Mass spectrometric characterization of human serum albumin dimer: A new potential biomarker in chronic liver diseases.

Human serum albumin (HSA) undergoes several structural alterations affecting its properties in pro-oxidant and pro-inflammatory environments, as it occurs during liver cirrhosis. These modificationsinclude the formation of albumin dimers. Although HSA dimers were reported to be an oxidative stressbiomarker, to date nothing is known about their role in liver cirrhosis and related complications. Addi-tionally, no high sensitive analytical method was available for HSA dimers assessment in clinical settings.Thus the HSA dimeric form in human plasma was characterized by mass spectrometry using liquidchromatography tandem mass spectrometry (LC-ESI-Q-TOF) and matrix assisted laser desorption timeof flight (MALDI-TOF) techniques. N-terminal and C-terminal truncated HSA, as well as the native HSA,undergo dimerization by binding another HSA molecule. This study demonstrated the presence of bothhomo- and hetero-dimeric forms of HSA. The dimerization site was proved to be at Cys-34, forming adisulphide bridge between two albumin molecules, as determined by LC–MS analysis after tryptic diges-tion. Interestingly, when plasma samples from cirrhotic subjects were analysed, the dimer/monomer ratioresulted significantly increased when compared to that of healthy subjects. These isoforms could repre-sent promising biomarkers for liver disease. Additionally, this analytical approach leads to the relativequantification of the residual native HSA, with fully preserved structural integrity.