Abstract
The kinetic properties of purified calf lens phosphofructokinase (PFK) have been studied under physiological concentrations of ATP, fructose-6-phosphate and cyclic AMP. The rate of the PFK reaction is characterized by a high sensitivity to inhibition by ATP. PFK rates measured as a function of fructose-6-phosphate concentration can be described by the Hill equation. Values of K F6P−0·5, the concentration of fructose-6-phosphate for half-maximal velocity, increase with increasing ATP concentration. Values of the Hill coefficient remain constant at 2 for all ATP concentrations studied. Values of V max also remain constant. Thus, ATP inhibition is competitive with fructose-6-phosphate. Inhibition can be reversed by cyclic AMP, which decreases the K F6P-0·5 values, but does not change the V max value or the Hill coefficient. The regulation of lens PFK activity can be described by a modification of the Hill equation which includes terms for the increase of K F6P−0·5 by ATP and the decrease of K F6P−0·5 by cyclic AMP.
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