Thymulin is a nine-peptide hormone (Glu-Ala-Lys-Ser-Gln-Gly-Gly-Ser-Asn) secreted by thymic epithelial cells. Zn is bound to thymulin in a 1:1 stoi-chiometry via the side-chain of asparagine and the hydroxyl groups of the two serines. The binding of Zn results in a conformational change, which produces the active form of thymulin. Thymulin binds to high-affinity receptors on T-cells and promotes T-cell maturation, cytotoxicity and interleukin (IL)-2 production (see Shankar and Prasad, 1998). Thymulin activity in vitro and in vivo in both animals and humans is dependent on plasma Zn levels, such that marginal changes in Zn intake or availability affect thymulin activity (Prasad et al., 1988; Shankar and Prasad, 1998). Thymulin is readily detectable in the serum of Zn-deficient patients, but is not active. The overall role of thymulin in the immuno-logical lesions caused by Zn deficiency has not been well studied. The use of thymulin as an indicator of Zn deficiency has been suggested, although thymulin concentration is also modulated by Zn-independent factors. Assays regarding Zn status and thymulin Zn saturation may prove more useful, much as the index of transferrin saturation provides useful information regarding iron status.
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