The study shows that the peptide called thymosin‑alpha‑1 isn’t actually found in rat organs; instead a larger protein, prothymosin‑alpha, is present and likely gets cut into thymosin‑alpha‑1 when needed. It’s most abundant in the thymus and also shows up in brain, liver, kidney, lung and spleen at lower levels. This means the body’s natural source of thymosin‑alpha‑1 is a precursor, not the active peptide itself.

A radioimmunoassay, using a rabbit antiserum directed against thymosin alpha 1, was employed to detect the presence of crossreacting peptides in rat tissues. Highest concentrations were present in thymus, but thymosin alpha 1 cross-reacting material was also detected in brain, liver, kidney, lung, and spleen, in amounts ranging from 15% to 65% of the quantities found in thymus. In each case, the major immunoreactive peptide, after extraction and purification by a procedure that avoids proteolytic modification, was identified as prothymosin alpha, a peptide containing approximately equal to 112 amino acid residues. Prothymosin alpha is believed to be the endogenous peptide from which thymosin alpha 1 and other fragments are formed by proteolytic modification during the preparation of thymosin fraction 5. No peptides corresponding in size and chromatographic behavior to thymosin alpha 1 were detected with the extraction procedure employed.