Methionine dependency of cultured human lymphocytes.

Human peripheral blood lymphocytes stimulated with phytohemagglutinin and a lymphocyte model consisting of the RPMI 6410 cell, a human virus-transformed B cell, required added methionine (Met) for growth of the cultures. This failure to meet all needs for Met via endogenous synthesis, which is characteristic of oncogenic transformation, occurred even in the presence of adequate homocysteine, methylfolate (5-CH3-H4PteGlu) and cobalamin (Cbl)-dependent methionine synthetase activity. Folinic acid (5-CHO-H4PteGlu), which provides available folate independently of Cbl, improved growth only slightly in the absence of Met. Free Cbl at 222 nM, an amount great enough to alter other intracellular events, failed to increase growth in the absence of Met, but 0.22 nM Cbl bound to transcobalamin II did, however, enhance growth.