Uptake of Vitamin B 12 by Phytohaemagglutinin‐Transformed Lymphocytes

S ummary . Phytohaemagglutinin (PHA)‐transformed lymphocytes have been used as a model cell system to study the uptake of radioactive vitamin B 12 by haemopoietic cells. Both mature granulocytes and PHA‐transformed lymphocytes took up more vitamin B 12 than mature, non‐transformed lymphocytes. Uptake of vitamin B 12 by PHA‐stimulated lymphocytes was greatest at 48–72 hr of culture, i.e. at about the time or just before the time of peak DNA synthesis. Vitamin B 12 deficient lymphocytes took up significantly less vitamin B 12 than normal lymphocytes. Folate deficient lymphocytes took up an average of about 50% more vitamin B 12 than normal but the difference was not statistically significant for the numbers tested. Vitamin B 12 uptake by PHA‐stimulated lymphocytes was related to their rate of RNA synthesis (measured by 3 H‐uridine uptake) and was closely related to active cytoplasmic protein synthesis since it was rapidly inhibited by puromycin and cycloheximide.