Human glutathione transferase P1-1 and nitric oxide carriers - New role for an old enzyme

S-Nitrosoglutathione and the dinitrosyl-diglutathionyl iron complex are inv olved in the storage and transport of NO in biological systems. Their inter actions with the human glutathione transferase P1-1 may reveal an additiona l physiological role for this enzyme. In the absence of GSH, S-nitrosogluta thione causes rapid and stable S-nitrosylation of both the CyS47 and Cys(10 1) residues. Ion spray ionization-mass spectrometry ruled out the possibili ty of S-glutathionylation and confirms the occurrence of a poly-S-nitrosyla tion in GST P1-1. S-Nitrosylation of Cys(47) lowers the affinity 10-fold fo r GSH, but this negative effect is minimized by a half-site reactivity mech anism that protects one Cys(47)/dimer from nitrosylation. Thus, glutathione transferase P1-1, retaining most of its original activity, may act as a NO carrier protein when GSH depletion occurs in the cell. The dinitrosyl-digl utathionyl iron complex, which is formed by S-nitrosoglutathione decomposit ion in the presence of physiological concentrations of GSH and traces of fe rrous ions, binds with extraordinary affinity to one active site of this di meric enzyme (K-i < 10(-12) M) and triggers negative cooperativity in the v acant subunit (K-i = 10(-9) m). The complex bound to the enzyme is stable f or hours, whereas in the free form and at low concentrations, its life time is only a few minutes. ESR and molecular modeling studies provide a reason able explanation of this strong interaction, suggesting that Tyr(7) and enz yme-bound GSH could be involved in the coordination of the iron atom. All o f the observed findings suggest that glutathione transferase P1-1, by means of an intersubunit communication, may act as a NO carrier under different cellular conditions while maintaining its well known detoxificating activit y toward dangerous compounds.