The antioxidant activities of aqueous phase beta-lactoglobulin (beta-Lg) and its chymotryptic hydrolysates (CTH) were compared in this study. Proteins and peptides have been shown to inhibit lipid oxidation reactions in oil-in-water emulsions; however, a more fundamental understanding of the antioxidant activity of these compounds in dispersed food lipid systems is lacking. CTH was more effective than an equivalent concentration of beta-Lg in retarding lipid oxidation reactions when dispersed in the continuous phase of Brij-stabilized oil-in-water emulsions (pH 7). Furthermore, it was observed that CTH had higher peroxyl radical scavenging and iron-binding values than beta-Lg. Liquid chromatography-mass spectrometry (LC-MS) was used to measure the rate of oxidation of three oxidatively labile amino acid residues (Tyr, Met, and Phe) in certain CTH peptide fragments. Significant oxidation of specific Tyr and Met residues present in two separate 12 amino acid peptide fragments was observed in the days preceding lipid oxidation (39 and 55% of Tyr and Met were oxidized, respectively, by day 4 of the study); however, no significant oxidation of the Phe residue present in a specific 14 amino acid peptide fragment could be observed during the same time period. These data could suggest that Met and Tyr residues are capable of scavenging radical species and have the potential to improve the oxidative stability dispersed food lipids.
Abstractβ-2-microglobulin (β2m) deposits as amyloid fibrils in the musculoskeletal system of patients undergoing long-term dialysis treatment as a result of kidney failure. Previous work has shown that Cu(II) binding causes β2m to organize into native-like dimers and tetramers that precede amyloid formation. Cu(II) is then released from higher order oligomers before mature Cu(II)-free amyloid fibrils are formed. While some of the Cu(II)-induced structural changes that enable β2m self assembly are starting to be revealed, the details of how the Cu(II) binding site evolves from the monomer to the dimers and tetramers are not known. Here, we report results from three mass spectrometry (MS) based methods that provide insight into the changing Cu-β2m interactions. We find that monomeric β2m binds Cu(II) via the N-terminal amine, the amide of Gln2, His31, and Asp59. In the dimer and tetramer, Asp59 is no longer bound to Cu(II), but the other residues still comprise a well-defined albeit weaker binding site that is better able to release Cu(II). Consistent with this is the observation that a fraction of the tetrameric species no longer binds Cu(II) at this weakened binding site, which agrees with a previous report that suggested the tetramer as the first Cu(II)-free oligomer. Our results also provide some insight into structural changes caused by Cu(II) binding that facilitate oligomer formation. Specifically, binding by Asp59 in the monomer requires significant movement of this residue, and we propose that this repositioning is important for establishing a pair of dimer-stabilizing salt bridges between this residue and Lys19. We also find evidence that Cu(II) binding in the Nterminal region of the monomer repels Arg3, which likely allows this residue to form a pair of dimerstabilizing salt bridges with Glu16. Overall, our measurements suggest that the previously proposed conformational switch caused by Cu(II) binding includes not only a cis-trans isomerization at Pro32 but also the repositioning of residues that are critical for the formation of new electrostatic interactions.β-2-microglobulin (β2m) is a 12 kDa subunit of the class I major histocompatibility complex and is a structural unit essential for the cell-surface expression of this complex. During normal turnover, β2m is released into serum and is eventually catabolized by the kidney. In patients undergoing hemodialysis as a result of kidney failure, β2m concentrations become elevated in the serum, and after as little as 18 months, β2m amyloids begin to form in the joints of these patients [1]. The cause of β2m amyloid formation in vivo is not precisely known, but several † This material is based upon work supported by the National Institutes of Health Grant RO1 GM 075092 *Department of Chemistry, University of Massachusetts, Amherst, rwvachet@chem.umass.edu, Telephone: (413) 545-2733, Fax: (413) 545-4490. ‡ Current address: Department of Chemistry, U-3060, University of Connecticut, 55 North Eagleville Road, Storrs, CT 06269-3060Supporting Information Ava...
We have identified conditions that allow metal-catalyzed oxidation (MCO) reactions and mass spectrometry (MS) to correctly identify binding sites of first-row transition metal ions to model peptides. This work extends the applicability of the MCO/MS method to metals other than Cu(II). When the appropriate reducing agent (ascorbate, 10 mM) and oxidizing agent concentrations (1 mM persulfate, atmospheric O2, or both) are used, metal-bound amino acids can be sufficiently and specifically oxidized for clear identification by MS. The MCO reactions with Mn(II), Fe(II), Co(II), and Ni(II) occur to lesser extents than with Cu(II), but oxidation is still extensive enough to allow easy identification of the metal-bound residues. With the exception of aspartic acid, the known metal-binding amino acids of angiotensin I and bacitracin A are oxidized, while no oxidation is observed at nonbinding residues. Failure to oxidize aspartic acid is likely due to the relatively slow reactivity of its carboxylic acid side chain with reactive oxygen species, suggesting that the current MCO/MS protocol is transparent to such acidic residues. Overall, this study indicates that, just as is possible for Cu(II), the MCO/MS method should be suitable for determining the Mn(II)-, Fe(II)-, Co(II)-, and Ni(II)-binding sites of metalloproteins.
Oxidative modifications to the side chains of sulfur-containing amino acids often limit the number of product ions formed during collision-induced dissociation (CID) and thus make it difficult to obtain sequence information for oxidized peptides. In this work, we demonstrate that electron-transfer dissociation (ETD) can be used to improve the sequence information obtained from peptides with oxidized cysteine and methionine residues. In contrast to CID, ETD is found to be much less sensitive to the side-chain chemistry, enabling extensive sequence information to be obtained in cases where CID fails to provide this information. These results indicate that ETD is a valuable technique for studying oxidatively modified peptides and proteins. In addition, we report a unique and very abundant product ion that is formed in the CID spectra of peptides having N-terminal cysteine sulfinic acid residues. The mechanism for this unique dissociation pathway involves a six-membered cyclic intermediate and leads to the facile loss of NH 3 and SO 2 , which corresponds to a mass loss of 81 Da. While the facile nature of this dissociation pathway limits the sequence information present in CID spectra of peptides with N-terminal cysteine sulfinic acid residues, extensive sequence information for these peptides can be obtained with ETD. M ass spectrometry (MS) is widely used for sequencing and identifying amino acid modifications in peptides and proteins. Identifying modifications to proteins is important for a variety of reasons. Post-translational modifications (PTMs) of proteins are necessary for a wide range of cellular functions such as protein trafficking, protein-protein interactions, and transcription. Identifying and pinpointing these modification sites are important for more deeply understanding protein function, both normal and abnormal. In this context, PTMs such as phosphorylation, acetylation, glycosylation, sulfonation, and methylation are important to characterize. Oxidation is another important protein modification that is typically associated with oxidative stress [1-4], but recent work has also shown that protein oxidation can play a regulatory role as well [5]. Furthermore, an increasing number of techniques make use of oxidative labeling to study protein structure. These methods use radicals (e.g., · OH) to modify solvent-exposed [6 -9] or metal-bound amino acids [10 -17], and MS n to identify oxidatively modified residues, typically in conjunction with proteolytic digestion.Very often side-chain modifications to peptides can make sequencing by collision-induced dissociation (CID) difficult. Perhaps the most well known example is the effect of phosphorylation on peptide ion dissociation. The CID spectra of phosphorylated peptides are commonly dominated by a neutral loss of H 3 PO 4 , often with little other sequence information present. Similarly, side-chain oxidation can dramatically affect peptide dissociation patterns and limit sequence information that is available by CID. For example, oxidation of cysteine ...
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