The synthesis of rat liver lysosomes: III. chemical composition of microsomal and lysosomal β-glucuronidases purified from rat liver

The microsomal (pI 6.9-7.6) and lysosomal (pI 5.8-6.6) β-glucuronidases [EC 3.2.1.31] purified from rat liver by the method of Himeno et al. (1) were subjected to chemical analyses. The enzymes had very similar amino acid compositions and did not contain sulfur-containing amino acids. The microsomal β-glucuronidase contained 6.5% carbohydrate, consisting of mannose, glucose, fucose, galactose, and glucosamine in a ratio of 69; 12; 3; 3; 25. Sialic acid was not detected in this enzyme. The lysosomal enzyme contained 5.9% carbohydrate, consisting of mannose, glucose, fucose, galactose, and glucosamine in a ratio of 57; 14; 3; 4; 23, with a trace of sialic acid. To confirm the presence of sialic acid in the lysosomal enzyme, more acidic forms of lysosomal β-glucuronidase with pI values ranging from pH 5.4 to 6.1 were also purified. They contained 0.11% sialic acid and were neuraminidase-sensitive. This suggest that a very small part of the lysosomal enzyme may be a sialoglycoprotein. Since it has been suggested that sialic acid in the lysosomal glycoproteins is cleaved rapidly by autolytic degradation (Goldstone, A. & Koening, H. (1974) Biochem. J. 141, 527-535), the sugar acceptor activity of the purified lysosomal β-glucuronidase was determined using sialyltransferase in the Golgi fraction of rat liver. As the lysosomal β-glucuronidase was not a substrate for the sialyltransferase, the possibility of cleavage of terminal sialic acid by autolytic degradation in probably ruled out. The microsomal β-glucuronidase was also not a substrate for the sialyltransferase. This suggests that sialylation may occur very seldom in β-glucuronidase in the Golgi region. The relationship between the carbohydrate moieties of the microsomal and lysosomal β-glucuronidases is discussed in connection with intracellular transport of β-glucuronidase.