Differences in the Binding of Thyroid Hormones and Indoles by Rat α1‐Fetoprotein and Serum Albumin

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Abstract
The transport of small molecules in the blood, normally assured by serum albumin in the adult, is not well known in the fetus since the albumin concentration is low in fetal serum and inversely related to the .alpha.1-fetoprotein concentration. To investigate whether rat .alpha.1-fetoprotein might be a fetal counterpart to albumin, the binding properties of these 2 proteins were compared with respect to a series of molecules of biological importance, especially during fetal development: thyroid hormones and indole analogs. Though high-affinity binding of thyroxine was found with both rat .alpha.1-fetoprotein and albumin, a significant difference in the number of binding sites for this hormone was found with the 2 proteins. While rat serum albumin strongly bound L-tryptophan and indolyl-3-acetic acid (Ka .apprxeq. 105 M-1), rat .alpha.1-fetoprotein did not bind any of the indoles tested. These results are discussed with respect to the physiological and pharmacological significance of the transport role of these proteins.