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B Foder

Publications and source records attributed to B Foder.

26 records · Page 2Linked to original sources
PubMed1982

Rate constants for calmodulin binding to Ca2+-ATPase in erythrocyte membranes.

The Ca2+-ATPase (ATP phosphohydrolase, EC 3.6.1.3) in human erythrocyte membranes, which is part of the Ca2+ pump, can be activated by binding of calmodulin. Rate constants (k1) for association of calmodulin and enzyme, which depends on the Ca2+ concentration, have been determined by the aid of an enzyme model. k1 increased from 0.25 . 10(6) to 17.3 . 10(6) M-1 . min-1 (70 times) when the free Ca2+ concentration was raised from 0.7 to 20 microM. The binding of calmodulin to the Ca2+-ATPase is reversible. The rate constants (k-1) for dissociation of enzyme-calmodulin complex decreased from 6.0 to 0.044 min-1 (135 times) when the free Ca2+ concentration was increased from 0.1 to 2-20 microM. The apparent dissociation constant Kd = k-1/k1 accordingly increased from 2.5 nM to 25 microM (or higher) when the Ca2+ concentration was reduced from 20 to 0.1 microM. Therefore, at 10(-7) M free Ca2+ most of the Ca2+-pump enzyme will not bind calmodulin. For the intact cell the time dependences of activation and deactivation of the Ca2+-pump enzyme have been estimated from the rate constants above. The results suggest that the Ca2+ pump is well suited to maintain a cytosolic concentration of 10(-7) M free Ca2+ (or lower) in the unstimulated cell and, when the cell is stimulated, to allow transient Ca2+ signals up to approx. 10(-5) M in the cytosol.

Calcimycin↗
PubMed1981

Decrease of apparent calmodulin affinity of erythrocyte (Ca2+ + Mg2+)-ATPase at low Ca2+ concentrations.

The calmodulin activation of the (Ca2+ + Mg2+)-ATPase (ATP phosphohydrolase, EC 3.6.1.3) in human erythrocyte membranes was studied in the range of 1 nM to 40 microM of purified calmodulin. The apparent calmodulin-affinity of the ATPase was strongly dependent on Ca2+ and decreased approx. 1000-times when the Ca2+ concentration was reduced from 112 to 0.5 microM. The data of calmodulin (Z) activation were analyzed by the aid of a kinetic enzyme model which suggests that 1 molecule of calmodulin binds per ATPase unit and that the affinities of the calcium-calmodulin complexes (CaiZ) decreases in the order of Ca3Z greater than Ca4Z greater than Ca2Z greater than or equal to CaZ. Furthermore, calmodulin dissociates from the calmodulin-saturated Ca2+-ATPase in the range of 10(-7)-10(-6) M Ca2+, even at a calmodulin concentration of 5 microM. The apparent concentration of calmodulin in the erythrocyte cytosol was determined to be 3 to 5 microM, corresponding to 50-80-times the cellular concentration of Ca2+-ATPase, estimated to be approx. 10 nmol/h membrane protein. We therefore conclude that most of the calmodulin is dissociated from the Ca2+-transport ATPase in erythrocytes at the prevailing Ca2+ concentration (probably 10(-7)-10(-8) M) in vivo, and that the calmodulin-binding and subsequent activation of the Ca2+-ATPase requires that the Ca2+ concentration rises to 10(-6)-10(-5) M.

Ca(2+) Mg(2+)-ATPase↗
PubMed1980

Activator-associated Ca2+-ATPase in erythrocyte membranes from cystic fibrosis patients.

Erythrocyte membranes were prepared by a method which should ensure binding of an activator protein (calmodulin) to the calcium dependent membrane ATPase involved in calcium transport. The level of enzyme activity, assayed at optimum conditions, was 5-400 times higher than that found in previous investigations on cystic fibrosis patients. The Ca2+-ATPase activity of the cystic fibrosis patients was reduced by 15% compared to control subjects, whereas patients suffering from chronic pulmonary diseases did not deviate from controls. Even if a reduction of Ca2+ pumping activity occurs in other cells, a 15% decrease could hardly be the only cause of the changed calcium concentrations in secretions from cystic fibrosis patients.

Adolescent↗
PubMed1978

Reversible shift between two states of Ca2+-ATPase in human erythrocytes mediated by Ca2+ and a membrane-bound activator.

The (Ca2+ + Mg2+)-dependent ATPase (ATP phosphohydrolase, EC 3.6.1.3) from human erythrocytes occurred in two different states, A-state and B-state, depending on the membrane preparation. The A-state showed low maximum activity (V) and the Ca2+ activation was characterized by a Hill coefficient, nH, of about 1 and a Michaelis constant, KCa, about 30 micron. The B-state showed high V, a nH above 1, which indicates positive cooperativity of Ca2+ activation, and KCa of about 1 micron. With varying ATP concentrations, both the A-state and B-state showed negative cooperativity and slightly different values of Km. The B-state was shifted to A-state when the membranes were exposed to low Ca2+ concentration. The shift reached 50% at approx. 0.5 micron Ca2+. At the low Ca2+ concentrations an activator was released from the membranes. The A-state was shifted to the B-state when the membranes were exposed to Ca2+ in the presence of the activator. The shift reached 50% at about 30 micron Ca2+. The recovery of high V was time dependent and lasted several minutes. Increasing concentrations of Ca2+ and activator accelerated the recovery. It is suggested that the A-state and the B-state correspond to enzyme free of activator and enzyme associated with activator, respectively. Furthermore, the two states may respresent a resting and an active state, respectively, of the calcium pump.

Adenosine Triphosphatases↗
PubMed1975

Decreased (Ca2+ + Mg2+)-stimulated ATPase activity in erythrocyte membranes from polycythemia vera patients.

Erythrocytes were hemolyzed in hypotonic phospate buffer containing 0.5 mmol/l Ca2+ and the membranes subsequently washed twice in hypotonic tris buffer. The centrifugation was performed in a continuous flow system, which was necessary to obtain maximal ATPase activity. The Mg2+-dependent Ca2+-stimulated ATPase activity of 14 patients with polycythemia vera was only 67 per cent (P less than 0.001) of the activity of a control material consisting of 10 donors and 11 bank blood specimens. Five patients with secondary polycythemia and four patients with an increased erythrocyte fraction did not differ significantly from the controls. The polycythemia vera patients with the highest leukocyte count showed the lowest ATPase activity. The apparent calcium dissociation constant of the ATPase in polycythemia vera was about 10(-6) mol/l, as in controls. The relation between the reduced ATPase activity and the abnormal hemopoiesis of polycythemia vera patients is discussed.

Adenosine Triphosphatases↗