Evidence for metabolism of 3-methylindole by prostaglandin H synthase and mixed-function oxidases in goat lung and liver microsomes

Abstract

3-Methylindole (3MI) is the causative agent of a naturally occurring lung disease in cattle. The effects of 3MI on the prostaglandin H synthase (PHS) and mixed-function oxidase (MFO) systems in lung and liver microsomes were investigated. Addition of 3MI in goat lung microsomes resulted in a pronounced increase in PHS activity as indicated by both the initial rate and total oxygen consumption. The effect of 3MI on PHS activity was dependent on arachidonic acid and inhibited by indomethacin. PHS was capable of activating [ 14C]3MI to a reactive intermediate as indicated by the covalent binding of 3MI to microsomal protein. [ 14C]3MI was converted to water-soluble 3MI metabolites in the PHS system. Biosynthesis of total prostaglandins was enhanced 69% with 0.5 mM 3MI. PHS-catalyzed co- oxidation of 3MI was shown to be independent of the MFO-catalyzed metabolism of 3MI. PHS and MFO enzyme activities were found to exist in both goat lung and liver microsomes. PHS activity was significantly higher in lung microsomes than in liver microsomes. Pulmonary MFO activity, unlike hepatic MFO activity, was enhanced with addition of 3MI. This indicates a tissue selectivity for the metabolism of 3MI. Lung microsomal PHS and MFO systems were capable of activating 3MI to a greater extent than liver microsomes. Thus, the combined effects of PHS and MFO systems in the activation of 3MI, the selectivity of pulmonary MFO to metabolize 3MI, and the capacity for altered prostaglandin biosynthesis in lung tissue may explain the tissue specificity of 3MI-induced toxicity.