Abstract
While the hydrophobicity of a molecule principally determines the Bioconcentration potential of an organic chemical in fish, lack of bioconcentration would be also expected for a chemical with large molecular size due to its restricted permeability through biological membranes. In this review, the later factor relevant to the steric hindrance of a chemical in bioconcentration process is surveyed. It is clarified that a hydrophobic chemical is uptaken mainly via the transcellular pathway through respiratory cells in the secondary lamellae of gills, and thus, the steric size of a molecule determines the permeability controlled by the simple diffusion. There are many kinds of threshold values proposed for uptake in terms of the steric size of chemicals such as molecular weight (MW), molecular volume (MV), effective cross sectional diameter (Deft), and maximum diameter (Dmax) . Among them, precise predictions for lack of bioconcentration are represented when MW of 700 is assumed to be a threshold value for uptake of a variety of chemicals, such as the existing chemicals under the Chemical Substances Control Law Japan, etc. Thus, this MW of 700 could be one of the most practical, but still simple trigger values for the prediction of uptake limitation. When a chemical has high density atoms such as fluorine in a molecule with high substitution ratio, however, MW would overestimate the steric size of the molecule. In such a case, combined use of more direct steric descriptors such as MV would be needed to make a precise prediction.
