Molecular mechanisms of the cellular internalization of human thioredoxin (TRX) have been largely unknown. Although TRX has no typical signal sequence, TRX is released from cells and enters into cells. To analyze the mechanisms of releasing or internalization of TRX, we prepared a mutant TRX (TRX-C35S), in which active site Cys35 is substituted by serine. Flowcytometric analysis revealed that Alexa488-labeled TRX-C35S binds more on the cell surface of the HTLV-I positive T cell lines comparing with the HTLV-I negative cell lines. The binding of TRX-C35S was inhibited by DTT or excess amount of wild type TRX. The binding and internalization of TRX-C35S was observed more on the cell surface of CD25 positive cells. Since CD25 is a marker of activated T cells, the binding and the internalization of TRX may be associated with the activation of T cells.
