Selective hepatic uptake of human β-hexosaminidase A by a specific glycoprotein recognition system on sinusoidal cells

Intravenously administered ¹²⁵I-labeled human β-hexosaminidase A (β-N-acetylglucosaminidase; 2-acetamido-2-deoxy-β-D-glucoside acetamidodeoxyglucohydrolase, EC 3.2.1.30) was rapidly cleared from the circulation of rats and accumulated in the liver. When hepatic cells were subsequently isolted, the label was recovered from both sinusoidal cells and, to a lesser extent, hepatocytes. Clearance was inhibited by the simultaneous infusion of mannan but not by a galactose-terminated glycoprotein. Studies in vitro, in which ¹²⁵I-β-hexosaminidase was incubated with isolated hepatic cells, detected no uptake of the labeled ligand by hepatocytes. In contrast, uptake by sinusoidal cells was shown to be temperature dependent and approached saturability. Prior treatment of sinusoidal cells with Pronase resulted in markedly decreased uptake of ¹²⁵I-β-hexosaminidase by these cells. Mannan and partially deglycosylated glycoproteins bearing terminal nonreducing N-acetylglucosamine or mannose residues were shown to be potent inhibitors of the cellular uptake of ¹²⁵I-β-hexosaminidase; native orosomucoid and desialylated (galactoseterminated) orosomucoid were not inhibitory. Of six simple sugars tested, including N-acetylglucosamine, or mannose was an effective inhibitor of the cellular uptake of ¹²⁵I-β-hexosaminidase. The kinetics of uptake of β-hexosaminidase and mannose-terminated orosomucoid by sinusoidal cells were shown to be similar. These findings suggest that the hepatic uptake of the lysosomal glycosidase β-hexosaminidase A is mediated by a receptor on sinusoidal cells which recognizes and binds mannose-terminated glycoproteins.