Thymosin alpha 1 is a native peptide in several tissues.

Failure to detect thymosin alpha 1 (T alpha 1) in tissue extracts prepared by procedures that prevent proteolytic activity has hitherto supported the suggestion that T alpha 1 is not a natural peptide, but the product of uncontrolled proteolysis of prothymosin alpha (ProT alpha), a polypeptide that includes T alpha 1 at its NH2 terminus. In this work, purification by isoelectric focusing of a product with the same isoelectric point as synthetic T alpha 1, and its further characterization, demonstrated that T alpha 1 is present as a native peptide in calf thymus and in several lymphoid and non-lymphoid rat tissues. T alpha 1 shows abnormal chromatographic behaviour which appears to be due to association with other components in tissue extracts. In all the tissues studied, T alpha 1 was present in higher concentration than ProT alpha (80-183 and 44-123 micrograms per gram of tissue, respectively). The ProT alpha/T alpha 1 ratio did not change when no measures were taken to prevent proteolysis during tissue homogenization.