The effect of trypsin treatment on rabbit muscle phosphofructokinase

The consequences of trypsin treatment of rabbit muscle phosphofructokinase, in terms of the physical and kinetic properties of the enzyme, have been investigated. At 1% trypsin (w/w) and 25 °C, no activity is lost over a period of 60 min. The complex sedimentation behavior at pH 8 (three peaks) is unchanged by this treatment as is the extent of dissociation of the enzyme when the pH is lowered from 8 to 6 or reassociation when the pH is raised back to 8. However, the trypsin-treated enzyme shows a subunit molecular weight, determined in the guanidine HCl or 0.5 m acetic acid, of 35,000-40,000 compared to the subunit molecular weight of the untreated enzyme at 75,000-80,000. Similarly, SDS gels give only a single species of about 80,000 for the native enzyme but two species, 42,000 and 48,000, for the trypsin-treated enzyme. Kinetic studies showed no differences in the regulatory properties of the enzyme including fructose 6-phosphate cooperativity, ATP inhibition, NH_4⁺ activation, and cAMP activation. Small differences in stability and inhibition by citrate and creatine phosphate were observed.