Effects of weakly basic amines on proteolytic processing and terminal glycosylation of secretory proteins in cultured rat hepatocytes

Abstract

We examined the effects of weakly basic amines on the secretion and post-translational modifications of secretory proteins in cultured rat hepatocytes. Weakly basic amines such as methylamine, chloroquine and NH4Cl strongly inhibited not only protein secretion, but also the proteolytic conversion of a proform of complement C3, allowing the precursor to be released into the medium. The amines, however, had no effect on the proteolytic conversion of prohaptoglobin into its subunits. Since available evidence indicates that the conversion of pro-C3 occurs at the Golgi complex while that of prohaptoglobin takes place in the endoplasmic reticulum, it is most likely that the weak bases specifically affect the proteolytic event occurring at the Golgi complex. Electron microscopic observations confirmed that the amines caused morphological changes of the Golgi complex, consisting of dilated cisternae and swollen vacuoles. When the glycosylation of alpha 1-protease inhibitor and haptoglobin was examined, it was found that the amines caused a marked accumulation in the cells of both glycoproteins corresponding to the mature secreted forms. Neuraminidase digestion demonstrated that the glycoproteins accumulating in response to the amines had acquired terminal sialic acid. The results indicate that the amines do not significantly affect terminal glycosylation, in contrast with their definite effect on proteolytic processing, despite the fact that both modifications take place in the Golgi complex.