On the Nature of Plasma Iodine

Despite the fact that crystalline thyroxine simulates all of the effects of the thyroid hormone, investigators have hesitated to state categorically that thyroxine is the actual form in which the circulating thyroid hormone exists.Since the minute amount of iodine in plasma makes it very difficult to isolate an iodine-containing compound, we have employed the radio-.activeisotope of iodine, I 131, to study the nature of the iodine contained in plasma.Rats were injected with 75 microcuries of carrier-free 1131 and plasma was removed 24 hours later.The plasma was extracted three times with 5 volumes of butyl alcohol at room temperature.Clear butyl alcohol extracts were obtained by centrifugation.Plasma F was found to pass quantitatively into the butyl alcohol fraction under these conditions.Moreover, only about 10 per cent of this iodine could be reextracted from the butyl alcohol with 4 N NaOH-5 per cent NazC03, a reagent which extracts inorganic iodine and diiodotyrosine iodine.These experiments demonstrate that approximately 90 per cent of the plasma iodine of the rat behaves like thyroxine in its solubility properties.Further identification of plasma iodine with thyroxine was effected by two separate procedures.25 mg. of three times recrystallized thyroxine (Squibb) were added as carrier to the butyl alcohol extract and the latter was concentrated to dryness on a boiling water bath under reduced pressure.The thyroxine in the residue was repeatedly recrystallized and the specific activity of the I"' determined after each recrystallization.The constancy of the specific activity is shown in Table I.It appears from these results that the 1131 in the butyl alcohol fraction of plasma is in the same chemical form as the material which underwent recrystallization.Especially is this true since all recrystallizationswere not carried out with the same solvent.The first recrystallization was made from hot 0.1 N K2C03 by the addition of glacial acetic acid, the second from a more concentrated 0.1.N EC+303 solution by cooling to 0" in an ice bath, the third by addition of glacial acetic acid to a 70 per cent alkaline alcohol solution.10 mg. of three times recrystallized thyroxine were added as carrier to the butyl alcohol extract and the latter concentrated to dryness, as described above.The thyroxine residue was then dissolved in 0.1 N NaOH and the solution shaken with either butyl alcohol or isoamyl alcohol.The distribution of the 1131 between the two immiscible solvents was compared with that of the I?*' (Table 11).Within experimental error it was found