Kinetic studies on a glutathione S-transferase from the larvae of Costelytra zealandica

Of the glutathione S-transferases from the New Zealand grass grub (Costelytra zealandica) active in conjugating the model substrate 1-chloro-2,4-dinitrobenzene, the most active was isolated in a functionally homogeneous form. This had an isoelectric point of 8.7. Preliminary evidence suggests that it is a homodimer with subunits of Mr 23 500. The dependence of the enzyme-catalysed reaction on substrate concentration was analysed in terms of the rate equation characteristic of Ordered Bi Bi or Rapid-Equilibrium Random mechanisms. Evidence was found for a critical ionizing event at pH 9.3 at 37 degrees C. This event appears to involve a twofold change in charge on the enzyme, which may be the result of co-operative ionizations rather than independent ionizations. This appears to affect neither the binding of the aromatic substrate to the enzyme, nor the maximum catalytic velocity of the enzyme-catalysed reaction. The variation of the kinetics with temperature was studied. Apparent thermodynamic parameters characteristic of the reaction were derived. The possible relevance of the temperature-dependence of the enzyme-catalysed reaction in vivo is discussed.