Myofibrils of skeletal muscle: the activity coefficient of orthophosphate

In the myofibrils of skeletal muscle, at 22 degrees C, pH 7.1 and at the physiological protein osmotic pressure of 1.8 x 10(5) dynes/cm2, orthophosphate behaves quite ideally, the activity coefficient being 0.85. Under the same conditions and at saturation, 2.67 mumoles of orthophosphate are bound per gram of dry myofibrils, with a dissociation constant of 7 x 10(-5) molal. Work is in progress to determine the activity coefficients of adenine nucleotide analogues. This work is needed to assess the actual value of the free energy of hydrolysis of ATP in muscle.