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.