In eukaryotic cells, transmembrane gradients for Na+, K+ and Ca2+ are established and maintained by ATP-dependent ion pumps. Deterioration of these systems with age can lead to alterations in cellular ion homeostasis and loss of cell function. This study focuses on the basic transport mechanisms utilized by ion pumps to gain insight into the molecular basis of altered transport function during aging. The biochemical expression of the plasma membrane Na,K pump is an ATPase which cycles through phosphorylated and dephosphorylated intermediate states. The kinetic behavior of the ATPase partial reactions is complex and cannot be explained by a simple consecutive mechanism. Recent rapid mixing experiments have shown that some of these complex effects can be eliminated by treatment with non- solubilizing concentrations of n-dodecyl beta, D maltoside, a non-ionic detergent. We propose that interactions between alpha catalytic subunits are responsible for the complex behavior and that the detergent disrupts hydrophobic subunit-subunit contacts which are necessary for optimal pumping rates and energy utilization. Rapid mixing and time-resolved EPR studies of the Ca2+ pump in sarcoplasmic reticulum have produced evidence for a substrate-activated conformational state which participates in Ca2+ translocation. Relaxation of this conformational state is associated with a large free energy change resulting in Ca2+ release into an occluded compartment near the inner membrane surface. The kinetic behavior of the Ca-ATPase is compatible with a dimer in which energy-yielding and energy- requiring reactions in adjacent subunits are coupled to optimize Ca2+ pumping.
