Regulation of enzyme activity in the hibernator. A kinetic and spectroscopic study of muscle pyruvate kinase from the Arctic ground squirrel.

Pyruvate kinase skeletal muscle of the Arctic ground squirrel was purified to homogeneity. The purified enzyme variants from the summer-active and winter hibernating squirrel appear to be identical with a near-neutral pI of 6.9 and a molecular weight of 234,000 as determined by gel filtration chromatography on Bio-Gel A-0.5m. Evidence for subunit interaction during inhibition by L-phenylalanine is demonstrated with ultraviolet derivative spectroscopy. A model for this interaction and its importance for a regulatory role are discussed. The absence of a temperature break in the Arrhenius plot for the pyruvate kinase reaction, the kinetic and physical data, and the near-neutral pI, suggest an amino acid composition that conserves the overall geometry and resultant kinetic behavior which render regulation of the enzyme insensitive to temperature.