KINETICS OF ATP TO ADP BETA-PHOSPHORYL CONVERSION IN CONTRACTING SKELETAL-MUSCLE BY IN-VIVO P-31 NMR MAGNETIZATION-TRANSFER

The rate constant of the beta-adenosine triphosphate to beta-adenosine diphosphate conversion was measured using P-31 nuclear magnetic reson ance magnetization transfer in resting and contracting in vivo rat ske letal muscle, Theoretically, the rate constant should be the sum of th e rate constants of the reactions catalyzing ATP-ADP exchange, In rest ing muscle, the conversion rate constant was 0.4 s(-1) and beta-ATP in trinsic T-1 was 1.7 s, The velocity of conversion was 3.8 mM s(-1). Du ring stimulation, phosphocreatine fell to 36% and ATP to 82% of initia l values, The rate constant and velocity of beta-phosphoryl conversion increased to 0.8 s(-1) and 6.3 mMs(-1), respectively, but did not rea ch expected levels, i,e, the product of the ATP concentration with the sum of pseudo first-rate constants of the individual reactions, These conversion velocities should be higher than reverse creatine kinase v elocities, previously measured to be 10 mM s(-1) in resting muscle and 7.5 mM s(-1) in contacting muscle and confirmed in this work, The dis crepancy between expected and observed data could be due either to com partmentation of part of the beta-ATP in pools exchanging slowly with the bulk of cellular ATP, or to ADP binding to macromolecules thus pre venting full ADP saturation during magnetization transfer, (C) 1997 by John Whey & Sons, Ltd.