Previous studies showed that the lipid peroxidation product 4-hydroxy-trans-2-nonenal (HNE) stimulates the rat neutrophil oriented migration in vitro within a micromolar range. Its effect is weak with respect to other known chemoattractants, but highly significant on the basis of a two-way analysis of variance. Other hydroxyalkenals were found to be chemotactic within a pico-micromolar range, and their chemotactic power was not correlated to their lipophilicity. The structural requirements for the chemotactic activity of the hydroxyalkenals were studied by testing the influence of alkanals and 2-alkenals on oriented migration. Alkanals are lacking both the trans double-bond and the hydroxy group, while 2-alkenals are lacking only the hydroxy group. The 2-alkenals (2-octenal, 2-nonenal) were found to be chemotactic, whereas alkanals (hexanal, octanal, nonanal) were ineffective. Therefore the chemotactic activity of the aliphatic aldehydes is dependent on the -C = C-CHO part of their molecule. The possibility that unsaturated aliphatic aldehydes are present in an inflammatory site at a concentration at which they are chemotactic in vitro was also investigated. Carbonyls in pleural exudates were analysed at different times after a pleurisy induction and HNE was detected both in the cells and in the cell-free supernant of the exudate at increasing concentrations during the 4 hours of the experiment. The exact source of HNE is unknown, but since HNE has been identified among the degradation products of peroxidized lipids it is likely that this aldehyde is formed consequent on lipid peroxidation reactions which occur at the phlogistic site. The possibility that HNE is involved, at least in part, in the recruitment of neutrophils in the inflammatory area is suggested.
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