A nonincinerative rate-sensing method for the determination of iodine in iodoproteins

Abstract

The method described here for iodine determination in iodoproteins deals both with a nonincinerative treatment of samples and with the application of a kinetic method of analysis to the iodine-catalyzed ceric-arsenite redox reaction. This method represents an improvement of existing methods, with new significant modifications, which applies especially to thyroid iodoproteins in the range of 10–1000 pmol of iodine. The procedure proposed is essentially composed of two steps: (1) treatment of samples with bromine in an acidic solution; (2) colorimetric estimation of the iodine produced by measuring the rate of decrease of the cerium(IV) color. The change in absorbance at 410 nm, due to the reduction reaction, is monitored photometrically. The reaction rate, which is proportional to the concentration of iodine, is derived from the slope of the absorbance change plotted versus time. The method allows: (i) a significant time saving, since the procedure requires less than 30 min; (ii) the use of optical plastic cuvettes as reaction cells, so that any transfer of sample is avoided; (iii) the use of common laboratory equipment, no special ashing or analytical apparatus being necessary. Moreover, the kinetic approach ensures precision and accuracy of the measurements. This procedure is proposed as a simple process for the sensitive determination of the iodine in iodoproteins without separation or the need for alkaline or acid digestion. The procedure is also suitable for iodine salts and iodoamino acids.