Endocrine Abstracts

The influence of the plant lectins on binding of IGF-I, IGF-II and insulin to the cognate receptors from solubilised human placental cell membranes was examined. The lectins (wheat germ agglutinin, WGA; concanavalin A, Con A; phytohaemagglutinin, PHA and Sambucus nigra agglutinin, SNA) were chosen according to their sugar specificity and were expected to bind to the most abundant saccharides present as terminal residues on N-glycan moieties of insulin receptor (IR) and IGF receptors (IGFRs): GlcNAc, Man, Gal and Sia. The lectin effects were tested using competitive ligand binding assays (CLBA) in which the binding of ¹²⁵I-labelled insulin or IGFs (tracers) to solubilised receptors, as competed by unlabelled insulin and IGFs, was carried out in the absence or presence of the lectins. WGA, Con A and PHA exerted different effects on the binding of the different tracers to the receptors, ranging from an inhibition to a significant enhancement of binding. These effects were also lectin specific and they could be blocked by specific sugars. The effect of a particular lectin depended on the order of addition of the receptors, ligands and lectins. The competition curves had different shapes for different lectins indicating the different mechanisms involved in the change of receptor affinity. The results of CLBA were in accordance with the results obtained from the lectin affinity chromatography and electrophoresis, where multiple glycoforms of the IR and IGFRs were shown to exist. The data from this work strongly suggested the change in affinity and specificity of placental IR and IGFRs for the homologous and heterologous ligands, under the influence of specific plant lectins. The fact that some plant lectins can modulate the interaction of placental IR and IGFRs with their ligands may imply the existence of placental lectins of the similar sugar specificity with the same function.