Changes in phosphofructokinase isozymes during development of myoblasts to myotubes.

The regulation of phosphofructokinase during development of C2C12 myoblasts to myotubes was investigated. Enzyme activity was markedly increased during myogenic development. The increase was observed when enzyme activity was measured under optimal conditions and was not due to changes in the allosteric kinetic properties of the enzyme. Immunoprecipitation of phosphofructokinase from [35S]methionine-labeled myogenic cells revealed that equal amounts of liver and muscle isozymes are present in myoblasts, while in myotubes there was a much higher level of the muscle isozyme. These results were confirmed using an immunoblotting technique. The increase in the level of muscle isozyme in myotubes is due to an increase in the rate of synthesis of the muscle isozyme and occurs in spite of a measurably small increase in its degradation rate. Northern blot analysis using a synthetic oligonucleotide probe showed a 25-fold increase in the level of muscle phosphofructokinase mRNA in myotubes. The conclusion is drawn that the increase in muscle isozyme in myotubes during myogenesis is due to an increase in its mRNA level.