Kupffer cells carry on the cell surface a lectin-like receptor with specificity for D-galactose/D-glucose residues. After tryptic digestion of receptors, new lectin activity appears on the plasma membrane within 15 min. Protein biosynthesis de novo is not involved. This indicates the transfer of new lectin molecules stored within the cell to the cell surface. Kupffer cells can enter a physiological state where receptor recycling does not take place.
Kupffer cells isolated from the rat liver are able to bind neuraminidase-treated rat erythrocytes via a D-galactose-specific receptor on the cell surface. Binding of desialylated erythrocytes was inhibited by several mono- and oligo-saccharides related to D-galactose, but not by unrelated sugars. However, after phosphorylation at position 6 D-glucose was as good an inhibitor as D-galactose. Two synthetic glycoproteins, D-galactosyl-albumin and, at a higher concentration, D-glucosyl-albumin, strongly inhibit cell contacts. Lectin-mediated binding of desialylated erythrocytes is dependent on the presence of Ca2'ons, but independent of ATP formation and cell motility. It is concluded that binding of desialylated erythrocytes by rat Kupffer cells is mediated by a Ca2-dependent D-galactosyl/D-glucosyl-recognition system.
The enzyme-substrate complex formed between pyridoxamine-pyruvate transaminase (EC 22.214.171.124) and pyridoxal was reduced with NaBH4. After carboxymethylation and tryptic digestion, pyridoxyl-lysine-containing peptides were isolated by a combination of Sephadex and Dowex 50 chromatography. Analysis of these peptides shows the structure around the pyridoxal-binding lysine residues to be Ala-Asp-Ile-Tyr-Val-Thr-Gly-Pro-Asx-Lys(Pxy)-Cys-Leu(Pro2, Gly2, Ala2, Met)(Thr, Leu2)Gly-Val-Ser-Glu-Arg. This structure differs from those found for the corresponding peptides from pyridoxal phosphate-dependent enzymes.