Scientists made a sensitive test to measure a protein called parathymosin in tissues. They found it’s most abundant in liver and kidney, lower in other organs, and barely present in blood, and the test doesn’t pick up related proteins like thymosin‑alpha‑1.

Parathymosin, a polypeptide that is structurally related to prothymosin alpha, has been shown to block the in vivo immunoenhancing effects of prothymosin alpha (Haritos, A.A., Salvin, S.B., Blacher, R., Stein, S. and Horecker, B.L. (1985) Proc. Natl. Acad. Sci. U.S.A. 82, 1050). To evaluate the content of parathymosin in tissues and fluids, we have developed a radioimmunoassay using an antiserum raised in a rabbit against parathymosin isolated from rat liver. Neither thymosin alpha 1 nor prothymosin alpha show significant cross-reactivity with this antiserum. Based on competition experiments with peptide fragments derived from parathymosin and on the lack of cross-reactivity with prothymosin alpha, the major epitope appears to comprise a region in parathymosin including amino acid residues 26-32. Using this radioimmunoassay, which detects as little as 4 pmol of parathymosin, it was confirmed that highest concentrations, ca. 230-240 micrograms/g of fresh tissue, are present in liver and kidney, followed by brain, lung, thymus and spleen. The content of parathymosin in circulating human leukocytes, 2-3 pmol/ml whole blood, was one-fifth of the quantity of prothymosin alpha. With this radioimmunoassay, parathymosin could not be detected in human plasma.