Dependence on intracellular Ca2+ on mass and turnover of phosphoinositides and phosphatidate in human erythrocytes.
Effects of a calcium-load on mass and turnover of phosphoinositides and phosphatidate were investigated in human erythrocytes by short-term labeling with [32P]Pi. The labeling of phosphatidate was accelerated at normal mass by short-term elevation of free intracellular [Ca2+] up to 1 microM and inhibited by the reduction of normal free [Ca2+]. Thus, the labeling of phosphatidate is a Ca2+-regulated process and not only the consequence of a net synthesis of diacylglycerol by other Ca2+-dependent reactions. Persisting elevation of free intracellular [Ca2+] between 1-40 microM induced an increase of the mass of phosphatidylinositol 4-phosphate with a concomitant decrease of the mass of phosphatidylinositol 4,5-bisphosphate. Under these conditions, the normal steady-state turnover of phosphoinositides was not altered by Ca2+, but mass and turnover of phosphatidate continuously rose. The increase in phosphatidate mass by far exceeded the decrease of the mass of phosphoinositides, indicating that phosphatidate was generated to a great extent by hydrolysis of other phospholipids in addition to the action of phosphoinositidase C with subsequent phosphorylation of diacylglycerol to phosphatidate. The results demonstrate that different phospholipid phosphodiesterases of human erythrocytes are activated by Ca2+-concentrations in the microM range as is known from various other cell types. In contrast to current explanations, Ca2+-dependent phospholipid phosphodiesterases of human erythrocytes did not exhibit an unusually low affinity against rising cytosolic Ca2+-concentrations.
Müller
E
E
Institute of Biochemistry II, Medical Faculty, Friedrich Schiller University, Jena, Germany.
Hegewald
H
H
Klinger
R
R
Frunder
H
H
D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D004075:Diglycerides / Q000096:biosynthesis / Q000737:chemistry; D004912:Erythrocytes / Q000378:metabolism*; D006801:Humans; D006868:Hydrolysis; D010712:Phosphatidic Acids / Q000737:chemistry / Q000378:metabolism*; D019269:Phosphatidylinositol 4,5-Diphosphate / Q000737:chemistry / Q000378:metabolism*; D018129:Phosphatidylinositol Phosphates / Q000737:chemistry / Q000378:metabolism*; D010727:Phosphoric Diester Hydrolases / Q000378:metabolism; D010761:Phosphorus Radioisotopes; D010766:Phosphorylation
D004075:Diglycerides; D010712:Phosphatidic Acids; D019269:Phosphatidylinositol 4,5-Diphosphate; D018129:Phosphatidylinositol Phosphates; D010761:Phosphorus Radioisotopes; C037178:phosphatidylinositol 4-phosphate; D010727:Phosphoric Diester Hydrolases; C021491:glycerophosphoinositol glycerophosphodiesterase; D002118:Calcium