Isopentenyladenosine Stimulates and Inhibits Mitosis of Human Lymphocytes Treated with Phytohemagglutinin

The plant cytokinin isopentenyladenosine, a component of yeast and mammalian transfer ribonucleic acid, is both a potent inhibitor and stimulator of DNA synthesis, transformation, and mitosis of the phytohemagglutinin-stimulated lymphocyte. The stage of the cell cycle and the concentrations used are critical for these effects. The addition of isopentenyladenosine within the first 12 hours after phytohemagglutinin at a concentration above 10(-6) molar results in inhibition, while lower concentrations (between 10(-7) and 10(-6) molar), added at 24 hours or later, have a stimulatory effect.