Kinetic and allosteric cooperativity in L-adenosine transport in chromaffin cells. A mnemonical transporter

In cultured chromaffin cells and plasma membrane vesicles from chromaffin tissue, the transport of D-[3H]adenosine followed Michaelis-Menten saturation kinetics, with Km values of 1.5 +/- 0.3 microM and 1.9 +/- 0.2 microM, respectively. The transport of the isomer, L-[3H]adenosine, showed sigmoidal kinetics in both preparations. In plasma membrane vesicles the S0.5 was 2.5 +/- 0.2 microM with a Hill coefficient of 2.8 and the Vmax value of 0.26 +/- 0.01 pmol s-1 (mg of protein)-1. In cultured chromaffin cells the kinetic parameters for L-[3H]adenosine were S0.5 = 6.2 +/- 0.2 microM and a Vmax 19.7 +/- 0.5 pmol/min per 10(6) cells, with a pronounced positive cooperativity. The Hill coefficient was 4.9. The transport of the L-isomer in cultured cells followed Michaelis-Menten kinetics at the lowest concentrations employed, below 2 microM. On the basis of these results, we propose a kinetic model whereby the adenosine transporter functions mnemonically.