Neutrophil gelatinase associated lipocalin (NGAL), a constituent of neutrophil granules, is a member of the lipocalin family of binding proteins. NGAL can also be highly induced in epithelial cells in both inflammatory and neoplastic colorectal disease. NGAL is proposed to mediate inflammatory responses by sequestering neutrophil chemoattractants, particularly N-formylated tripeptides and possibly leukotriene B(4) and platelet activating factor. The crystal structures of NGAL display a typical lipocalin fold, albeit with an unusually large and atypically polar binding site, or calyx. The fold of NGAL is most similar to the epididymal retinoic acid-binding protein, another lipocalin, though the overall architecture of the calyces are very different. The crystal structures also reveal either sulfate ions or an adventitiously copurified fatty acid bound in the binding site. Neither ligand is displaced by added N-formylated tripeptides. The size, shape, and character of the NGAL calyx, as well as the low relative affinity for N-formylated tripeptides, suggest that neither the copurified fatty acid nor any of the proposed ligands are likely to be the preferred ligand of this protein. Comparisons between the crystal structures and the recently reported solution structure of NGAL reveal significant differences, in terms of both the details of the structure and the overall flexibility of the fold.

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Cathelicidins are a family of antibacterial and lipopolysaccharide-binding proteins. hCAP-18, the only human cathelicidin, is a major protein of the specific granules of human neutrophils. The plasma level of hCAP-18 is >20-fold higher than that of other specific granule proteins relative to their levels within circulating neutrophils. The aim of this study was to elucidate the background for this high plasma level of hCAP-18. Plasma was subjected to molecular sieve chromatography, and hCAP-18 was found in distinct high molecular mass fractions that coeluted with apolipoproteins A-I and B, respectively. The association of hCAP-18 with lipoproteins was validated by the cofractionation of hCAP-18 with lipoproteins using two different methods for isolation of lipoproteins from plasma. Furthermore, the level of hCAP-18 in delipidated plasma was <1% of that in normal plasma. Immunoprecipitation of very low, low, and high density lipoprotein particles with anti-apolipoprotein antibodies resulted in coprecipitation of hCAP-18. hCAP-18 belongs to the cathelicidins, a group of antimicrobial peptides found in mammalian neutrophils (1). The cathelicidins share a highly conserved N-terminal prosequence that is homologous to cathelin, a protein first isolated from porcine leukocytes (2). The active antimicrobial domains of the cathelicidins generally reside in their C termini. The antimicrobial activity is observed only when the C-terminal domain is cleaved from the holoprotein (3-5). The C termini of the cathelicidins show great variability in amino acid sequence, but they are all highly cationic and hydrophobic. The antimicrobial part of many cathelicidins, including the C terminus of hCAP-18 (also named LL-37), has been shown to bind lipopolysaccharide (6).Porcine and bovine neutrophils contain a variety of cathelicidins, whereas hCAP-18 is the only cathelicidin identified in humans (6 -8). hCAP-18 is a major protein of the specific granules of human neutrophils (9), but is also present in squamous epithelia (10) and in keratinocytes during inflammatory skin diseases (11). Transcripts for hCAP-18 have been found in lung tissue by in situ hybridization (12).We have previously shown that the relative plasma levels of specific granule proteins from neutrophils are very low compared with the levels in circulating neutrophils (Ͻ1%) (13). In contrast, the concentration of hCAP-18 in plasma is ϳ1.2 g/ ml, which is Ͼ20% of the amount present in circulating neutrophils (14). In general, neutrophils are activated to release their granule proteins only when present outside the circulation. Thus, degranulation is unlikely to be the cause of this high plasma level of hCAP-18 since other granule proteins localized in the same granule subset would be expected to have equally high plasma levels as hCAP-18. This is not the case. Thus, a specific mechanism must exist to sequester hCAP-18 in the circulation and provide the relatively high concentration of this pro-bactericidal protein in plasma. Since hCAP-18 partitions mainly in the hydrop...