MATRIX-ASSISTED-LASER-DESORPTION IONIZATION MASS-SPECTROMETRY ANALYSIS AND THIOL-GROUP DETERMINATION OF ISOFORMS OF BOVINE CYTOCHROME-C-OXIDASE, A HYDROPHOBIC MULTISUBUNIT MEMBRANE-PROTEIN/

Matrix-assisted laser desorption/ionization-mass spectra (MALDI-MS) ar e obtained from entire bovine heart (fl) and liver (L) cytochrome c ox idase membrane protein complexes. Molecular masses of most of the subu nits are in excellent agreement with the published sequences. Some cor rections are necessary fur the nuclear coded subunit IX, which is N-ac etylated, and X, with a corrected C-terminal peptide sequence, The mas s values of two of the three tissue-specific subunits (VIII-L and X-L) are not in agreement with the DNA-deduced sequences and]have been cor rected by protein sequencing. For the investigation of the cysteine st atus hyl-amino-3-(4'-maleimidylphenyl)-4-methylcoumarin proved to be a n excellent site-specific reagent. MALDI-MS with the SH-reacted enzyme indicates disulfide bridges only in subunit VII and a distorted tetra hedral S coordination of the zinc in subunit VI. (C) 1998 Academic Pre ss.