title: Steady-state properties of calcium binding to parvalbumins from bullfrog skeletal muscle: effects of Mg2+, pH, ionic strength, and temperature.
abstract: To improve our understanding of the physiological roles of parvalbumins, PA-1 (pI 4.78) and PA-2 (pI 4.97) parvalbumins were prepared from bullfrog skeletal muscle and their calcium binding properties were examined in a medium of constant ionic strength (I = 0.106, pH 6.80, at 20 degrees C) containing various concentrations of Mg2+ by using a metallo-indicator, tetramethylmurexide. Apparent binding constants for Ca2+ in the presence of Mg2+ changed in the manner expected if Ca2+ and Mg2+ compete for two independent homogeneous binding sites. The following values were obtained: for PA-1, KCa = 1 X 10(7) M-1, KMg = 900 M-1; for PA-2, KCa = 6 X 10(6) M-1, KMg = 830 M-1 (I = 0.106, pH 6.80, at 20 degrees C). The apparent binding constants are strongly dependent on temperature: at 10 degrees C for PA-1, KCa = 2 X 10(8) M-1, KMg = 10(4) M-1; for PA-2, KCa = 5 X 10(7) M-1, KMg = 5 X 10(3) M-1 (I = 0.106, pH 6.80). The dependence of the affinities for Ca2+ on ionic strength is similar to or less than that of GEDTA (EGTA). The affinities for Ca2+ and Mg2+ of parvalbumins are unchanged between pH 6.5 and 7.2.
mesh_terms: D000596:Amino Acids / Q000032:analysis; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology*; D009124:Muscle Proteins / Q000378:metabolism*; D009132:Muscles / Q000378:metabolism*; D009994:Osmolar Concentration; D010320:Parvalbumins / Q000378:metabolism*; D011485:Protein Binding; D011892:Rana catesbeiana; D013696:Temperature
chemical_list: D000596:Amino Acids; D009124:Muscle Proteins; D010320:Parvalbumins; D008274:Magnesium; D002118:Calcium