Effects of pH on 5-methyltetrahydrofolic acid transport in human erythrocytes

We measured the effects of external pH on influx, efflux and net steady-state levels for 14CH3H4PteGlu1. Initial rates of uptake were inversely proportional to external pH. Lowering the pH of the suspending medium increased influx by enhancing the affinity of the carrier: the apparent Km values at pH 6, 7.2 and 8.1 were 0.14, 0.25 and 0.63 microM respectively. In contrast, Vmax was independent of external pH. Efflux rate constants at pH 6, 7.5 and 8.5 were 0.034, 0.059 and 0.067/min respectively. consequently, lowering the external pH increased steady-state levels of 14CH3H4PteGlu1.