Em. Klamo et al., KINETICS AND STOICHIOMETRY OF A PROTON MYO-INOSITOL COTRANSPORTER/, The Journal of biological chemistry, 271(25), 1996, pp. 14937-14943
Voltage clamp recording was used to measure steady-state and presteady
state currents mediated by a myoinositol transporter cloned from Leis
hmania donovani and expressed in Xenopus oocytes, Application of myoin
ositol resulted in inward currents, which did not require external sod
ium and which were increased by increasing the extracellular proton co
ncentration and by membrane hyperpolarization, Alkalinization of the e
xtracellular space occurred concomitantly with myo-inositol influx. Co
rrelation of membrane currents with radiolabeled myo-inositol flux rev
ealed that one positive charge is translocated with each molecule of m
yo-inositol, consistent with cotransport of one proton, The transport
concentration dependence on both species suggested ordered binding of
a proton followed by a molecule of myo-inositol, In the absence of myo
-inositol, a voltage-dependent capacitance was observed that correlate
d with the transporter expression level, This charge movement obeyed a
Boltzmann function, which was used to estimate a turnover of 0.70 +/-
0.06 s(-1) at -60 mV. The pH and voltage dependence of the charge mov
ements were simulated with a model involving alternating access of int
ernal and external protons to sites within an occluded pore.