Purification and properties of γγ-enolase from pig brain

Isoelectric focusing revealed three enolase isoforms in pig brain, which were designated as αα(pI = 6.5), αγ- (pI = 5.6), and γγ-enolase (pI = 5.2). The pI of purified γγ-enolase was also 5.2. The γγ-enolase isoform of enolase was purified from pig brain by a purification protocol involving heating to 55°C for 3 min, acetone precipitation, ammonium sulfate precipitation (40%-80%), DEAE Sephadex ion-exchange chromatography (pH 6.2), and Sephadex G200 gel filtration. The final specific activity was 82 units/mg protein. As with other vertebrate enolases, γγ-enolase from pig proved to be a dimer with a native mass of 85 kDa and a subunit mass of 45 kDa. The pH optimum for the reaction in the glycolytic direction is 7.2. The K(m) values for 2-PGA, PEP, and Mg²⁺ were determined to be 0.05, 0.25, and 0.50 mM, respectively, similar to K(m) values of other vertebrate enolases. The amino acid composition of pig γγ-enolase, as determined by amino acid analysis, shows strong similarity to the compositions of γγ-enolases from rat, human, and mouse, as determined from their amino acid sequences. Despite the differences seen with some residues, and considering the ways that the compositions were obtained, it is assumed that pig γγ-enolase is more similar than the composition data would indicate. Moreover, it is likely that the sequences of pig γγ-enolase and the other γγ-enolases are almost identical. Li⁺ proved to be a noncompetitive inhibitor with either 2-PGA or Mg²⁺ as the variable substrate. This enolase crystallized in the monoclinic space group P2, or P2₁. An R(symm) <5% was obtained for data between 50 and 3.65 Å, but was a disappointing 30% for data between 3.65 and 3.10 Å, indicating crystal disorder.