Overestimate of 125I-protein uptake from the adult mouse gut.

Gastric instillation of 125I-labelled protein in mice leads to rapid appearance of blood radioactivity which is precipitable with 10% trichloroacetic acid (TCA). The same occurs when Na125I is given by mouth. Parenteral administration of non-radioactive NaI before feeding Na125I or 125I-labelled protein inhibits the appearance of TCA-precipitable radioactivity in the blood. The results presented in this study show that measurement of TCA-precipitable radioactivity in the blood after feeding 125I-labeled protein is not a reliable method for studying the passage of macromolecules across the gastrointestinal barriers unless an excess of iodide is given.