Receptor-mediated endocytosis of apamin by liver cells

Authors


Correspondence to P. N. Strong MRC Receptor Mechanisms Research Group, Department of Pharmacology, University College London, Gower Street, London WC1 6BT, England

Abstract

The binding and uptake of the bee venom toxin apamin, by guinea-pig and rat liver were studied.

Guinea-pig liver plasma membranes contain inhibitable, high-affinity binding sites for [125I]monoiodoapamin: Kd= 12.6±0.8 pM (SE); Bmax= 4.2±0.2 fmol/mg protein. No binding sites for [125I]monoiodoapamin on rat liver plasma membranes were detected in agreement with the absence of a physiological response to the toxin by rat hepatocytes.

[125I]Monoiodoapamin, injected into the portal vein of guinea-pigs, was recovered in an undegraded form in a liver endosome fraction. The uptake of [125I]monoiodoapamin by rat livers was less than 4% of that taken up by guinea-pig livers and there was little evidence of radiolabelled toxin appearing in isolated rat endocytic vesicles.

Inhibitable, high-affinity binding sites for [125I]monoiodoapamin were also identified on isolated guinea-pig liver endosomal membranes; Kd= 10.6±3.3 pM; Bmax= 2.5±0.6 fmol/mg protein. No inhibitable apamin binding sites were detected on rat endosomal membranes.

Plasma membranes and endosomal membranes isolated from guinea-pig liver showed a similar spectrum of polypeptides to that previously reported for plasma membranes and endosomal membranes isolated from rat liver. The enzymatic composition of guinea-pig endosomes was also similar to that previously reported for rat endosomes.

The results indicate that apamin was internalised by receptor-mediated endocytosis by guinea-pig liver cells in an analogous manner to that already shown for a variety of endogenous ligands.

Ancillary