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Structure-Activity Relationships for Induction of Peroxisomal Enzyme Activities By Phthalate Monoesters in Primary Rat Hepatocyte Cultures

Rat hepatocytes were cultured for 70 hours with a series of four isomeric octyl and five isomeric hexyl phthalate monoesters, and their effects on peroxisomal fatly acid β-oxidation (palrnitoyl-CoA oxidation) and carnitine acetyltransferase activities determined. All nine monoesters produced dose-related increases in enzyme activities and marked quantitative compound potency differences were observed. Generally octyl isonters were more potent than hexyl isomers and 2- and 3-etlryl substituted isomers were more potent than their straight chain and 1-ethyl substituted analogs. For example, mono(2-ethylhexyl)phthalate was more potent than mono(1-ethylhexyl)phthalate, and this was also observed after oral administration of the two isomers to rats for seven days. The cell culture data for induction of palmitoyl-CoA oxidation were used to generate quantitative structure-activity relationships. Relatively poor correlations were observed between biological activity and simple hydrophobic parameters, but a good correlation was obtained when compound electronic structural parameters, obtained by molecular orbital calculations, were employed. These studies demonstrate relationships between biological activity and chemical structure for a series of phthalate monoesters and indicate the potential usefulness of primary rat hepatocyte cultures to screen compounds for peroxisome proliferation.