Rabbit Muscle Phosphofructokinase: Studies of the Subunit Molecular Weight and Structure: ISOLATION OF CARBOXYMETHYLATED CYSTEINYL PEPTIDES AND SEDIMENTATION EQUILIBRIUM STUDIES

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Abstract The molecular weight of the subunit of rabbit muscle phosphofructokinase has been investigated by techniques which include sodium dodecyl sulfate gel electrophoresis, sedimentation equilibrium experiments, and the isolation of carboxymethylcysteine-containing tryptic peptides. Sedimentation equilibrium experiments in high concentrations of guanidine, in 0.5 m acetic or propionic acid, yield values of 75,000 to 85,000 for the subunit molecular weight. Sodium dodecyl sulfate gel electrophoresis resulted in values usually about 75,000 with the protein migrating as a single band. On the other hand, only eight to nine peptides containing carboxymethylcysteine were isolated from the trypsin digestion mixture, which would indicate a molecular weight of about 42,500 to 47,500. These peptides comprise about 40% of the amino acid residues for this molecular weight. From the data presented here, we have concluded that the subunit molecular weight of phosphofructokinase is 75,000 to 85,000. Not only are these subunits identical, but each subunit seems to consist of 2 similar units. Furthermore, these units appear to be linked in such a way as to be resistant to denaturing conditions.