A tandem MS precursor-ion scan approsch to identify variable covalent modification of albumin Cys34 : a new tool for studyng vascular carbonylation

A LC-ESI-MS/MS approach based on the precursor ion scanning technique was developed and evaluated to characterize the covalent modifications of Cys34 human serum albumin (HSA) induced by oxidative stress and in particular by reactive carbonyl species (RCS). The approach involves the isolation and enzyme digestion of HSA, thus to generate a suitable length peptide (LQQCPF) containing the modified tag residue. The adducted LQQCPF peptides are identified by LC-ESI-MS/MS in precursor ion scan mode and then characterized in product ion scan mode. The product ions used in the precursor ion scanning were selected by studying the MS/MS fragmentation of a series of LQQCPF derivatives containing Cys34 adducted by different ,-unsaturated aldehydes and di- and keto-aldehydes. The specificity of the method was increased by a Boolean logic which reconstitutes a virtual Current Trace showing the peaks present in the three precursor ion scans and characterized by the same parent ion. The method was firstly evaluated to identify and characterize the Cys34 covalent adducts of HSA incubated in the presence of 4-hydroxy-hexenal, 4-hydroxy-trans-2-nonenal (HNE) and acrolein (ACR). Then, it was applied to study the Cys34 modification of human plasma incubated in the presence of mildly oxidized LDL, and the LQQCPF adducts with HNE and ACR were easily identified. In other experiments, plasma was oxidized by AAPH or by Fe2+/H2O2. In both conditions, the sulfinic derivative of LQQCPF was identified and characterized, to demonstrate that the method here reported is suitable not only to study RCS modified albumin, but also to monitor the oxidative state of Cys34 as a marker of oxidative damage.