Uptake of tyramine cellobiose by rat liver

The uptake of ¹²⁵I-tyramine cellobiose (TC) by isolated rat hepatocytes and by total rat liver is markedly higher than that of ¹⁴C-sucrose and ¹²⁵I-PVP, suggesting that TC does not enter the cells by fluid phase endocytosis. The distribution of radioactivity after differential centrifugation shows that the compound is shared out amongst sedimentable structures and unsedimentable fraction. Analysis by isopycnic centrifugation indicates that quickly after its penetration into the cells, most of sedimentable ¹²⁵I-TC is associated with lysosomes. Such an intracellular localization is confirmed by the distributions observed after free flow electrophoresis and by the fact that radioactivity and cathepsin C, a lysosomal hydrolase, are simultaneously released from a mitochondrial fraction treated with glycyl-L-phenylalanine-2-naphthylamide. Pretreatment of the rats with chloroquine, an acidotropic drug that accumulates in lysosomes, prevents to some extent the entry of ¹²⁵I-TC into these organelles. Experiments performed with purified lysosomes show that ¹⁴C-sucrose does not cross the lysosomal membrane when ¹²⁵I-TC accumulates linearly with time in the fractions. These results are explained by supposing that the linkage of tyramine to cellobiose allow the disaccharide to diffuse through the plasma and the lysosome membranes, and that the accumulation of the molecule in these organelles results from its weak basic properties. ¹²⁵I-TC could be an interesting molecule with which to study acidotropism in the whole animal and in isolated and cultured cells.