Effects of ammonia on processing and secretion of precursor and mature lysosomal enzyme from macrophages of normal and pale ear mice: evidence for two distinct pathways.

Abstract

Lysosomal enzymes have been shown to be synthesized as microsomal precursors, which are processed to mature enzymes located in lysosomes. We examined the effect of ammonium chloride on the intracellular processing and secretion of two lysosomal enzymes, beta-glucuronidase and beta-galactosidase, in mouse macrophages. This lysosomotropic drug caused extensive secretion of both precursor and mature enzyme forms within a few hours, as documented by pulse radiolabeling and molecular weight analysis. The normal intracellular route for processing and secretion of precursor enzyme was altered in treated cells. A small percentage of each precursor was delivered to the lysosomal organelle slowly. Most precursor forms traversed the Golgi apparatus, underwent further processing of carbohydrate moieties, and were then secreted in a manner similar to secretory proteins. The lag time for secretion of newly synthesized beta-galactosidase precursor was notably longer than that for the beta-glucuronidase precursor. The source of the secreted mature enzyme was the lysosomal organelle. Macrophages from the pale ear mutant were markedly deficient in secretion of mature lysosomal enzyme but secreted precursor forms normally. These results suggest that ammonia-treated macrophages contain two distinct intracellular pathways for secretion of lysosomal enzymes and that a specific block in the release of lysosomal contents occurs in the pale ear mutant.