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14,15-Dehydroleukotriene A4: a specific substrate for leukotriene C4 synthase
1Center for Cardiopulmonary Pharmacology, Institute of Pharmacological Sciences, University of Milan, Italy.
The Biochemical Journal
|January 10, 1998
Summary
14,15-dehydro-leukotriene A4 (14,15-dehydro-LTA4) is metabolized by human platelets to 14,15-dehydro-leukotriene C4 (14,15-dehydro-LTC4). However, it does not form 14,15-dehydro-leukotriene B4 in neutrophils and irreversibly inhibits leukotriene A4 hydrolase.
Area of Science:
- Biochemistry
- Enzymology
- Lipid Metabolism
Background:
- Leukotrienes are key inflammatory mediators derived from arachidonic acid.
- Leukotriene A4 (LTA4) is a pivotal intermediate in leukotriene biosynthesis.
- Understanding LTA4 metabolism is crucial for developing anti-inflammatory therapies.
Purpose of the Study:
- To investigate the metabolic fate of 14,15-dehydro-LTA4 in human platelets and polymorphonuclear leukocytes (PMNLs).
- To determine the enzyme kinetics and inhibition patterns of 14,15-dehydro-LTA4 metabolism.
- To elucidate the role of the C-14,15 double bond in LTA4 metabolism and enzyme interactions.
Main Methods:
- Reversed-phase High-Performance Liquid Chromatography (HPLC) with diode-array UV detection for metabolite separation and identification.
- Enzymatic assays using human platelet leukotriene C4 (LTC4) synthase and PMNL leukotriene A4 (LTA4) hydrolase.
- Inhibition studies using MK-886, a known LTC4 synthase inhibitor.
- Use of purified neutrophil and eosinophil preparations to assess cell-specific metabolism.
Main Results:
- Human platelets efficiently metabolize 14,15-dehydro-LTA4 to 14,15-dehydro-LTC4, with kinetics similar to authentic LTA4.
- This metabolism is inhibited by MK-886, indicating LTC4 synthase involvement.
- 14,15-dehydro-LTA4 is not converted to 14,15-dehydro-leukotriene B4 by human red blood cells or PMNLs.
- Minor 14,15-dehydro-LTC4 in PMNLs is attributed to eosinophil contamination.
- 14,15-dehydro-LTA4 irreversibly inhibits PMNL LTA4 hydrolase (IC50 = 0.73 microM).
- The methyl terminus of LTA4 does not affect LTC4 synthase metabolism, but the C-14,15 double bond is crucial for LTA4 hydrolase activity, not binding.
Conclusions:
- Human platelet LTC4 synthase metabolizes 14,15-dehydro-LTA4 similarly to LTA4.
- PMNLs do not significantly metabolize 14,15-dehydro-LTA4, and it acts as an irreversible inhibitor of LTA4 hydrolase.
- The C-14,15 double bond is essential for LTA4 hydrolase catalytic function.