Functional Role of Sialic Acid in IgG Binding to Microvillus Membranes in Neonatal Rat Intestine

A close parallelism exists between sialylation and endocytotic activity of the small intestine during postnatal development in rats. Thus, the binding of ¹²⁵I-labelled IgG to microvillus membranes and its relationship to membrane sialic acid has been studied in suckling rat intestine, during (a) postnatal development; (b) cortisone-induced precocious maturation, and (c) after desialylation of brush borders by neuraminidase treatment. Neuraminidase-treated membranes exhibited low (42%, p < 0.001) IgG binding. The observed decrease in IgG binding to desialylated membranes was associated with a decrease in the value of affinity constant, (–Ka = 0.4 × 10⁶ <i>M</i>^–1 in control and 0.23 × 10⁶ <i>M</i>^–1 in desialylated membranes). The number of IgG-binding sites (2.3 nmol/mg protein) was unchanged under these conditions. A similar decrease (50%) in IgG binding to brush borders was also observed in cortisone-injected pups. This was associated with reduced sialic acid (37%) content of the membranes compared to the controls. The value of –Ka was reduced from 0.4 × 10⁶ <i>M</i>^–1 in the control to 0.3 × 10⁶ <i>M</i>^–1 in the hormone-injected pups. The number of binding sites (n) was decreased from 2.2 to 1.4 nmol/mg protein under these conditions. Low concentrations of calcium (0.1–1.6 m<i>M</i>) in the incubation medium enhanced IgG binding (p < 0.001) to brush borders in pups but there was no change in binding of IgG to the membranes at 2 m<i>M</i> Ca²⁺ concentration compared to controls. Addition of Zn²⁺ or Mg²⁺ did not affect IgG binding under these conditions. These findings suggest a functional role of Ca²⁺ and sialic acid residues of the membrane glycans in IgG-receptor interactions in suckling rat intestine.