Acid hydrolases in early and late endosome fractions from rat liver

We have examined the distribution of the cation-independent mannose 6-phosphate receptor and five acid hydrolases in early and late endosomes and a receptor-recycling fraction isolated from livers of estradiol-treated rats. Enrichment of mannose 6-phosphate receptor mass relative to that of crude liver membranes was comparable in membranes of early and late endosomes but was even greater in membranes of the receptor-recycling fraction. Enrichment of acid hydrolase activities (aryl sulfatase, N-acetyl-beta-glucosaminidase, tartrate-sensitive acid phosphatase, and cholesteryl ester acid hydrolase) and cathepsin D mass was also comparable in early and late endosomes but was considerably lower in the receptor-recycling fraction. The enrichment of two acid hydrolases, acid phosphatase and cholesteryl ester acid hydrolase, in endosomes was severalfold greater than that of the other three examined, about 40% of that found in lysosomes. Acid phosphatase and cholesteryl ester acid hydrolase were partially associated with endosome membranes, whereas cathepsin D was found entirely in the endosome contents. These findings raise the possibility that lysosomal enzymes traverse early endosomes during transport to lysosomes in rat hepatocytes and suggest that the greater enrichment of some acid hydrolases in endosomes is related to their association with endosome membranes. Despite the substantial enrichment of lysosomal enzymes in hepatocytic endosomes, we found that two, cholesteryl ester acid hydrolase and cathepsin D, did not degrade cholesteryl esters and apolipoprotein B-100 of endocytosed low density lipoproteins in vivo, presumably because they are inactive at the pH within endosomes.