Figure 16-12. The model indicates the position of the iron atom. Close to the iron atom are two adjacent unoccupied positions, which are thought to be the location of interaction of the 1,4-diene system of the substrate fatty acid with molecular oxygen during catalysis. The lipoxins (LP) derive directly from arachidonic acid through the 15-lipoxygenase pathway (Figure 1613). Little is known about lipoxin actions. However, LPA dilates the microvasculature, and both LPA and LPB inhibit the cytotoxic effects of natural killer cells.
As can be seen from Figure 16-11, there are at least seven known LTs. Importantly, LTCj, LTGj, and LTC5 contain glutathione, and LTD4 contains cysteinylgly-cine. The D series contains sulfur amino acid residues as well. None of these very active compounds arise via a cycloendoperoxide, but all arise from the direct peroxidation of the fatty acid substrate on C-5. These compounds are very active bronchoconstrictors in the lungs and airways and are of importance in the etiology of asthma.
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If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.