No abstract

The ability of human serum albumin to capture unbound copper under different clinical conditions is an important variable potentially affecting homeostasis of this element. Here, we propose a simple procedure based on size-exclusion chromatography with on-line UV and nitrogen microwave-plasma atomic-emission spectrometry (MP-AES) for quantitative evaluation of Cu(II) binding to HSA upon its glycation in vitro. The Cu-to-protein molar ratio for non-glycated albumin was 0.98 ± 0.09; for HSA modified with glyoxal (GO), methylglyoxal (MGO), oxoacetic acid (GA), and glucose (Glc), the ratios were 1.30 ± 0.22, 0.72 ± 0.14, 0.50 ± 0.06, and 0.95 ± 0.12, respectively. The results were confirmed by using ICP-MS as an alternative detection system. A reduced ability of glycated protein to coordinate Cu(II) was associated with alteration of the N-terminal metal-binding site during incubation with MGO and GA. In contrast, glycation with GO seemed to generate new binding sites as a result of tertiary structural changes in HSA. Capillary reversed-phase liquid chromatography with electrospray-ionization quadrupole-time-of-flight tandem mass spectrometry enabled detection and identification of Cu(II) coordinated to the N-terminal metal-binding site (Cu(II)-DAHK) in all tryptic digests analyzed. This is the first report confirming Cu(II)-DAHK species in HSA by means of high-resolution tandem mass spectrometry, and the first report on the use of MP-AES in combination with chromatographic separation.

It has been reported that glycation of human serum albumin (HSA) changes its capability for copper binding whereas the increase of free copper might have an impact on protein glycation - a key process in diabetes progression. In this work, proteomic analysis of non-glycated HSA and HSA glycated with methylglyoxal (MGo) in the absence or in the presence of Cu(ii) (0.1; 1.0; 5.0 mg Cu L) has been undertaken. Trypsin hydrolysates were subjected to capillary HPLC-ESI-QTOF-MS and MS/MS. Raw data were analyzed using two proteomic platforms: MaxQuant () and ProteinScape (Bruker). Considering seven MGo-derived modifications, the sequence coverage was 98% for non-modified HSA and ≥93% for HSA incubated with MGo or MGo + Cu(ii). Peptide mapping yielded 76 identical peptides in all samples though important differences were found between non-modified HSA and protein glycated with or without Cu(ii). Overall, 46 peptides with residues from 1 to 3 modified were detected/sequenced; the MGo-derived modifications found were: hydroimidazolone, argpyrimidine, N-carboxyethyl-lysine and S-carboxyethyl-cysteine; 39 modified sites were identified (22 on arginine, 12 on lysine, and 5 on cysteine) and among them, 27 were common for ProteinScape and MaxQuant. The count of the modified peptides and the comparative analysis of their abundance in different samples indicated that Cu(ii) at physiological and sub-physiological concentrations inhibited HSA glycation as compared to the glycation of the Cu-devoid protein; at higher concentrations (5 mg Cu L), this inhibitory effect tends to be inverted. The results obtained suggest that increased protein glycation might be associated with Cu-deficiency and with excessive Cu(ii) concentrations, calling for more detailed studies performed on real-world samples with a strict control of copper concentration.