A Reduced Model for Bioconcentration and Biotransformation of Neutral Organic Compounds in Midge

A bioconcentration factor (BCF) database and a toxicokinetic model considering only biota-water partitioning and biotransformation were constructed for neutral organic chemicals in midge. The database contained quality-reviewed BCF and toxicokinetic data with variability constrained to within 0.5 to 1 log unit. Diverse conditions in exposure duration, flow set-up, substrate presence, temperature, and taxonomic classification did not translate into substantial variability in BCF, uptake rate constant (k₁ ), or depuration rate constant (k_T ), and no systematic bias was observed in BCFs derived in unlabeled versus radiolabeled studies. Substance-specific biotransformation rate constants k_M were derived by difference between the calculated biota-water partitioning coefficient (K_BW ) and experimental BCF for developing a midge biotransformation model. Experimental midge BCF was modeled as BCF = K_BW /(1 + k_M/ k₂ ) with log k_M (k_M in h^-1 ) = -0.37 log K_OW - 0.06T (in K) + 18.87 (root mean square error [RMSE] = 0.60), log k₁ (k₁ in L kg_wet.wt^-1  h^-1 ) = -0.0747 W (body weight in mg_wet.wt ) + 2.35 (RMSE = 0.48). The K_BW value was estimated using midge biochemical composition and established polyparameter linear free energy relationships, and the diffusive elimination rate constant (k₂ ) was computed as k₂  = k₁ /K_BW. The BCF model predicted >85% of BCFs that associated with neutral organic compounds (log K_OW  = 1.46 - 7.75) to within 1 log-unit error margin and had comparable accuracy similar to amphipod or fish models. A number of outliers and critical limitations of the k_M model were identified and examined, and they largely reflected the inherent limitation of difference-derived k_M , the lack of chemical diversity, and inadequate temperature variation in existing data. Future modeling efforts can benefit from more BCF and toxicokinetic observations of BCF on structurally diverse chemicals for model training, validation, and diagnosis. Environ Toxicol Chem 2021;40:57-71. © 2020 SETAC.