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The metabolism of 20:4 (arachidonic acid) in alkenylacyl, alkylacyl and diacyl lipid classes in choline glycerophospholipids (CGP) and ethanolamine glycerophospholipids (EGP) in rabbit alveolar macrophages was examined. [3H]20:4 was very rapidly incorporated into diacyl glycerophosphocholine (GPC). After the removal of free 20:4, the radioactivity was gradually lost from diacyl GPC. Concomitantly, the radioactivities in alkylacyl GPC and alkenylacyl glycerophosphoethanolamine (GPE) were increased, indicating that 20:4 was mobilized from diacyl GPC to alkylacyl GPC and alkenylacyl GPE. The mobilization was considered to be a 20:4‐specific event. The gradual accumulation of 20:4 in ether phospholipids leads to a high abundance of 20:4 in these lipids. These results suggest metabolic relationships between 20:4 and ether phospholipids, including platelet‐activating factor (PAF).

High levels of ether phospholipids were found in rabbit alveolar macrophages. Choline phosphoglycerides (CPG) contained a significant amount of alkylacyl compound (32.5%). On the other hand, ethanolamine phosphoglyceride (EPG) included a very large amount of alkenylacyl compounds (61.2%). Small amounts of alkenylacyl CPG and alkylacyl EPG were also observed. The occurrence of a high amount of alkylacyl CPG may be related to the synthesis or release of platelet‐activating factor (PAF) from macrophages. Fatty chains at the 1‐ and 2‐positions in each lipid class of CPG or CPG or alkenylacyl EPG were several other. Particularly, the levels of 20∶4 (arachidonic acid) in alkylacyl CPG or alkenylacyl EPG were several times higher than those in corresponding diacyl phospholipids. Large portions of 20∶4‐containing species have alkenyl or alkyl ether moieties at their 1‐position in both CPG (73.6%) and EPG (85.9%). These results suggest the importance of ether‐containing phospholipids in rabbit alveolar macrophages.

Identification of low-abundance, low-molecular-weight native peptides using non-tryptic plasma has long remained an unmet challenge, leaving potential bioactive/biomarker peptides undiscovered. We have succeeded in efficiently removing high-abundance plasma proteins to enrich and comprehensively identify low-molecular-weight native peptides using mass spectrometry. Native peptide sequences were chemically synthesized and subsequent functional analyses resulted in the discovery of three novel bioactive polypeptides derived from an epidermal differentiation marker protein, suprabasin. SBSN_HUMAN[279–295] potently suppressed food/water intake and induced locomotor activity when injected intraperitoneally, while SBSN_HUMAN[225–237] and SBSN_HUMAN[243–259] stimulated the expression of proinflammatory cytokines via activation of NF-κB signaling in vascular cells. SBSN_HUMAN[225–237] and SBSN_HUMAN[279–295] immunoreactivities were present in almost all human organs analyzed, while immunoreactive SBSN_HUMAN[243–259] was abundant in the liver and pancreas. Human macrophages expressed the three suprabasin-derived peptides. This study illustrates a new approach for discovering unknown bioactive peptides in plasma via the generation of peptide libraries using a novel peptidomic strategy.

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