Quantitative characterization of homo- and heteroassociations of muscle phosphofructokinase with aldolase

Abstract

Dissociation of purified phosphofructokinase accompanied with inactivation was analyzed in the absence and presence of aldolase and the data were compared with those obtained with muscle extract. The kinetics of the decrease in enzymatic activity was highly dependent on the dilution factor in both cases, but the inactivation appeared to be biphasic only with extract. The inactivation of the phosphofructokinase was impeded by addition of excess of aldolase. Time courses of kinase inactivation were fitted by alternative kinetic models to characterize the multiple equilibria of several homo- and hetero-oligomers of phosphofructokinase. The combination of modeling data obtained with purified and extract systems suggests that aldolase binds to an intermediate dimer of phosphofructokinase and within this heterocomplex the kinase is completely active. The intermediate dimer is stabilized by association with microtubules and the kinase activity decreased due to dilution can be recovered by addition of excess aldolase. In extract, the phosphofructokinase is of sigmoidal character (Hill coefficient of 2.3); the addition of excess exogenous aldolase to phosphofructokinase resulted in heterocomplex formation displaying Michaelian kinetics. The possible physiological relevance of heterocomplex formation of phosphofructokinase in muscle extract is discussed.