Effect of pH on calcium ion dependence of dog cardiac sarcoplasmic reticulum adenosine triphosphatase activity.

ABSTRACT

The effect of [Ca2+] on (Ca2+ + Mg2+)-ATPase activity from dog heart sarcoplasmic reticulum fragments was evaluated under three different pH values 6.0, 6.8 and 7.6. Considering pH 6.8 as reference, under pH 7.6 an increase of the activity at saturating [Ca2+], an increase of pCa needed for half-maximal activation, and a decrease of the Hill coefficient, were observed. Opposite results were obtained under pH 6.0. The positively cooperative effect of Ca2+ was only observed at the lowest pH. The results are in line with previous suggestions regarding the existence of two enzymatic states with different Ca2+ affinities; this paper suggests as well, that equilibrium between these states is affected by pH. The results are compared with data obtained with skeletal sarcoplasmic reticulum fragments under the same experimental conditions.