Oxidative modification of protein structures under the action of myeloperoxidase and the hydrogen peroxide and...

Myeloperoxidase of neutrophilic leukocytes (MPO) at pH 4.0 to 6.5 mediated oxidation of Cl- ions, yielding hypochloride (OCl-) which then reacted with amino acids and polypeptides. Thiol and thioether groups may be oxidized to disulfide or to sulphoxides and sulphonic acids respectively. Tryptophanyl residues yielded 2-oxoindole. Epsilon amino groups of lysine produced chloramine which, however, decomposed, yielding aldehyde residues. Bovine serum albumin treated with MPO-Cl-H2O2 system yielded derivatives with a decreased affinity to antialbumin antibodies and increased electrophoretic mobility. Albumin aldehyde derivatives were also obtained. At H2O2 molar ratio with albumin 20:1, a precipitation of albumin occurred, due to the formation of new polymeric albumin derivatives. The lysozyme (LZM) lost its enzyme activity when 1.4 to 1.8 mol of H2O2 per 1 mol of LZM was used. Addition of H2O2 above molar ratio 5:1 produced LZM polymerization to di-, tri-, tetra and pentameric derivatives. IgA exposed to the MPO-Cl-H2O2-Cl- system split into light chains (molecular weight: 25.8 kDa), heavy chains (molecular weight: 81.8 kDa) and a third polypeptide which size was half the light chain size (molecular weight: 13.9 kDa). The IgA exceeding the HOCl ratio 1:350 (mg/mumol) produced both precipitation and degradation of the IgA polypeptide structure. The treatment of IgG with HOCl released a fragment corresponding to half the light chain size, the light chain, and the heavy chain, whereas HOCl treatment of IgM released only a fragment which size was smaller than the heavy chain and another fragment which size was the same as the light chain. The MPO-Cl-H2O2 system produced many specific changes in protein structures.(ABSTRACT TRUNCATED AT 250 WORDS)