Studies on the inhibitory effect of bacitracin on 125I-labelled insulin internalization in the rat hepatocyte.

Previous studies have suggested that transglutaminase has a role in the internalization of some polypeptide hormones and is inhibited by the antibiotic, bacitracin. Bacitracin has been used in insulin-receptor studies to inhibit extracellular degradation of 125I-labelled insulin. The aim of this study was to investigate bacitracin's effect on 125I-labelled insulin-receptor interactions in isolated rat hepatocytes. 1 g/l bacitracin increased cell-associated 125I-labelled insulin insulin at 20, 30 and 37 degrees C (P less than 0.001, 0.0005 and 0.0005, respectively). At 5 and 15 degrees C (internalization does not occur), bacitracin did not affect cell-associated 125I-labelled insulin. The bacitracin effect was concentration dependent, increasing to 2 g/l. Scatchard analysis showed that bacitracin did not alter insulin receptor affinity or number. 1 g/l bacitracin abolished the effect of chloroquine. The increased cell-associated radioactivity with bacitracin was surface-bound in nature. 0.5 g/l bacitracin decreased 125I-labelled insulin degradation in hepatocyte suspensions (P less than 0.001) and in buffer previously incubated with hepatocytes (P less than 0.0005). More 125I-labelled insulin remained associated with cells during dissociation studies at 37 degrees C when the buffer contained 1 g/l bacitracin. Label that appeared in the buffer after 60 min was significantly more intact in the presence of bacitracin (P less than 0.025). These results suggest that bacitracin retards the internalization of 125I-labelled insulin in isolated rat hepatocytes.