Kinetic Characterization of Ca²⁺ Transport in Synaptic Membranes

Abstract: Lysed synaptosomal membranes were prepared from brain cortices of HA/ICR Swiss mice, and the ATP‐stimulated Ca²⁺ uptake, Ca²⁺‐stimulated Mg²⁺ ‐dependent ATPase activity, and the Ca²⁺‐stimulated acyl phosphorylation of these membranes were studied. The K_m values for free calcium concentrations ([Ca²⁺]_f) for these processes were 0.50 μM, 0.40 μM, and 0.31 μM, respectively. Two kinetically distinct binding sites for ATP were observed for the ATP‐stimulated Ca²⁺ uptake and the Ca²⁺‐stimulated Mg²⁺‐ATPase activity. The high‐affinity K_m values for ATP for these two processes were 16.3 μM, and 28 μM, respectively. These results indicate that the processes studied operate in similar physiological concentration ranges for the substrates [Ca²⁺]_f and ATP under identical assay conditions and, further, that these processes may be functionally coupled in the membrane.