Effect of ammonium, sodium, and potassium ions on rabbit muscle phosphofructokinase-1 and adenylate kinase activities.

This report shows that 30 nM PFK-1 and 30 nM AK were both affected by the presence of NH(4)(+), Na(+), and K(+) salts but with opposite consequences. Low concentrations of PFK-1 lose about half of its activity as a result of dilution and become susceptible to further activity losses owing to the presence of monovalent salts. On the other hand low concentrations of AK lose about 75 percent of its activity but regains activity losses owing to the presence of monovalent salts. It was determined that regain of AK activity did not appear to be a reflection of a major effect on the K(m) value of either AMP or ATP. Dilution to 30 nM AK resulted in no increase K(m) values compared to K(m) values at 140 nM AK. Dilution caused major decreases in the maximum velocities, V(max), when ATP or fructose 6-phosphate was the variable substrate. It was shown in earlier reports that these same low concentrations of PFK-1 and AK were susceptible inhibitions by ascorbate. These attributes are discussed as they may relate to the role of ascorbate facilitation glycogen synthesis in resting muscle and the role that the cytoskeleton infrastructure scaffold may play is also discussed.

Nanomechanics of multiple units in the erythrocyte membrane skeletal network.

Erythrocytes undergo deformations when they transport O(2) and CO(2) across the membrane, yet the 3D nanomechanics of the skeletal network remains poorly understood. Expanding from our previous single isolated unit, we now simulate networks consisting of 1-10 concentric rings of repeating units in equibiaxial deformation. The networks are organized with (1) a 3D model for a single unit, (2) a wrap-around mode between Sp and actin protofilament in the intra-unit interaction, and (3) a random inter-unit connectivity. These assumptions permit efficient five-degrees-of-freedom (5DOF) simulations when up to 30 pN of radial forces are applied to the boundary spectrin (Sp) and the center and other units are analyzed. As 6 Sp balance their tensions, hexagonal units become irregular. While actin protofilaments remain tangent to the network, their yaw (Phi) angles change drastically with addition of neighboring units or an Sp unfolding. It is anticipated that during deformation, transmembrane complexes associated with the network move laterally through the lipid bilayer and increase the diffusion of molecules across the membrane. When protofilament/Sp sweeps under the lipid bilayer, they mix up the submembrane concentration gradient. Thus, the nanomechanics of actin protofilaments and Sp may enhance the transport of molecules during erythrocyte deformation.