the superreactivity of the cysteine side chain in histone H3. Characterization of a 32-amino-acid peptide including Cys-110.

Tryptic digestion of the whole histone mixture from chicken erythrocytes is used to isolate the hydrophobic portion (residues 84-115) of histone H3. The phenomenon of a superreactive thiol at Cys-110 was reinvestigated by the reaction of a neutral, fluorogenic reagent, N-[p-(2-benzimidazolyl)phenyl]maleimide, with this peptide and with the intact histone. Removing the highly basic portions of H3 leads to a strong reduction of Cys-110 reactivities, even if a stoichiometric complex with histone H4 is reconstituted. An apparent superreactivity is found only for a non-degraded histone H3 under conditions of low ionic strength but not for the histone complex in its native conformation. It is concluded that thiol activation is due to an artifactual cluster of positive charges around Cys-110 rather than to a stable microenvironment.