On the molecular size of thymosins.

The immunoregulatory polypeptide prothymosin alpha and its biologically active N-terminal fragment thymosin alpha 1m, with relative molecular masses of 12,500 and 3108 respectively, were found to behave as oligomers (trimers to hexamers) in gel-filtration measurements. This phenomenon of an apparent association of polypeptides has been reported for other thymosins--parathymosin alpha, thymosin beta 4 and thymosin beta 10. In contrast, sedimentation equilibrium ultracentrifugation shows that thymosin alpha 1 is a monomer with a relative molecular mass of 3000 +/- 200. Measurement of the diffusion coefficient as 221 micron2/s suggests that the molecule is approximately spherical. The implications for the molecular species of prothymosin alpha, parathymosin alpha, and beta-thymosins are discussed.