pH‐Dependence of the ATP‐Driven Uptake of Noradrenaline by Bovine Chromaffin‐Granule Ghosts

Abstract

The kinetic parameters of noradrenaline uptake by chromaffin granule ghosts have been measured at external pH values between 6.5 and 8.5. The log of the K_m for noradrenaline decreased linearly with pH with a slope of –1.0, indicating that the observed affinity increase originated in deprotonation of a single chemical group. This result is interpreted as showing that the neutral form of monoamines is the true substrate for the amine carrier. The K_m of the carrier for the neutral form of noradrenaline was calculated as 0.1 μM. The maximal velocity, V, of the uptake reaction was constant from pH 6.5 to 8.0 and decreased at more alkaline pH values. Since the proton electrochemical gradient Δμ_H+ generated by the membrane H⁺‐pump was independent of the pH in the range 6.5–9.5, the pH dependence of the maximal velocity of uptake reflects the pH profile of the monoamine transporter.