Abstract
1 Diethylpyrocarbonate reacts specifically with histidine residues in human ceruloplasmin at pH 6. 2 Carbethoxylation of imidazole groups in ceruloplasmin does not affect the optical absorption in the visible region which is due to paramagnetic Type-1 copper or diamagnetic copper, but has a pronounced effect on electron paramagnetic resonance (EPR) spectra of the protein. The ERP-spectral changes do not involve the Type-1 copper signal or a conversion of diamagnetic copper into paramagnetic copper but indicate a change in the binding situation of Type-2 copper. ERP spectra of the modified protein are pH-dependent over the range pH 5–7. 3 Carbethoxylation of ceruloplasmin and the concomitant EPR-spectral changes parallel a decrease in oxidase activity towards ferrous ions and dimethyl-p-phenylenediamine. Inhibition by diethylpyrocarbonate can be (at least partially) reversed by treatment with hydroxylamine at pH 7, which also restores the EPR spectrum of native ceruloplasmin.
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