Immunochemical studies on thymosin: radioimmunoassay of thymosin alpha 1

A sensitive and specific radioimmunoassay (RIA) has been developed to measure thymosin alpha 1 in human blood and tissue extracts. Thymosin alpha 1, originally isolated from bovine thymosin fraction 5, has an identical primary structure in bovine and human species. Antiserum to synthetic thymosin alpha 1 was prepared in rabbits. A synthetic analogue, N-Ac-(Tyr1)thymosin alpha 1 was radioiodinated by utilizing Na125I and soluble lactoperoxidase. The RIA is capable of detecting as small an amount as 40 pg thymosin alpha 1 in serum/plasma and does not significantly cross-react with other putative thymic hormones or other purified blood proteins that have been assayed. Proteolytic degradation of thymosin alpha 1 does not occur during the collection, storage, or assay of serum or plasma. The concentration of thymosin alpha 1 in the blood is highest in utero, decreases sharply after birth, and appears to remain fairly constant during the first 15 yr of life. Maternal blood levels of thymosin alpha 1 appear to be above normal during pregnancy.