The serum thymic factor (FTS) utilized in its synthetic or natural form loses its biological activity in a rosette assay after treatment with a metal ion-chelating agent, Chelex 100. This activity is restored by the addition of Zn salts and, to a lesser extent, certain other metal salts. FTS activation is secondary to the binding of the metal to the peptide. The metal-to-peptide molar ratio of 1:1 provides the best activation. These data indicate the existence of two forms of FTS. The first one lacks Zn and is biologically inactive; the second one contains Zn and is biologically active, for which we propose the name of "thymulin" (FTS-Zn). The presence of Zn in synthetic FTS was confirmed by atomic absorption spectrometry. The interaction between Zn and FTS was fiurther suggested by microanalysis demonstrating the presence of this metal in thymic reticuloepithelial cells.

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These results suggest that hyperparathyroidism is associated with a compensatory increase in apoptosis, possibly favored by a diminished Bcl-2/Bax ratio. This renders highly improbable the hypothesis that parathyroid hyperplasia is due to a decreased rate of apoptosis.

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