IODOTYROSINE-LIKE SUBSTANCES IN HUMAN SERUM

Relative rates of transcapillary movement of free thyroxine, protein-bound thyroxine, thyroxine-binding proteins, and albumin

The rate of appearance of labeled thyroxine (T4) and albumin in lymph from various areas after simultaneous i.v. injection of the labeled substances in conscious ambulatory sheep has been used to estimate the relative rates of transcapillary movement of stable T4 and albumin. Labeled T4 appeared in hepatic lymph at the same rate as albumin. In intestinal and leg lymph, labeled T4 appeared eight and four times as rapidly as albumin indicating that T4 crosses capillaries in these areas independently of and much more rapidly than albumin and other proteins having similar distribution kinetics. The lymph:plasma ratios for all the T4-binding proteins including albumin were very similar in any one area showing that the relative fractional rates of transcapillary movement of these proteins were very similar. Therefore in extrahepatic areas, transcapillary movement of T4 in the protein-bound form was quantitatively much less important than in the free form. The findings support earlier views, recently questioned, that free T4 is of considerable physiological significance.

Materials indistinguishable from iodotyrosines in normal human serum and human serum albumin

Materials indistinguishable from authentic mono- and diiodotyrosines were identified in extracts of normal human serum as well as in extracts of purified human serum albumin. These materials were not found in association with the other serum proteins. Identification of MIT and DIT was made by a technique using rechromatography to constant specific activity, as well as by the Barker wet ash distillation method, which established the compounds in question as being iodinated ones. By two different extraction and chromatographic methods we estimated the amounts of both MIT and DIT present in normal human serum or albumin; the estimates were in good agreement. These compounds together constituted between 19% and 25% of the extractable serum iodine.