The A2B2 hybrid isozyme of phosphofructokinase

The hybrid isozyme of phosphofructokinase, A2B2, was formed by incubation of rabbit muscle enzyme. A4, and rabbit liver enzyme, B4, in the presence of sodium citrate at neutral pH. The enzyme composition of the resulting mixture of A2B2 and the homoprotomeric forms was identical to that found in rabbit adipose tissue extracts. Hybrid formation, which apparently proceeds by way of dimers, can be blocked by fructose-1,6-P2, fructose-6-P, and high concentrations of MgATP. The A2B2 isozyme was separated from A4 and B4 by ion exchange chromatography. The kinetic regulatory properties of A2B2 were compared with those of A4, B4, and a 1:1 mixture of A4 and B4. ATP inhibition of A2B2 was intermediate between that observed with A4 and B4 and was clearly not identical to a simple summing of the effects of A and B subunits. Similar comparisons were made using other phosphofructokinase inhibitors, citrate, 2,3-P2-glycerate, and P-creatine. In each case the observed inhibition was intermediate between the observed with A4 and B4. The existence in a number of tissues of phosphofructokinase A2B2 provides added diversity to the regulatory mechanisms of glycolysis.