An EPR study of structural perturbations induced by methylindole in the protein and lipid regions of erythrocyte membranes.

3-Methylindole has been shown in previous work to cause pulmonary edema and emphysema in cattle and goats. In this paper, evidence is presented to show that 3-methylindole induces structural perturbations in bovine erythrocyte membranes. The structural perturbations which were induced as a function of 3-methylindole concentration in the membranes were measured by EPR using the attachment of maleimide spin label to the sulfhydryl groups of membrane proteins and by intercalation of methyl-5- doxylstearate, methyl-12-doxylstearate, and methyl-16-doxylstearate into the lipid region. The EPR spectra of the malemide spin-labeled membrane proteins became more immobilized as the concentration of 3-methyl-indole increased. The order parameter describing the EPR spectra of methyl-5-doxylstearate decreased from 0.69 to 0.55 as the concentration of 3-methylindole increased. The acyl chains in the region of the carbon 5 position were converted to a less ordered structure. The EPR-spectra of methyl-12-doxylstearate was a superposition representing at least three tumbling rates. As the concentration of 3-methylindole increased, the major fraction of the methyl-12-doxylstearate probes experienced an increase in tumbling rate and a smaller fraction is observed a strongly immobilized state. The EPR spectra of methyl-16-doxylstearate were not perceptibly changed in the presence of 3-methylindole. The concentration dependence suggests that 3-methylindole preferentially intercalates into the ordered region of the alkyl chains sampled by the methyl-5-doxylstearate. These results confirm that 3-methylindole induced structural changes at the molecular level.