3-Methylindole (skatole) is a metabolite of L-tryptophan formed in the rumen of cattle. There is considerable evidence that 3-methylindole is the cause of fog fever (acute bovine pulmonary emphysema) in cattle. Experimentally, administration of 3-methyl-indole causes acute pulmonary oedema and emphysema in cattle, goat and sheep. The aim of this study was to investigate the pharmaco-kinetics and mechanism of action of 3-rethylindole with special reference to fog fever, the most important respiratory disease problem of grazing adult cattle in Britain. Samples of ruminal fluid taken from different animals and from the same animal on different days showed considerable variations in their ability to metabolize L-tryptophan to 3-rethylindole, (8-78% conversion of L-tryptophan to 3-methylindole, during 24 hours incubation). These variations provided an explanation for the considerable individual variations in the severity of respiratory distress seen in cattle after oral administration of L-tryptophan and observed after sudden change to better grazing. It was suggested that the increased rate of conversion of L-tryptophan to 3-methylindole, rather than the excessive intake of the amino acid may be responsible for the production of the disease. Certain carbohydrates (fructose, glucose, lactose, galactose, sucrose, mannitol, starch and inulin) substances related to carbohydrates (citrate, lactate, acetate and glycerin) and antibacterial agents (ampicillin, penicillin, streptomycin, tetracycline, chlorar-phenicol, sulfaguanidine and sulfamethoxypyridazine) inhibited, to different degrees, the conversion of L-tryptophan to 3-methylindole. An oral dose of L-tryotophan (0.5 g/kg body weight) did not produce respiratory distress in any of four treated adult cattle. Failure of L-tryptophan to produce the disease was attributed to failure of conversion of the amino acid to 3-rrothylindole which was not detected, in plasma of the treated cattle. The absorption of 3-methylindole, after oral administration in cattle, is rapid. 3-Methyloxindole is a metabolite of 3-methylindole in cattle. Mean plasma half lives of 3-methyloxindole, 3-methylindole and 3-methylindole metabolites were estimated to be 11, 16 and 48 minutes respectively. It was concluded that measurement of the concentration of 3-methyl-indole or 3-methylindole metabolites in single plasma samples would not reflect the magnitude of production of 3-rethylindole in the rumen of cattle. 3-Methylindole (40 mug/ml) and 3-methyloxindole (50 mug/ml) did not cause the release of mediators of anaphylaxis from chopped bovine lung preparations (in vitro). 3-Methylindole and 3-methylindole analogues (3-methyloxindole, 3-phenylindole, 5-methylindole and 7-methylindole) (5-640 mug/ml) did not cause contraction of the isolated pulmonary artery, pulmonary vein, trachea bronchus of calves. It was concluded that the initial step in the pathogenesis of the 3-methylindole-induced pulmonary damage does not involve an action on the pulmonary vein or the release of mediators of anaphylaxis in cattle. Radioactivity from tritiated 3-methylindole became covalently bound to tissues of several organs when tritiated 3-methylindole was administered, intravenously, to calves. Lung tissue showed the highest concentration of covalently bound metabolites. In vitro studies showed that this covalent binding is catalyzed by a microsomal enzyme system with the classical characteristics of a cytochrome P-450 dependent mixed function oxidase. This enzyme system activates 3-methylindole to a chemically reactive highly electrophilic metabolite which becomes covalently bound to nucleophilic sites on cellular macromolecules. The 3-methylindole-reactive metabolite can be detoxified by spontaneous and glutathione-S-transferase-catalyzed conjugation with glutathione. Pretreatment of sheep with diethylmaleate (depletes glutathione) and L-cysteine (increases glutathione) was shown to increase and decrease, respectively, the severity of pneumotoxic effect of 3-methylindole. 3-Methyloxindole did not become covalently bound to microsomal proteins, in vitro, and did not cause acute pulmonary toxic effects similar to those produced by 3-methylindole, in vivo, suggesting that the 3-methylindole-reactive metabolite is intermediate between 3-methylindole and 3-methyloxindole. The liver possess an enzyme system capable of catalyzing the covalent binding of 3-methylindole metabolites to microsomal proteins, in vitro. This enzyme system was qualitatively identical to that of the lung. The Michaelis constants of the lung and liver systems were 0.37 and 0.44 mnol 3-methylindole and maximal velocities of microsomal alkylation were 100 and 556 pmol covalently bound 3-methylindole/mg microsomal protein/minute, respectively. These findings suggest that the specificity of 3-methylindole towards the lung is not due to higher rate of reactive metabolite formation in the lung, but is probably due to deficiency of the lung in defence mechanisms against this reactive metabolite
