Abstract
Three classes of cobalamin-binding proteins are known -intrinsic factor which is secreted from gastric mucosa and is necessary for intestinal absorption of a physiological dose of cobalamin, transcobalamin (formerly designated as transcobalamin II) which transports cobalamin in blood stream to peripheral tissues and haptocorrin (also called as R-binder) which is found in various body fluids and binds endogenous cobalamin. We estimated concentrations of cobalamin and cobalamin-binding proteins in serum of normal and various disorders and observed that both increased markedly in chronic myelogenous leukemia. Physicochemical properties of these cobalamin-binders were investigated by gel filtration and various electrophoretic techniques. Shift of isoelectric point of intrinsic factor toward acidic side on binding of cobalamin was demonstrated by elaborate devices. Haptocorrin was purified by affinity chromatography from human colostrum. Using antiserum produced against the purified haptocorrin, we established a radioimmunoassay system for haptocorrin which enabled estimation of concentration of serum haptocorrin even after an injection of a large dose of cobalamin to patients for the purpose of therapy. Intestinal receptor for intrinsic factor was purified using solidified cobalamin-intrinsic factor. Antiserum obtained by immunizing a rabbit with the purified receptor made a radioimmunoassay for the receptor sensitive and specific. Studies on characteristics of solubilised receptor for transcobalamin from cortex of hog kidney suggested the existence of two mechanisms to take up cobalamin-transcobalamin in kidney - a low-specific mechanism with high capacity reported by Lindemans et al. and a specific mechanism for transcobalamin with low capacity reported by us. Human seminal plasma showed high capacity to bind cobalamin which derived from both transcobalamin and haptocorrin. The haptocorrin comprised of many isoproteins with extremely low isoelectric points. Physiological significances of them are, however,m still largely unknown.
