Identification of cadmium containing metabolites in HepG2 cells after treatment with cadmium-selenium quantum dots

The transformation of quantum dots (QDs) by organisms has attracted broad attention but remains unclear. Understanding of the metabolites helps to reveal the transformation pathway of QDs. Cd containing-metallothionein (MT) are the main species formed by Cd released from CdSe QDs in HepG2 cells, while speciation analysis of Cd containing MTs remains a challenge because MTs has several subisoforms and can bind with several metals. Herein, we built a hyphenated platform for speciation analysis of QDs in HepG2 cells after treatment with CdSe/ZnS QDs. The Cd-containing MTs were separated in reversed phase high performance liquid chromatography (RP-HPLC) and subsequently online detected by inductively coupled plasma mass spectrometry (ICP-MS) and electrospray ionization quadrupole time-of-flight mass spectrometry (ESI-Q-TOF-MS) parallelly. Four groups of Cd-containing metabolites were found by detecting Cd in ICP-MS. Their structures were identified in ESI-Q-TOF-MS and further confirmed with standards of four subisoforms of MT, including N-terminal acetylation MT2a, N-terminal acetylation MT1e, N-terminal acetylation MT1g and MT1m. Each group of them contains various stoichiometry of Cd/Zn. The metabolites of QDs remain same while the concentrations of each metabolite and its stoichiometry of Cd/Zn vary for different incubation concentration/time. This work provides a new parallel hyphenation technique of HPLC-ICP-MS/ESI-MS with high separation resolution and powerful detection ability, and the obtained results provide detailed metabolism information of QDs in HepG2 cells after treatment of CdSe/ZnS QDs, contributing to deep exploration of the functional mechanisms of QDs in organisms.