A sphingomyelin transfer protein in rat tumors and fetal liver.

The binding of the disaccharides methyl beta-D-lactoside and 2-acetamido-2-deoxy-3-O-(beta-D-galactopyranosyl)-beta-D-galactopyranose [beta-D-Gal-(l leads to 3)-D-GalNAc] to peanut agglutinin was studied by ultraviolet difference spectroscopy. The magnitude of the difference spectra varied with the concentration of the carbohydrates; association constants and thermodynamic parameters were determined from titration experiments at different temperatures. The enthalpy and entropy changes for binding of methyl beta-D-lactoside were found to be delta H degree = -65 +/- 4 kJ mol-1, delta S degree = -156 +/- 14 J mol-1 K-1. For beta-D-Gal-(1 leads to 3)-D-GalNAc the observed thermodynamic parameters were delta H degree = -78 +/- 5 kJ mol-1, delta S degree = -177 +/- 16 J mol-1 K-1. For both disaccharides, the enthalpy change upon binding to the lectin is much larger than found for the binding site on peanut agglutinin. The observed parameters are compared with those found for the binding of monosaccharides and oligosaccharides to other lectins and to lysozyme. Molecular models of the minimum energy conformers of beta-D-Gal(1 leads to 3)-D-GalNAc and methyl beta-D-lactoside are used to interpret the interaction of these, and structurally related ligands, with the peanut agglutinin binding site.