The apparent binding constants of Ca2+ to EGTA and heavy meromyosin

The apparent binding constant (K E app) of EGTA to Ca2+ was determined using a color indicator, murexide, or a Ca2+-selective electrode in the pH range between 6.5 and 7.5, at 5 degrees C and at 20 degrees C. K E app values obtained at two different temperatures (5 degrees C and 20 degrees C) were essentially the same when measured at the same pH. However, K E app depended markedly on pH. In the presence of 0.1 M KCl and 20 mM MOPS-KOH buffer (pH 6.8), K E app was 6.3 X 10(5) M-1. The absolute binding constant (K) was calculated as 10(10.48). Addition of 10 mM MgCl2 did not change the K E app. The binding of Ca2+ to heavy meromyosin was studied in 0.1 M KCl and 20 mM MOPS-KOH buffer (pH 7.2), at 5 degrees C. The binding constant and maximum binding number (mol/mol) were obtained as 1.8 X 10(6) M-1 and 1.4, respectively.