Characterization of quinone derived protein adducts and their selective identification using redox cycling based chemiluminescence assay.

The cytotoxic mechanism of many quinones has been correlated to covalent modification of cellular proteins. However, the identification of relevant proteins targets is essential but challenging goals. To better understand the quinones cytotoxic mechanism, human serum albumin (HSA) was incubated in vitro with different concentration of menadione (MQ). In this respect, the initial nucleophilic addition of proteins to quinone converts the conjugates to redox-cycling quinoproteins with altered conformation and secondary structure and extended life span than the short lived, free quinones. The conjugation of MQ with nucleophilic sites likewise, free cysteine as well as ɛ-amino group of lysine residue of HSA has been found to be in concentration dependent manner. The conventional methods for modified proteins identification in complex mixtures are complicated and time consuming. Herein, we describe a highly selective, sensitive, simple, and fast strategy for quinoproteins identification. The suggested strategy exploited the unique redox-cycling capability of quinoproteins in presence of a reductant, dithiothreitol (DTT), to generate reactive oxygen species (ROS) that gave sufficient chemiluminescence (CL) when mixed with luminol. The CL approach is highly selective and sensitive to detect the quinoproteins in ten-fold molar excess of native proteins without adduct enrichment. The approach was also coupled with gel filtration chromatography (GFC) and used to identify adducts in complex mixture of proteins in vitro as well as in rat plasma after MQ administration. Albumin was identified as the main protein in human and rat plasma forming adduct with MQ. Overall, the identification of quinoproteins will encourage further studies of toxicological impact of quinones on human health.
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