Reversible dissociation and inactivation of phosphofructokinase in the ischemic rat heart
To determine if shifts in the ratio of active phosphofructokinase (PFK) tetramers to the inactive dimeric form of the enzyme occur in vivo in the ischemic rat heart, we have developed a rocket immunoelectrophoretic (IEP) assay that provides a sensitive means by which to measure relative differences in this ratio among crude heart extracts. In ischemic hearts, in the face of a drop in intracellular pH from 6.95 to 6.25, there is a time-dependent decrease (63%) in the ratio of tetramer to dimer IEP rocket height relative to perfused controls. Concomitant with this hysteretic depolymerization is a 50% loss of PFK catalytic activity. Realkalinizing extracts of ischemic hearts fosters a recovery of 86% of the activity lost during ischemia and a return of the tetramer-to-dimer ratio to near control value. The amount of reactivation is directly dependent on the degree of enzyme dissociation that occurred during ischemia. Importantly, ischemia-induced dimerization is also reversed in vivo by postischemic reperfusion. The data are consistent with those in the previous study [Hand and Carpenter, Am, J. Physiol. 250 (Regulatory Integrative Comp. Physiol. 19): R505-R511, 1986] that characterized the pH-dependent hysteretic dissociation of heart PFK in vitro, and together they represent the first demonstration that this molecular behavior is operative in intact tissue. Other vertebrate muscle systems in which this mechanism might be functioning during pH-dependent glycolytic inhibition are discussed.