RESUMO
Effect of different calcium concentrations on erythrocyte ghost shape and size has been studied. Erythrocyte ghosts are smallest at approximately 10 microM of total calcium content and in the absence of EDTA; both decrease and increase of free Ca2+ results in expansion of ghosts. Changes in erythrocyte ghost size occur at equal Ca2+ concentrations which, as it has been already found, alter ATPase activity of contractile membrane protein. The possibility is discussed that under physiologic conditions contractile membrane protein causes weak membrane contraction which differs from aphysiologic membrane shrinkage brought about particularly by cross-bridges at high Ca2+ concentrations.
Assuntos
Cálcio/farmacologia , Membrana Eritrocítica/fisiologia , Trifosfato de Adenosina/metabolismo , Proteínas Contráteis/fisiologia , Membrana Eritrocítica/citologia , Humanos , Magnésio/metabolismoRESUMO
The aim of this report was to find which part of the membrane is responsible for the Ca2+ dependence of membrane permeability to K+. We found that the enzyme activity of large contractile complex of membrane proteins, the so called spectrin-dependent ATPase (sp-ATPase) increases at certain Ca2+ concentrations when K+ permeability decreases and vice versa. Ca2+ apparent dissociation constant for sp-ATPase is 6 X 10(-7) M which is the value corresponding to findings of Porzig and Stoffel (1978) for Ca2+ binding to membrane with low K+ permeability. Moreover, 20 chemically quite different substances which inhibit sp-ATPase activity simultaneously increase in the same concentrations K+ permeability and vice versa. No exception was found. The results show that low membrane permeability to K+ occurs at such conformation of sp-ATPase at which its enzyme activity may fully be manifested whereas at other conformations permeability to K+ increases. The conformation of sp-ATPase seems to affect gating mechanism of a respective channel for passive K+ transport through membrane.
Assuntos
Adenosina Trifosfatases/sangue , Cálcio/farmacologia , Membrana Eritrocítica/metabolismo , Potássio/sangue , Transporte Biológico Ativo/efeitos dos fármacos , Permeabilidade da Membrana Celular/efeitos dos fármacos , Humanos , Técnicas In Vitro , Canais Iônicos/efeitos dos fármacos , Canais Iônicos/metabolismo , Cinética , Conformação Proteica , Espectrina/farmacologiaRESUMO
The paper reports on the relationship between spectrin-dependent ATPase and erythrocyte shape. It can be seen from various experiments that different treatment of erythrocyte membrane which brings about alteration in activity of spectrin-dependent ATPase, also changes erythrocyte shape. We suggest that spectrin-dependent ATPase is a large actomyosin-like protein complex with some characteristics of contractile protein which, under suitable conditions, causes a physiological tension (contraction) of the membrane. Energetically rich state of spectrin-dependent ATPase seems to be responsible for this phenomenon.
Assuntos
Adenosina Trifosfatases/sangue , Membrana Eritrocítica/ultraestrutura , Espectrina/sangue , Actomiosina/metabolismo , Cálcio/farmacologia , Membrana Eritrocítica/efeitos dos fármacos , Membrana Eritrocítica/enzimologia , HumanosRESUMO
The dependence of saponin-stimulated Mg-ATPase activity in the erythrocyte membrane on Ca2+ concentration was studied. In the membrane of freshly sampled human erythrocytes we found for this enzyme and Ca2+ an apparent dissociation constant of 0.611 mumol/l (SE +/- 0.106 mumol/l) and Hill coefficient of 0.93 (SE +/- 0.05). The enzyme is in most probability identical with Ca,Mg-ATPase of high affinity to Ca2+ described also as spectrin-dependent Ca,Mg-ATPase.
