Leukotriene B4 (LTB4) is a dihydroxy fatty acid derived from arachidonic acid through the 5-LO pathway. It promotes a number of leukocyte functions including aggregation, stimulation of ion fluxes, enhancement of lysosomal enzyme release, superoxide anion production, chemotaxis, and chemokinesis. 12-oxo LTB4 is an initial metabolite of LTB4 formed via the LTB4 12-hydroxydehydrogenase pathway. It is rapidly converted to 10,11-dihydro-12-oxo-LTB4, followed by reduction of the 12-oxo group to give 10,11-dihydro-LTB4. 12-oxo-LTB4 (EC50 = 33 nM) is about 70-fold less potent than LTB4 (EC50 = 0.46 nM) at stimulating Ca2+ mobilization in human neutrophils. It is also significantly less potent than LTB4 at stimulating neutrophil migration with EC50 values of 170 and 2.7 nM for 12-oxo-LTB4 and LTB4, respectively.
The effects of Leukotriene B4 (LTB4) are mediated by two known receptors, BLT1 and BLT2. LTB4 is a high affinity ligand for BLT1, and many of its pro-inflammatory effects are believed to be transduced through this receptor. The BLT2 is more enigmatic, in that LTB4 is not a high-affinity ligand, nor is it clear that BLT2 activation promotes inflammation. LTB4 ethanolamide (LTB4-EA) is a theoretical 5-LO metabolite of arachidonoyl ethanolamide (AEA). In CHO cells transfected with human BLTR1, LTB4-EA was a potent antagonist with about three times greater affinity for the receptor than LTB4 (Ki = 1.22 nM versus 3.88 nM). LTB4-EA antagonizes the LTB4-induced contractions of guinea pig lung parenchyma with an EC50 of 10 nM. LTB4-EA thus represents a potential endogenous anti-inflammatory compound functioning as a natural antagonist of BLTR1.
Leukotriene B4 (LTB4) compounds are produced by both enzymatic and non-enzymatic processes. The products of enzymatic origin, via Leukotriene A4 (LTA4) hydrolase, are stereospecifically 12(R). Non-enzymatic hydrolysis products are 50:50 mixtures at C-12, but are almost exclusively trans at C-6. Thus, the non-enzymatic hydrolysis product of LTA4 is 6-trans-12-epi LTB4. 12-epi LTB4 is an isomer which would not be expected to occur in either non-enzymatic hydrolysis products, or in the enzymatic products of LTA4 hydrolase. Compared to LTB4, 12-epi LTB4 has significantly reduced activity for the LTB4 receptor on human neutrophils (IC50 of 7.5 mM), and on guinea pig lung membranes with a (Ki of 4.7 mM). 12-epi LTB4 is an weak agonist at both the recombinant human BLT1 and BLT2 receptors, requiring approximately 10 mM for full activation of the receptor.
Leukotriene B4 (LTB4) is a dihydroxy fatty acid derived from arachidonic acid through the 5-lipoxygenase pathway. It promotes a number of leukocyte functions including aggregation, stimulation of ion fluxes, enhancement of lysosomal enzyme release, superoxide anion production, chemotaxis, and chemokinesis. At least two LTB4 receptors, termed BLT1 and BLT2, have been identified. 14,15-dehydro LTB4 is a LTB4 receptor antagonist that has a higher binding affinity for BLT1, demonstrating a Ki value of 27 nM, compared to BLT2, which has a Ki value of 473 nM. 14,15-dehydro LTB4 inhibits LTB4-induced release of lysozymes from rat polymorphonuclear leukoctyes with an IC50 value of 1 μM.
20-hydroxy LTB4 is a metabolite of LTB4 in human neutrophils. In human leukocytes, LTB4 is inactivated by the enzyme LTB4 20-hydroxylase. 20-hydroxy LTB4 is not only much less active (~5%) compared to LTB4 in causing degranulation of PMNL, but actually inhibits LTB4-induced degranulation of human neutrophils (Ki = 13.3 nM). However, 20-hydroxy LTB4 is as active as LTB4 in contracting parenchymal strips from guinea pig lung. 20-hydroxy LTB4 retains considerable ligand binding affinity at the BLT2 receptor, but does not appear to function as an agonist.
LTB4 dimethyl amide is a moderate inhibitor of LTB4-induced degranulation of human neutrophils (Ki = 130 nM) and lysozyme release from rat PMNL. LTB4 dimethyl amide appears to be an antagonist of the LTB4 receptor on guinea pig lung membranes.
Leukotriene B4 (LTB4)-3-aminopropylamide is an analog of LTB4 that exhibits potent and selective binding to the BLT1 receptor with Ki values of 5.1 nM at BLT1 and 1,227 nM at BLT2, indicating its high affinity for BLT1 over BLT2. This compound's effects are mediated through interactions with two receptors, BLT1 and BLT2.
18-Carboxy dinor Leukotriene B4 (18-carboxy dinor LTB4) represents a β-oxidation metabolite of Leukotriene B4 (LTB4). Initially, LTB4 is metabolized in the liver to 20-carboxy LTB4, which subsequently undergoes β-oxidation to form 18-carboxy dinor LTB4.