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The effect of metal ions on mitochondrial pyridine dinucleotide transhydrogenase
Abstract:
Bovine heart submitochondrial particle transhydrogenase is inhibited by cations in a concentration and pH-dependent manner, and non-energy-linked transhydrogenation is inhibited to a greater extent by metals than the energy-linked reaction. The inhibition of the enzyme by Mg2+ is competitive with the NADP substrate and non-competitive with the NAD substrate. Mg2+ stimulates inactivation of the enzyme by 5,5'-dithiobis(2-nitrobenzoic acid), and protects against thermal and proteolytic inactivation. This suggests that Mg2+ binding in the NADP site alters transhydrogenase to a more thermostable conformation, which is less susceptible to attack by trypsin and more reactive with 5,5'-dithiobis(2-nitrobenzoic acid). Other cation inhibitors mimic Mg2+ in these properties. The order of effectiveness of the inhibitors tested is La3+ greater than Mn2+ greater than Ca2+ congruent to Mg2+ greater than Sr2+ greater than Na+ congruent to K+. This order is described by the Irving-Williams order for the stability of metal-ligand complexes, suggesting that carboxylates or amines may comprise the inhibitory cation binding site.
Insights
Cations like magnesium (Mg2+) inhibit bovine heart transhydrogenase by binding to the NADP site, altering enzyme stability and activity. This inhibition is concentration and pH-dependent, with varying effects on energy-linked reactions.
Area of Science:
- Biochemistry
- Enzymology
- Mitochondrial research
Background:
- Bovine heart submitochondrial particle transhydrogenase is crucial for cellular energy metabolism.
- Understanding enzyme inhibition mechanisms is key to elucidating metabolic pathways.
Purpose of the Study:
- To investigate the inhibitory effects of cations on bovine heart transhydrogenase.
- To characterize the mechanism of inhibition by magnesium (Mg2+) and other metal ions.
Main Methods:
- Enzyme kinetics assays to determine inhibition patterns (competitive, non-competitive).
- Studies on enzyme inactivation by chemical agents (DTNB), heat, and proteolysis.
- Analysis of cation inhibition order using the Irving-Williams series.
Main Results:
- Cations inhibit transhydrogenase in a concentration and pH-dependent manner.
- Mg2+ inhibition is competitive with NADP+ and non-competitive with NAD+, suggesting binding at the NADP+ site.
- Mg2+ enhances DTNB inactivation and protects against thermal and proteolytic degradation, indicating a conformational change.
Conclusions:
- Cation binding, particularly Mg2+, stabilizes transhydrogenase in a conformation less prone to degradation but more reactive to DTNB.
- The observed inhibition order aligns with the Irving-Williams series, suggesting carboxylate or amine ligands at the cation binding site.