Selective oxidation in vitro by myeloperoxidase of the N-terminal amine inapolipoprotein B-100

In contrast to the multiple low abundance 2,4-dinitrophenythydrazine-reacti ve tryptic peptides formed by oxidation of LDL with reagent HOCl in vitro, myeloperoxidase-catalyzed oxidation produces a dominant product in consider ably greater yield and selectivity. This modified peptide had a single amin o-terminal sequence corresponding to amino acids 53-66 of apolipoprotein B- 100 (apoB-100), but its mass spectra indicated a significantly higher mass than could be reconciled with simple modifications of this peptide. Subsequ ent studies indicate that this product appears to result from N-chlorinatio n of the N-terminal amino group of apoB-100 and dehydrohalogenation to the corresponding imine, which may form the hydrazone derivative directly, or a fter hydrolysis to the ketone. The methionine residue is oxidized to the co rresponding sulfoxide, and the primary sequence peptide (residues 1-14 of a poB-100) is linked by the intramolecular disulfide bond between C-12 and C- 61 to the peptide composed of residues 53-66, as we have observed previousl y (Yang, C-Y, T. W Kim, S. A. Weng, B. Lee, M. Yang, and A. M. Gotto, Jr. 1 990. Proc. Nad. Acad. Sci. USA. 87: 5523-5527) in unmodified LDL. The selec tive oxidation by myeloperoxidase of the N-terminal amine suggests strong s teric effects in the approach of substrate to the enzyme catalytic site, an effect that may apply to other macromolecules and to cell surface molecule s.