Iodinated fibroblast β-glucuronidase as a ligand for receptor-mediated endocytosis

Abstract

Antibodies raised to human placental beta-glucuronidase were shown to cross-react with the beta-glucuronidase secreted by mouse 3T3 fibroblasts, but did not react with other lysosomal enzymes. The beta-glucuronidase secreted by 3T3 cells was purified 15000-fold by chromatography on an affinity column made from this antibody and resolved into a single component, of Mr 68000, by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. Iodinated samples of purified enzyme were taken up into mouse peritoneal macrophages by receptor-mediated endocytosis at a rate similar to that calculated previously for unlabelled enzyme, and uptake was competitively inhibited by yeast mannan. Binding of beta-glucuronidase to macrophages was saturable, with a Kd of 7 X 10(-9)l/mol, an affinity comparable with that calculated for the binding of mannosylated bovine serum albumin (Kd 1.3 X 10(-9)l/mol), a ligand specific for mannose receptors. Four times as many molecules of mannosylated albumin (12000) as of beta-glucuronidase (3000), however, bound to each cell. This purification and iodination procedure did not therefore have any adverse effect on the uptake properties of secreted beta-glucuronidase, and provides a ligand with which to investigate binding and specific endocytosis into a range of different types of cell.

Intracellular localization of β-glucuronidase in fibroblasts after direct transfer from macrophages

“…The ,J-glucuronidase secreted by 3T3 fibroblasts was purified from culture medium by affinity chromatography on concanavalin A-Sepharose, followed by antibody affinity chromatography (Dean et al, 1985a). Batches containing 100 ,ug of protein were iodinated by using lodobeads and 1 mCi of Nal25I (Dean et al, 1985a).…”