Metal ion-binding properties of avian thymic hormone

Lanthanide ion luminescence studies and 45Ca2(+)-binding measurements were used to study the metal ion-binding properties of avian thymic hormone. The procedure used to isolate the protein--involving heat-treatment at 80 degrees C, trichloroacetic acid precipitation, DEAE-agarose chromatography, and gel filtration--affords material that is deemed homogeneous by sodium dodecyl sulfate-polyacrylamide gel electrophoresis as well as the absence of a detectable tryptophan signal in the fluorescence emission spectrum. Avian thymic hormone exhibits a pI = 4.35 when subjected to isoelectric focusing through polyacrylamide gels. The two ion-binding sites are indistinguishable in their interactions with Ca2+ and Mg2+, displaying KCa = 8 nM and KMg = 68 microM. The Eu3+ 7Fo----5Do excitation spectrum at pH 6 displays a peak at 5795.4 A, with a shoulder at 5792.8 A and is replaced at higher pH values by a broader spectrum with a maximum at 5784.8 A and a shoulder at 5777.1 A. The pKa governing this spectral interconversion is 8.21. All of these properties are very similar to those observed with other parvalbumins. However, polyclonal antibodies to avian thymic hormone do not cross-react with the parvalbumin from chicken leg muscle, as judged by Western blot analysis-further evidence that avian thymic hormone and the muscle-associated chicken parvalbumin are indeed distinct proteins.