There are as many as six peptides of the thymus called thymosins. These are described in Table 17-1 and appear to be acidic peptides in the molecular weight range 800-15,000. These "hormones" are produced in the thymus gland and control the development of the thymic-dependent lymphoid system as the thymus participates in the process of immune regulation. Among peptides isolated from the thymus, thymosin a-i, thymosin ft, and polypeptide ft have been characterized fully and sequenced. The primary sequences of thymosin thymosin ft, thymopoietin II, prothymosin a, and facteur thymique serique (FTS), now known as thymulin, are shown in Figure 17-6. The thymosins modulate immunological responses in vitro and in vivo as well as regulate the differentiation of thymic-dependent lymphocytes. Polypeptide ft is homologous with ubiquitin (see later) and the N-terminal 74 amino acids of a nonhistone chromosomal protein that resembles a histone.

Ubiquitin was discovered as a thymus polypeptide, but it was soon learned that it was present in a wide variety of tissues, hence the name, ubiquitin. Although this protein occurs in chromatin often conjugated with histone, it has been characterized in another context, being identical to the heat-stable polypeptide of a proteolytic system in which it plays an important role in cellular protein breakdown. As a consequence of these findings, ubiquitin is not considered to be a thymic hormone.

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