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-[H-3]adenosine followed Michaelis-Menten saturation kinetics, with K(m) values of 1.5 +/- 0.3 muM and 1.9 +/- 0.2 muM, respectively. The transport of the isomer, L-[H-3]adenosine, showed sigmoidal kinetics in both preparations. In plasma membrane vesicles the S0.5 was 2.5 +/- 0.2 muM with a Hill coefficient of 2.8 and the V(max) value of 0.26 +/- 0.01 pmol s-1 (mg of protein)-1. In cultured chromaffin cells the kinetic parameters for L-[H-3]adenosine were S0.5 = 6.2 +/- 0.2 muM and a V(max) 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 muM. On the basis of these results, we propose a kinetic model whereby the adenosine transporter functions mnemonically.