Quantification of in vivo biotransformation of the anionic surfactant C12-2-linear alkylbenzene sulfonate in fathead minnows

Linear alkylbenzene sulfonate (LAS) is a major surfactant in household detergents and enters the environment via the wastewater. Aquatic organisms are thus exposed to LAS and can bioaccumulate this compound. Even though the extent of bioaccumulation is determined by the organisms' capability of metabolizing LAS, little is known about metabolism of LAS in small aquatic organisms. In the present investigation, we present a novel approach to quantify in vivo biotransformation. Fish (fathead minnows [Pimephales promelas]) were exposed to the LAS constituent 2-n-(p-sulfophenyl)-dodecane (C-12-2-LAS). The parent surfactant and its biotransformation product 3-n-(p-sulfophenyl)-butyric acid (C-4-3-SPC) were determined in fish tissue. On the average, the concentration of C-4-3-SPC in fish was 70 to 80% of that of C-12-2-LAS. The first-order one-compartment model of bioconcentration was extended to include biotransformation as an explicit process. Analysis of the C-4-3-SPC/C-12-2-LAS concentration ratio in fish allowed estimating a rate constant for in vivo biotransformation of C-12-2-LAS in fathead minnows. With the estimates of the biotransformation rate constant (k(BT.LAS)) ranging between 0.31 and 0.72/d, biotransformation contributes to more than 40% of the elimination of C-12-2-LAS in fathead minnows. This indicates that biotransformation is a significant process in reducing the bioaccumulation potential of LAS. Moreover, the present investigation demonstrates that the combination of measurements of parent compound and metabolite with an extended bioaccumulation model is a viable approach for quantification of biotransformation in small aquatic test animals.