Chemical modification as a probe of the topography and reactivity of horse-spleen apoferritin

In apoferritin, but not in ferritin, 1.0 +- 0.1 cysteine residue per subunit can be modified. In ferritin 3.3 +- 0.3 lysine residues and 7.1 +- 0.7 carboxyl groups per subunit can be modified, whilst the corresponding values for apoferritin are 4.4 +- 0.4 lysine residues and 11.0 +- 0.4 carboxyl groups per subunit. Modification of lysine residues with maleic anhydride and of carboxyl groups with glycineamide in apoferritin which has been dissociated and denatured in guanidine hydrochloride leads to the introduction of 9.1 +- 0.5 maleyl groups per subunit and 22.0 +- 0.9 glycineamide residues per subunit. Whereas unmodified apoferritin subunit can be reassociated from guanidine hydrochloride to apoferritin monomer, the ability of maleylated apoferritin to reassociate is impaired. Apoferritin in which all the carboxyl group have been blocked with glycineamide cannot be reassociated to apoferritin and exists in solution as stable subunits. The modification of one cysteine residue per subunit, of 3 or 4 lysine per subunit or of 7 carboxyl groups per subunit has no effect on the catalytic activity of apoferritin. In contrast, the modification of 11 carboxyl groups per subunit completely abolishes the catalytic properties of the protein. We conclude that one or more carboxyl groups are essential for the catalytic activity of horse spleen apoferritin. (orig.)