A comparison of the rate equations, kinetic parameters, and activation energies for the initial uptake ofl-lysine,l-valine, γ-aminobutyric acid, and α-aminoisobutyric acid by mouse brain slices

Abstract

At substrate concentrations, in medium, of 0.2 to 20mm and at temperatures of 25 and 37°C, the initial concentrative influx of the amino acidsl-lysine (30 and 37°C),l-valine, and γ-aminobutyric acid into incubated mouse-cerebrum slices follows the rate equation for the initial influx of α-aminoisobutyric acid (Cohen,J. Physiol. 228:105, 1973),v=V _max/(1+K _t/S)+k _u S. Kinetic constants at 37°C are:V _max=0.089 μmoles/g final wet wt of slices, min,K _t=0.69mm,k _u=0.037 μmoles/g final wet wt,mm-substrate, min forl-lysine;V _max=0.60,K _t=1.30,k _u=0.067 forl-valine; andV _max=1.71,K _t=1.58,k _u=0.094 for γ-aminobutyric acid. The linear term,k _u S, is due to an unsaturable process of concentrative uptake, not diffusion. Comparison of temperature coefficients reveals a “reference” pattern for typical low affinity transport of amino acids into brain slices. Its characteristics are: Activation energies associated withV _max andk _u are in range 14 to 20 kcal/mole;K _t varies only slightly with temperature.l-Lysine and α-aminoisobutyric acid fit this pattern;l-valine and γ-aminobutyric acid deviate in part. The Akedo-Christensen plot (J. Biol. Chem. 237:118, 1962) does not distinguish between the rate equationv=V _max/(1+K _t/S)+k _u S for saturable uptake plus first-order unsaturable concentrative uptake, and the rate equationv=V _max/(1+K _t/S)+k _D(S−S _i) for saturable uptake plus first-order nonconcentrative “passive diffusion”.