Efficient Trace-labelling of Proteins with lodine

Journal/Book: (Reprinted from Nature Vol. 182 p. 53 only July 5 1958). 1958;

Abstract: National Institute for Medical Research The Ridgeway Mill Hill London N.W.7. IN the methods of iodination currently used only the cationic portion of the iodine molecule becomes bound to the ring structure of tyrosine so that the theoretical efficiency of labelling is 50 per cent. In practice efficiencies are always lower than this and may be only a few per cent when the ratio of iodine to protein used is less than one atom per molecule. Values greater than 50 per cent can be obtained by adding oxidizing agents to liberate iodine from iodide but most if not all of these appear to affect adversely the properties of the labelled protein. Iodine-131 monochloride was therefore prepared and used in dilute aqueous solutions for labelling proteins. Efficiencies were twice those obtained with labelled elementary iodine in similar circumstances. Essentially the same result was achieved by merely adding carrier-free radioactive iodine to inactive carrier iodine monochloride provided the radioiodine used was free of reducing agent. (Iodine-131 condensed into N/50 sodium hydroxide can be obtained for this purpose from the Radiochemical Centre Amersham subject to an extra handling charge. Very little appears to be known about the ionic equilibria obtaining in this solution.) The radioiodine may be added to the carrier iodine monochloride in dilute hydrochloric acid solution before or after conversion of the iodine monochloride to hypoiodite. This step which is indicated by the loss of the characteristic yellow colour of iodine monochloride appears to be a prerequisite for substitution of iodine in the benzene ring of tyrosine. The conversion is best carried out by injecting a glycine buffer of pH 8 5 into the iodine-131 monochloride solution just prior to rapid mixing with the protein. The pH of the protein solution should not be higher than 9 5 since hypoiodite is unstable above this pH. Using this procedure 60-80 per cent of the radioactivity used was bound to proteins provided the molar ratio of iodine monochloride to protein was greater than 2. This applied to plasma albumins and globulins of various species including some antibodies and pathological human globulins. schö

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