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Ascorbic acid deficiency and the flavin-containing monooxygenase
Abstract:
Activity of the flavin-containing monooxygenase (FMO) was reduced significantly in ascorbic acid deficient guinea pigs. Reduction in oxidation of dimethylaniline (DMA) and of thiobenzamide was associated with a decrease in the activity of the FMO. In both ascorbate supplemented and deficient guinea pig hepatic 12,000 g supernatant fractions, SKF-525A and n-octylamine did not inhibit DMA N-oxidation. Phenobarbital pretreatment did not increase the rate of N-oxidation of DMA. In addition, hepatic supernatant fractions thermally treated at 50 degree were unable to N-oxidize DMA, but 80% of the cytochrome P-450 activity was retained. Also, N-oxidation of DMA was reduced by 53% at pH 7.0, while oxidation of cytochrome P-450 specific substrates was inhibited by only 19%. Kinetic studies of DMA N-oxidation indicate no significant change in the apparent Km in ascorbate supplemented or deficient animals. The in vitro addition of ascorbic acid had no effect on the activity of the FMO. The toxicological implications of the reduction in FMO activity in ascorbic acid deficiency are discussed.
Insights
Ascorbic acid deficiency significantly reduces flavin-containing monooxygenase (FMO) activity in guinea pigs, impacting drug metabolism. This highlights potential toxicological risks associated with vitamin C deficiency.
Area of Science:
- Biochemistry
- Pharmacology
- Toxicology
Background:
- Flavin-containing monooxygenase (FMO) is crucial for metabolizing various xenobiotics.
- Ascorbic acid (vitamin C) is essential for many physiological processes, but its role in FMO activity is not fully understood.
- Ascorbic acid deficiency is known to affect enzyme activities.
Purpose of the Study:
- To investigate the impact of ascorbic acid deficiency on flavin-containing monooxygenase (FMO) activity in guinea pigs.
- To elucidate the specific enzyme responsible for dimethylaniline (DMA) N-oxidation in the context of ascorbic acid deficiency.
- To explore the toxicological implications of altered FMO activity due to vitamin C deficiency.
Main Methods:
- Assessing FMO activity in guinea pigs with varying ascorbic acid levels.
- Measuring the N-oxidation of model substrates like dimethylaniline (DMA) and thiobenzamide.
- Utilizing enzyme inhibitors (SKF-525A, n-octylamine) and thermal inactivation to differentiate enzyme activities.
- Conducting kinetic studies (apparent Km) to analyze enzyme behavior.
Main Results:
- Ascorbic acid deficiency significantly reduced FMO activity, correlating with decreased DMA and thiobenzamide oxidation.
- DMA N-oxidation was found to be mediated by an enzyme distinct from cytochrome P-450, as indicated by thermal stability and pH sensitivity.
- No significant changes in apparent Km for DMA N-oxidation were observed between supplemented and deficient groups.
Conclusions:
- Ascorbic acid deficiency impairs FMO-mediated drug metabolism in guinea pigs.
- The study suggests FMO, not cytochrome P-450, is primarily responsible for DMA N-oxidation in this model.
- Reduced FMO activity in vitamin C deficiency may have significant toxicological consequences.