Redox cycling of aromatic hydrocarbon quinones catalysed by digestive gland microsomes of the common mussel (Mytilus edulis L)

The NAD(P)H-dependent redox cycling of six aromatic hydrocarbon quinones (1,4-benzoquinone, tetramethyl-1,4-benzoquinone (duroquinone), 1,2- and 1,4-naphthoquinones, 9,10-phenanthrenequinone, anthraquinone) by digestive gland microsomes of M. edulis was assessed in terms of oxygen consumption (Clark electrode) and reactive oxygen species (ROS) production (conversion of superoxide anion radical, O-2(-) and hydrogen peroxide, H2O2 to hydroxyl radical, . OH-the latter detected by oxidation of 2-keto-4-methiolbutyric acid (KMBA) to ethylene); additionally, oxygen consumption was determined for the known ROS-generating 2-methyl-1,4-naphthoquinone (menadione) and benzo[a]pyrene diones (1,6-, 3,6- and 6,12-). Stimulated oxygen consumption was detectable for duroquinone, naphthoquinones, menadione and phenanthrenequinone only, whereas stimulated ROS production was evident for all ten quinones, indicating that the latter is a more sensitive measure of redox cycling. ROS production was greatest for 1,2-naphthoquinone and least for anthraquinone. Stimulated ROS production and/or oxygen consumption for the six aromatic hydrocarbon quinones was greater for the NADH- than the NADPH-dependent reactions. Michaelis-Menten kinetics with respect to quinone concentration were evident for both stimulated oxygen consumption and ROS production, and correlations between the two measurements provide strong evidence for the process of redox cycling (i.e. conversion of O-2 to O-2(-)) occurring in digestive gland microsomes. Differences were also seen between the profiles for the two measurements, particularly for 1,4-naphthoquinone, indicative of other processes of oxygen consumption and/or ROS production occurring. Comparison of different quinones indicates that enzyme specificity and quinone structure are factors in determining ROS production. Overall, the results indicate a wide pro-oxidant potential for quinones formed from aromatic hydrocarbons by biotransformation and photo-oxidation processes.