Removal of the terminal sialic acid residues from many serum glycoproteins results in exposure of their penultimate galactose residues and rapid clearance from circulation by the liver. Low-density lipoprotein is a glycoprotein containing 21 galactose and 9 sialic acid residues per particle. Studies in this laboratory and others have shown that both the liver and extrahepatic tissues contribute to the degradation of low-density lipoprotein. This study was undertaken to determine whether desialylation of pig low-density lipoprotein alters its removal from circulation. Low-density lipoprotein was incubated at 37 degrees C with an agarose-bound neuraminidase, proteinase-free, from Clostridium perfringens. After 18 h at pH 5.0, 70% of the sialic acid residues were removed. The desialylated 131I-labelled and native 125I-labelled low-density lipoproteins were simultaneously injected into a pig, and their disappearance from plasma was followed for 96 h. The turnovers of the two were identical. In contrast, neuraminidase-treated fetuin was cleared about 200-fold faster than native fetuin. Studies were also performed in cultured rat hepatocytes. Rates of degradation of native and neuraminidase-treated low-density lipoprotein were similar, whereas asialo-fetuin was degraded at six to ten times the rate of native fetuin. Thus desialylation does not appear to alter low-density-lipoprotein catabolism by hepatic or extrahepatic cells.

Human liver alpha-L-fucosidase was purified to apparent homogeneity and analyzed for carbohydrate content primarily by gas-liquid chromatography (glc). The enzyme is about 7% carbohydrate by weight and contains the following sugars (residues per 50, 000 molecular weight subunit): mannose (8.3), glucosamine (4.3) (presumably N-acetylated), sialic acid (1.6) and glucose (1.6). Galactose (0.8) and L-fucose (1.8) were also found but their presence may be due to artifacts of the purification procedure.

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