Metabolism to methionine and growth stimulation by 5'-methylthioadenosine and 5'-methylthioinosine in mammalian cells.

Viable human and murine lymphoblasts, and normal human tissue extracts, converted the thioether nucleosides 5'-methylthioadenosine (MeSAdo) and 5'-methylthioinosine (MeSIno) to methionine. Both MeSAdo and MeSIno, but not homocysteine, supported the short-term growth of human or murine lymphoblasts in methionine deficient medium. However, MeSAdo at concentrations greater than 25 microM inhibited cell growth. MeSIno was non-toxic at concentrations up to 200 microM, and supported the long-term growth of lymphoblasts in methionine-free medium.