Heart 6-phosphofructo-1-kinase. Subcellular distribution and binding to myofibrils

6-Phosphofructo-1-kinase (phosphofructokinase) (ATP:D-fructose-6-P 1-phosphotransferase, EC 2.7.1.11) can be identified in sheep heart homogenates in two forms, a soluble form and a form bound to the particulate fraction. Homogenates from immediately-dissected hearts have the enzyme in the soluble form, while those collected after a delay have the enzyme bound to the particulate fraction. Aldolase appears to show the same change in its location. Homogenization in a solution with concentrated macromolecular species (20% albumin) results in a greater association of phosphofructokinase and of aldolase to the particulate fraction in homogenates from immediately dissected hearts. Phosphofructokinase activity can be solubilized by two specific means: by high ionic strength, which is dependent upon specific salts; or by low ionic strength, which is dependent upon the presence of phosphofructokinase substrates or modifier ligands. These two means of solubilization are affected differently upon decreasing the pH below 6.9: the solubilization at low ionic strength is prevented, whereas phosphofructokinase is still solubilized by high ionic strength. Under the latter condition, the enzyme is in the inactive dimeric state, which can be activated at an alkaline pH. Myofibrils present in the particulate fraction can account for the binding of phosphofructokinase in heart homogenates. Purified myofibrils, when added to heart supernatant fluids, can bind phosphofructokinase at a slightly acidic pH. Conditions for phosphofructokinase binding to myofibrils, as well as its dissociation, follow what was observed with the binding of phosphofructokinase to the particulate fraction. At an acidic pH, and in the presence of a high concentration of ATP, phosphofructokinase exhibits low activity. However, if phosphofructokinase is assayed under these conditions while bound to myofibrils, the enzyme is activated.