Establishment of a sensitive radioimmunoassay for serum thymic factor with a special reference to extraction procedures

Concentrations of thymulin in unextracted serum from pigs, sheep and cattle as measured by ELISA

These studies were conducted to develop an ELISA for measurement of thymulin concentrations in unextracted blood serum (or plasma) from domestic animal species (pigs, sheep and cows). This assay was quite variable (intraassay C.V. of 13.3 and 6.4% at 12.6 and 50.5 pg/mL and interassay C.V. of 24.2%). Serial dilutions of serum from these species produced inhibition curves parallel to the reference standard, suggesting that there were no substances in serum causing non-specific interference in the assay. In addition, none of the other thymic peptides tested resulted in problematic displacement of thymulin binding to the antiserum. Using this assay, it was found that somatotropin (ST) treatment had no effect on serum thymulin concentrations in either pigs or cows. Chromatographic separation of thymulin activity in sheep serum showed three peaks with approximate MW estimates of 95, 80 and 1 kDa. Serum thymulin concentrations in a sheep injected with thymulin was cleared from blood with a half-life (t1/2) of 10.3 +/- 0.6 min. Serum thymulin concentrations increased between birth and 6 mo old in pigs. These data indicate that a rapid and reliable ELISA has been developed to measure thymulin in blood of these domestic animals. This assay should be of value in the study of thymulin function and factors regulating its secretion.