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Paraquat toxicity and pyridine nucleotide coenzyme synthesis: a data correction
1John M. Dalton Research Center, Columbia, MO.
Free Radical Biology & Medicine
|January 1, 1990
Summary
Pyridine nucleotide coenzyme loss in E. coli poisoned by oxygen or paraquat is not due to enzyme impairment. A naturally occurring inhibitor in E. coli extracts caused previously reported low enzyme activity.
Area of Science:
- Biochemistry
- Microbiology
- Enzymology
Background:
- Pyridine nucleotide coenzymes are vital for cellular metabolism.
- Previous studies suggested impaired nicotinatemononucleotide pyrophosphorylase (carboxylating) activity in Escherichia coli poisoned by hyperbaric oxygen or paraquat.
- This impairment was hypothesized to cause coenzyme depletion.
Purpose of the Study:
- To investigate the cause of pyridine nucleotide coenzyme decrease in E. coli exposed to hyperbaric oxygen or paraquat.
- To determine if nicotinatemononucleotide pyrophosphorylase (carboxylating) [EC 2.4.2.19] activity is impaired under these conditions.
- To identify factors contributing to altered enzyme activity measurements.
Main Methods:
- E. coli extracts were prepared after exposure to 1 mM paraquat or 4.2 atmospheres of oxygen.
- Nicotinatemononucleotide pyrophosphorylase (carboxylating) activity was directly assayed in these extracts.
- Enzyme activity was also tested in Neurospora crassa exposed to 1 mM paraquat.
- E. coli extracts were analyzed for the presence of enzyme inhibitors.
Main Results:
- Direct enzyme assays showed no impairment of nicotinatemononucleotide pyrophosphorylase (carboxylating) in E. coli exposed to paraquat or hyperbaric oxygen.
- Enzyme activity in Neurospora crassa was also unaffected by paraquat exposure.
- A non-dialyzable inhibitor of nicotinatemononucleotide pyrophosphorylase (carboxylating) was identified in E. coli extracts.
- This inhibitor explained the previously observed erroneous low enzyme activities.
Conclusions:
- The decrease in pyridine nucleotide coenzymes in poisoned E. coli is not caused by impaired nicotinatemononucleotide pyrophosphorylase (carboxylating) activity.
- A naturally occurring inhibitor present in E. coli extracts interferes with enzyme assays, leading to false low activity readings.
- This finding corrects previous interpretations regarding the mechanism of coenzyme depletion under oxidative stress.