BINDING OF THYROID HORMONE IN RABBIT SERUM: DEFINITION OF A PREALBUMIN AND EFFECTS OF STRAIN VARIATION ON BINDING

Abstract

SUMMARY: Sera from inbred rabbit strains have been studied to determine radio-thyroxine distribution after electrophoresis, to measure protein binding capacity for thyroxine (T₄) and to examine possible interrelationships of binding capacity and serum protein-bound iodine (PBI). In electrophoretic studies at pH 7·4, serum albumin and prealbumin, the latter a previously unrecognized carrier of T₄ in the rabbit, are the principal transport proteins. At pH 9·0, prealbumin is the major carrier (61% of tracer). Prealbumin also binds significant quantities of tri-iodothyronine. The mean binding capacity of serum prealbumin for T₄ in 12 rabbit strains was 500 μg/100 ml. Protein-bound iodine levels are known to be strain-dependent in the rabbit. In the current studies, however, there was no strain-specificity of prealbumin binding capacity, and no correlation between PBI and binding capacity of prealbumin, the principal T₄-specific transport protein. These observations suggest that factors other than the capacity of binding proteins for T₄ may be primary determinants of hormone levels (PBI) in blood in the rabbit.