Labelling of Inulin with Radioactive Iodine

INULIN is used in physiological experiments on renal function and for measuring the volume of extracellular water. Inulin consists of some thirty fructofuranose units linked through C(1) and C(2), corresponding with a molecular weight of about 5,000. The dispersion of molecular weight is thought to be such that nearly all molecules pass freely through capillary walls and the glomerular membrane; apparently inulin is not significantly metabolized during the experiments. Accurate determinations of inulin in biological fluids are required and these are usually colorimetric and involve subtraction of a ‘blank’ value, due to the presence of other carbohydrates. Radioactive labelling of the inulin could greatly facilitate such experiments. Ideally, labelling should cause the minimum change in the molecule. Carbon-14 could be introduced biosynthetically or tritium by exposure to the gas, but iodine-131 is more conveniently counted in many laboratories. We have experimented with the following reaction sequence on primary hydroxyl groups : A satisfactory product was obtained, but the final labelling step, consisting of refluxing the tosyl inulin with sodium iodide in diethyl ketone, is neither a convenient nor an efficient method of labelling. An alternative procedure is the introduction of an allyl ether group followed by addition of iodine at the double bond :