[Sulfhydryl groups of cyclic AMP-dependent histone kinase from pig brain]

A preparative method of isolation of the histone kinase regulatory subunit has been developed. The total number of cysteine residues in the enzyme (13 residues) and in each of its subunits (3 residues in the catalytic subunit and 10 residues in the regulatory one), has been determined. Two disulfide bridges have been found in the regulatory subunits. The relative availability of the sulfhydryl groups with respect to different modifying agents has been studied. It has been demonstrated that SH-groups blocking does not change essentially the enzyme activity.