"Exchange diffusion": rate equations for the influx of alpha-aminoisobutyric acid into mouse cerebrum slices containing this amino acid.

Rate equations for the gross influx of alpha-aminoisobutyric acid (AIB) into mouse cerebrum slices containing AIB have a first-order term for unsaturable concentrative influx, identical to the corresponding term for unloaded slices, and a modified Michaelis-Menten term, V'max/(1 + Kt/S), for saturable concentrative influx. [V'max identical to v'L(1 + Kt/S), where v'L = saturable component of influx, S = AIB concentration in medium, and Kt = Michaelis constant for unloaded slices.] Below a tissue AIB (T) of 19 mumol/g final wet weight, V'max increases linearly following V'max = V1 + m1T; above that value, Vmax is virtually constant. The transition is sharp. This equation is consistent with a carrier model for active transport. At the transition, intracellular AIB is about 1 molecule for every 70 amino acid residues of tissue protein, vastly more than could be accommodated by AIB-binding sites in cell membranes. The transition may come from a slow process that does not fill all sites when the tissue AIB is below the transition concentration, or from an AIB-induced phase transition in the membrane.