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  • Title: Inactivation of Phycomyces isocitrate lyase by thiol-reactive reagents. Evidence for an essential thiol group.
    Author: Olano J, de Arriaga D, Rúa J, Busto F, Soler J.
    Journal: Biochim Biophys Acta; 1992 Mar 12; 1119(3):287-95. PubMed ID: 1547274.
    Abstract:
    Isocitrate lyase from the mycelium of Phycomyces blakesleeanus was inactivated with thiol-reactive reagents, 5,5'-dithiobis-(2-nitrobenzoic)acid, p-hydroxymercuribenzoic acid, N-ethylmaleimide or iodoacetate, at pH 6.8 and 25 degrees C. In all cases the inactivation is characterized by a biphasic kinetic profile. The rapid initial phase of inactivation does not increase linearly with increasing reagent concentration, but exhibits an apparent saturation effect, suggesting the formation of a reversible complex between the enzyme and the reagent prior to the inactivation step. Re-activation of the enzyme was observed under thiol excess treatment. The pH dependence of the initial phase of inactivation suggests that a group on the enzyme with pKa = 6.8 is being modified. The effect of ligands was tested on the inactivation reaction. Mg(2+)-Ds-isocitrate and Ds-isocitrate provided total protection, whereas Mg2+ ions, succinate and oxalate provided only partial protection of the enzyme against inactivation. On the basis of these results, we would suggest that the thiol-reactive reagents modify at least one thiol group crucial for the enzymatic activity and probably located in the interface between succinate and glyoxylate subsite.
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