Purification and kinetic mechanism of the major glutathione S-transferase from bovine brain.

The major glutathione S-transferase isoenzyme from bovine brain was isolated and purified approx. 500-fold. The enzyme has a pI of 7.39 +/- 0.02 and consists of two non-identical subunits having apparent Mr values of 22,000 and 24,000. The enzyme is uniformly distributed in brain, and kinetic data at pH 6.5 with 1-chloro-2,4-dinitrobenzene (CDNB) as substrate suggest a random rapid-equilibrium mechanism. The kinetics of inhibition by product, by GSH analogues and by NADH are consistent with the suggested mechanism and require inhibitor binding to several different enzyme forms. Long-chain fatty acids are excellent inhibitors of the enzyme, and values of 1nKi for hexanoic acid, octanoic acid, decanoic acid and lauric acid form a linear series when plotted as a function of alkyl chain length. A free-energy change of -1900 J/mol (-455 cal/mol) per CH2 unit is calculated for the contribution of hydrophobic binding energy to the inhibition constants. The turnover number of the purified enzyme dimer is approx. 3400/min. When compared with the second-order rate constant for the reaction between CDNB and GSH, the enzyme is providing a rate acceleration of about 1000-fold. The role of entropic contributions to this small rate acceleration is discussed.