Glutathione-Coated Quantum Dots as Immobilized Substrates for the Evaluation of Glutathione-Dependent Enzymes: Glutathione Reductase and Glutathione S-Transferase.

Quantum dots (QDs), owing to their unique optical properties and tunable surface functionalities, have emerged as powerful tools in biomedical research, particularly in diagnostics, drug delivery, and bioimaging. Among them, glutathione (GSH)-coated silver sulfide (Ag2S) QDs represent a promising class of biocompatible nanomaterials with near-infrared (NIR) fluorescence, making them highly suitable for biological applications. This chapter presents a novel approach utilizing GSH-coated Ag2S QDs as immobilized substrates and substrate analogues for the evaluation of glutathione-dependent enzymes glutathione reductase (GR) and glutathione S-transferase (GST). Through kinetic analyses, both purified enzymes and biologically relevant enzyme sources, such as rat liver lysate and cultured cell lysates, were employed to investigate the interaction efficiency and enzymatic activity with QD-bound GSH. The results demonstrated that GSH-coated QDs not only participate in enzyme-catalyzed redox cycling but also retain functionality comparable to free GSH or oxidized glutathione (GSSG), particularly under physiological conditions. Additionally, microscopic imaging confirmed the internalization of these QDs in mammalian cells, indicating their potential for in vivo enzymatic assays and theranostic applications. Overall, this study introduces a robust and innovative nanobiotechnological platform to explore enzyme-substrate interactions and redox regulation, with significant implications for biosensing, cancer research, and nanomedicine.