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Inactivation of porcine heart cytoplasmic malate dehydrogenase by pyridoxal 5'-phosphate
Abstract:
Pyridoxal 5'-phosphate (pyridoxal-5'-P) has been found to act as a bifunctional reagent during the inactivation of porcine heart cytoplasmic malate dehydrogenase (L-malate: NAD+ oxidoreductase, EC 1.1.1.37). The biphasic kinetics and X-azolidine-like structure formed were similar to those observed for mitochondrial malate dehydrogenase (Wimmer, M.J., Mo, T., Sawyers, D.L., and Harrison, J.H. (1975) J. Biol. Chem. 250, 710-715). In the cytoplasmic enzyme, however, irreversible inactivation representing X-azolidine formation was found to be the dominant characteristic of the interaction with pyridoxal-5'-P. Spectral evidence indicated that at total inactivation 2 mol of pyridoxal-5'-P were incorporated per mol of enzyme or one pyridoxal-5'-P per enzymatic active site. The presence of NADH protected the enzyme from inactivation suggesting interaction of pyridoxal-5'-P at or near the enzymatic active centers of this enzyme. Fluorometric titrations indicated that pyridoxal-5'-P-inactivated enzyme failed to bind NADH or at least failed to bind NADH in the same fashion as native enzyme.
Insights
Pyridoxal-5'-phosphate inactivates porcine heart cytoplasmic malate dehydrogenase through X-azolidine formation. This process involves one pyridoxal-5'-phosphate per active site and prevents NADH binding.
Area of Science:
- Biochemistry
- Enzymology
Background:
- Porcine heart cytoplasmic malate dehydrogenase is a key enzyme in cellular metabolism.
- Pyridoxal 5'-phosphate (pyridoxal-5'-P) is a derivative of vitamin B6 with known cofactor roles.
Purpose of the Study:
- To investigate the mechanism of inactivation of porcine heart cytoplasmic malate dehydrogenase by pyridoxal-5'-P.
- To characterize the interaction between pyridoxal-5'-P and the enzyme's active site.
Main Methods:
- Enzyme kinetics studies to observe biphasic inactivation.
- Spectral analysis to identify reaction products and stoichiometry.
- Fluorometric titrations to assess cofactor binding.
Main Results:
- Pyridoxal-5'-P acts as a bifunctional reagent, leading to irreversible inactivation.
- X-azolidine formation occurs, with 1 mole of pyridoxal-5'-P incorporated per active site.
- NADH protects the enzyme, and inactivated enzyme shows impaired NADH binding.
Conclusions:
- Pyridoxal-5'-P irreversibly inactivates cytoplasmic malate dehydrogenase via X-azolidine formation at the active site.
- The enzyme's ability to bind NADH is compromised after inactivation.
- This interaction highlights the role of pyridoxal-5'-P in enzyme regulation and active site chemistry.
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