SKELETAL-MUSCLE PHOSPHOFRUCTOKINASE ACTIVITY EXAMINED UNDER PHYSIOLOGICAL CONDITIONS IN-VITRO

The in vitro activity of skeletal muscle phosphofructokinase (PFK) was determined over the full physiological range of citrate concentration s. Enzyme aggregation was enhanced with a crowding agent, as the regul atory properties of PFK are altered with dilution. Cuvette conditions simulated concentrations of effecters and substrates during rest, mode rate aerobic exercise, and intense aerobic exercise in human skeletal muscle. Citrate inhibition was not eliminated with enhanced enzyme agg regation, but activity was improved at all citrate concentrations. Max imal PFK activity with no citrate present was 0.27 +/- 0.01 mu mol . m in(-1) . mg(-1) protein with resting effectors and 1.64 +/- 0.07 and 7 .15 +/- 0.52 mu mol . min(-1) . mg(-1) protein with moderate aerobic a nd intense aerobic effector levels, respectively. Under resting condit ions, PFK activity decreased to 49% of maximal when citrate was increa sed from 0 to 0.15 mM and only a small further inhibition to 43% occur red at 0.5 mM. Citrate was a less potent inhibitor under both exercise conditions with the sharpest decline to 72-77% of maximal activity at 0.15 mM followed by a slower decline to 65-70 and 53% activity at 0.2 5 and 0.5 mM citrate, respectively. The present in vitro measurements predict that alterations in citrate around concentrations normally rep orted in resting and exercising muscle would have little effect on flu x through PFK. Therefore, the generally accepted concept that citrate is a potent inhibitor of PFK in all physiological situations has been exaggerated.