Table 3. Intrapeptide (adjacent) Cross-Links (K+36+K) Identified in the FPLC Monomer Fraction of ApoA-I Exposed to MDA^a.

X-linked Lys residues	peptide involved	theoretical mass (Da)	experimental mass (Da)	error (ppm)	protease	product yield (%)	relative intensity
NTb–K12	1–13	1604.73	1604.74	4.67	Glu-C	49.68	196.78
K12–K23	11–27	1914.03	1914.04	4.51	trypsin	49.92	197.74
K40–K45c	28–59	3626.80	3626.82	4.76	trypsin	27.52	108.99
K94–K96c	93–111	2415.23	2415.24	4.26	Glu-C	7.42	29.39
K106–K107c,d	97–116	2681.27	2681.28	4.31	trypsin	44.42	175.93
K133–K140d	132–149	2201.14	2201.15	4.71	trypsin	47.53	188.23
K195–K206d	189–215	3057.60	3057.62	5.30	trypsin	25.10	99.43
K206–K208c,e	206–212	792.46	792.46	0.14	Glu-C	56.77	224.86
K238–K239c,e	236–243	1030.55	1031.55	0.10	Glu-C	25.93	102.68

^aApoA-I (5 μM) was incubated with 100 μM MDA at 37 °C in phosphate buffer (pH 7.4) for 24 h. Reactions were initiated by adding MDA and terminated by adding a 20-fold molar excess (relative to MDA) of aminoguanidine. The reaction mixture was then concentrated, and the monomer was isolated by FPLC. The combined monomer fractions were further concentrated and digested with trypsin or Glu-C. Proteolytic digests were desalted by solid phase extraction prior to MS analysis and analyzed on a hybrid linear ion trap Orbitrap instrument.

^bN-Terminal amino group.

^cCross-linked peptides can be considered short-range with respect to the protein sequences.

^dAlso identified by Glu-C digestion.

^eAlso identified by trypsin digestion.