Label-free and de-conjugation-free workflow to simultaneously quantify trace amount of free/conjugated and protein-bound estrogen metabolites in human serum

Different chemical forms of sex hormones including free/conjugated metabolites as well as their protein/DNA adducts in human serum are a panel of important indicators of health conditions. It is, however, hard to quantify all species simultaneously due to the lack of general extraction, derivatization, and de-conjugation methods. Here we developed a label-free and de-conjugation-free workflow to quantify 11 free/conjugated estrogen metabolites including depurinating DNA and protein adduct forms of 4-hydroxyestradiol (4OHE2) in human serum. Aceto-nitrile acts as an excellent solvent to purify adducted and non-adducted human serum albumin (HSA) by pre-cipitation as well as to extract free/conjugated metabolites and depurinating DNA adducts from the supernatant by salting-out effect. The adduction level of 4OHE2 on HSA was determined by proteomics; free/conjugated metabolites were quantified by a newly developed microflow liquid chromatography (microflow LC)-nanoelectrospray ionization (nanoESI)-multiple reaction monitoring (MRM) method with high reproducibility (7-22% RSD, n > 3) and sub-picogram levels (0.6-20 pg/mL) of quantification limits (S/N = 8) by using non -pulled capillary as nano-ESI emitter. This workflow was demonstrated to reveal endogenous adduction level of 4OHE2 on HSA as well as circulation levels of free/conjugated metabolites in clinical samples. 4OHE2 in human serum were solely detected as protein-bound form, indicating the merit of such integrated platform covering unstable or active metabolites. Compared to traditional methods using labeling or de-conjugation reaction, this workflow is much simplier, more sensitive, and more specific. Moreover, it can be widely applied in omics to concurrently access various bio-transformed known and un-known markers or drugs.

Automated summary

The study developed a label-free and de-conjugation-free workflow to quantify different chemical forms of sex hormones in human serum. This workflow was found to be simpler, more sensitive, and more specific compared to traditional methods, and it can be widely applied in the study of various bio-transformed markers or drugs.