Thyroxine-Protein Interactions I. BINDING OF THYROXI?;E TO HUnIAN SERUhI ALBUiVIIN AIVD ~IODII~IED ALBUiWIKS*

In previous reports (1, 2), it was suggested that the binding of thyroxine to human serum albumin resulted primarily from an electrostatic interaction between anionic portions of the thyroxine molecule and cationic sites on t.he protein. From the reduction in thyroxine binding exhibited by acetylated albumin preparations, it seemed likely that part of the reactive cationic sites were made up of e-amino groups of lysyl residues in the protein (2). The results of an investigation of the effect of guanidination and acetylation of albumin on thyroxine binding are described in this paper. The data provide further support for the involvement of lysyl e-amino groups in the binding reaction.