The effects of some ions and chelating agents on the oxidase activity of caeruloplasmin

Abstract

The oxidase activity of purified human ceruloplasmin was studied with N N-dimethyl-p-phenylenedi-amine as substrate by measuring the extinction of the red reaction product. Anions caused inhibition in the order thiocyanate> fluoride> chloride > bromide > sulphate. The acetate buffer used also had some inhibitory effect. Ceruloplasmin, which is partly inhibited by acetate or oxalate, is protected against inhibition by chloride. Inhibition by chloride, and probably by acetate, is consistent with an anti-com-petitive mechanism. The effects on activity of some chelating agents were studied. Ethylenediaminetetraacetate (EDTA) inhibits non-competitively. The inhibition-log (EDTA concentration) curve, unlike the corresponding curves of oxalic acid and malonic acid, is sharply discontinuous. Activity is increased by Fe2+ ions and to a lesser extent by Ni2+, Co2+, Zn2+ and Fe3+ ions at low concentrations. The same metals decrease activity at higher concentrations. Activity is also decreased by low concentrations of Hg2+, Al3+ and In3+ ions.