Fig. 3: Two model compounds—1,4-epoxynaphthalene (selected to represent the epoxy-containing domain of GO) and 1,4-naphthoquinone (representing the carbonyl-containing domain)—can be effectively reduced in either Gamble’s solution or sodium citrate in buffer (pH 7.4), whereas benzoic acid and 2-naphthoic acid (representing the carboxyl-containing domain) cannot be transformed (a).

These results indicate that the bioreduction of GO in the simulated human lung fluids likely involves the ring-opening of epoxy groups (to form hydroxyl groups) and the transformation of carbonyl groups to hydroxyl groups. A schematic illustration of the mechanisms of GO biotransformation in the simulated human lung fluids (b) is proposed based on the reactions of these model compounds