Figure 4.

Quantitative analyses of the substrate specificity, pH dependence, and K_m myoinositol of recombinant AtINT2 in Xenopus oocytes. A, Quantitative analysis of inward currents elicited by different potential substrates of AtINT2 showing high specificity for inositol and a marginal capacity to transport Glc. B, Relative inward currents elicited by different inositol epimers and derivatives in AtINT2-expressing or AtINT4-expressing (inset) oocytes. C, pH dependence of myoinositol-elicited inward currents. D, The K_m value of AtINT2 for myoinositol was determined by analyzing H⁺ currents in the presence of different myoinositol concentrations. Normalized currents were plotted against the substrate concentration. Data were fitted with Michaelis-Menten-type kinetics. The voltage dependence of the K_m values is shown in the inset. Apparent K_m values ranged from 0.7 ± 0.08 at −160 mV to 1.75 ± 0.13 at −20 mV (n = 5). Myoinositol-dependent H⁺ currents were determined at a membrane potential of −60 mV (A–D) or at voltages indicated (inset in D). Results ± se are presented.