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Thiol dependence of nitric oxide synthase
1Medizinische Universitätsklinik, Klinische Biochemie und Pathobiochemie, Würzburg, Germany.
Biochemistry
|October 17, 1995
Summary
Glutathione (GSH) and other thiols significantly boost nitric oxide synthase (NOS) activity by maintaining essential protein thiols in a reduced state. This thiol-dependent regulation is crucial for optimal NOS function and stability.
Area of Science:
- Biochemistry
- Enzymology
- Molecular Biology
Background:
- Nitric oxide synthases (NOS) catalyze L-arginine to L-citrulline conversion, requiring NADPH and tetrahydrobiopterin (H4biopterin).
- The role of thiols, such as glutathione (GSH), in NOS activity and stability during experimental procedures is not fully understood.
- Conflicting reports exist regarding GSH's effect on NOS, with some suggesting inhibition in the presence of catalase.
Purpose of the Study:
- To investigate the effects of various thiols, thiol reagents, antioxidants, and H4biopterin regeneration systems on purified porcine cerebellum NOS.
- To elucidate the mechanism by which thiols influence NOS activity, stability, and cofactor requirements.
Main Methods:
- Assays of purified porcine cerebellum NOS activity in the presence of different thiols, thiol reagents, and antioxidants.
- Kinetic analysis of NOS activity, including determination of Vmax and apparent S0.5 for H4biopterin.
- Investigation of the interaction between GSH and protein disulfide isomerase (PDI).
- Assessment of the effects of ascorbate and dihydropteridine reductase on NOS kinetics.
Main Results:
- GSH and other thiols consistently stimulated NOS activity up to 4-fold, contrary to previous reports of inhibition.
- GSH increased Vmax, reduced the loss of activity during assays, and decreased the apparent S0.5 for H4biopterin.
- GSH's stabilizing effect on NOS was enhanced by PDI, suggesting reductive protection of protein thiols.
- Thiol reagents abolished NOS activity, and ascorbate/dihydropteridine reductase mimicked some of GSH's kinetic effects.
Conclusions:
- NOS activity is critically dependent on the reduced state of essential protein thiols, with GSH playing a significant role in maintaining this state.
- GSH enhances NOS activity and stability, potentially by preventing protein thiol oxidation and by influencing H4biopterin redox state.
- The findings clarify the complex role of thiols in NOS function, highlighting their importance for both enzyme kinetics and structural integrity.