The properties of phosphofructokinase after its slight purification from the mucosa of rat jejunum were studied. 2. The enzyme is inhibited by almost 100% by an excess of ATP (1.6mm), with 0.2mm-fructose 6-phosphate. AMP, ADP, P(i) and NH(4) (+) at 0.2, 0.76, 1.0 and 2mm respectively do not individually prevent the inhibition of phosphofructokinase activity by 1.6mm-ATP with 0.2mm-fructose 6-phosphate to any great extent, but all of them together completely prevent the inhibition of phosphofructokinase by ATP. 3. One of the effects of high concentrations of ATP on the enzyme was to increase enormously the apparent K(m) value for the other substrate fructose 6-phosphate, and this increase is largely counteracted by the presence of AMP, ADP, P(i) and NH(4) (+). At low concentrations of ATP the above effectors individually decrease the concentration of fructose 6-phosphate required for half-maximum velocity and when present together they decrease it further, in a more than additive way. 4. When fructose 6-phosphate is present at a saturating concentration (5mm), 0.3mm-NH(4) (+) increases the maximum velocity of the reaction 3.3-fold; with 0.5mm-fructose 6-phosphate, 4.5mm-NH(4) (+) is required for maximum effect. The other effectors do not change the maximum reaction velocity. 5. The results presented here suggest that NH(4) (+), AMP, ADP and P(i) synergistically decrease the inhibition of phosphofructokinase activity at high concentrations of ATP by decreasing the concentration of fructose 6-phosphate required for half-maximum velocity. Such synergism among the effectors and an observed, low ;energy charge' [(ATP+(1/2)ADP)/(AMP+ADP+ATP)] in conjunction with the possibility of a relatively high NH(4) (+) and fructose 6-phosphate concentration in this tissue, may keep the mucosal phosphofructokinase active and uninhibited by ATP under aerobic conditions, thus explaining the high rate of aerobic glycolysis and the lack of Pasteur effect in this tissue.
