ADJUSTMENT OF K' TO VARYING PH AND PMG FOR THE CREATINE-KINASE, ADENYLATE KINASE AND ATP HYDROLYSIS EQUILIBRIA PERMITTING QUANTITATIVE BIOENERGETIC ASSESSMENT

Physiologists and biochemists frequently ignore the importance of adju sting equilibrium constants to the ionic conditions of the cell prior to calculating a number of bioenergetic and kinetic parameters, The pr esent study examines the effect of pH and free magnesium levels (free [Mg2+]) on the apparent equilibrium constants (K') of creatine kinase (ATP: creatine N-phosphotransferase; EC 2.7.3.2), adenylate kinase (AT P:AMP phosphotransferase; EC 2.7.4.3) and adenosinetriphosphatase (ATP phosphohydrolase; EC 3.6.1.3) reactions, We show how K' can be calcul ated using the equilibrium constant of a specified chemical reaction ( K-ref) and the appropriate acid-dissociation and Mg2+-binding constant s at an ionic strength (I) of 0.25 mol l(-1) and 38 degrees C, Substit uting the experimentally determined intracellular pH and free [Mg2+] i nto the equation containing a known K-ref and two variables, pH and fr ee [Mg2+], enables K' to be calculated at the experimental ionic condi tions, Knowledge of K' permits calculation of cytosolic phosphorylatio n ratio ([ATP]/[ADP][P-i]), cytosolic free [ADP], free [AMP], standard transformed Gibbs energy of formation (Delta(f)G'degrees(ATP)) and th e transformed Gibbs energy of the system (Delta(f)G'(ATP)) for the bio logical system, Such information is vital for the quantification of or gan and tissue bioenergetics under physiological and pathophysiologica l conditions.