Effect of Penicillin on Thyroxine-Binding by Plasma Proteins

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Abstract
Penicillin increases the dialysis of thyroxine-I131 across a semipermeable membrane from a compartment of diluted plasma to a compartment consisting of aqueous buffer. Penicillin also enhances the uptake of thyroxine-I131 by red cells incubated with plasma. These observations indicate that penicillin interferes with the binding of thyroxine by plasma proteins. Paper electrophoresis of plasma containing thyroxine-I131 and penicillin indicates that penicillin inhibits primarily the binding of thyroxine to the prealbumin carrier of this hormone. Furthermore, penicillin in high concentrations induces a widening of the albumin band of stained electrophoretic strips of whole plasma and aqueous solutions of crystalline human albumin. All observed penicillin effects on human plasma can be reversed if penicillin is subsequently dialyzed out of the plasma sample. Nevertheless, the addition of progressive concentrations of nonradioactive thyroxine does not abolish the penicillin-induced inhibition of prealbumin binding. Thus, the interaction of penicillin and thyroxine at the prealbumin binding site, though reversible by dialysis, is not one of competitive binding.