Nine polypeptide peaks with antibiotic activity were resolved from human polymorphonuclear leukocyte azurophil granule membranes. All but 1 of the 12 constituent polypeptides were identified by N-terminal sequence analysis.Near quantitative recovery of protein and activity permitted an assessment of the contribution of each species to the overall respiratory-burst-independent antimicrobial capacity of the cell. Three uncharacterized polypeptides were discovered, including two broad-spectrum antibiotics. One of these, a defensin that we have designated human neutrophil antimicrobial peptide 4, was more potent than previously described defensins but represented less than 1% of the total protein. The other, named azurocidin, was abundant and comparable to bactericidal permeability-increasing factor in its contribution to the killing of Escherichia coli.

A 14-kD protein was purified from human PMNs and its NH2-terminal sequence was determined. Comparison of a portion of the NH2-terminal sequence of this protein to the recently reported NH2-terminal sequence of eosinophil major basic protein (MBP) showed them to be identical. To aid further characterization of the structural and functional properties of this molecule, we isolated from an HL-60 cDNA library a single class of cDNA clones whose sequence matched exactly the NH2-terminal amino acid sequence of the 14-kD polypeptide. Northern analysis of HL-60 cells suggests that MBP is constitutively expressed in HL-60 cells and is highly transcribed from a single copy gene. The sequence of the full-length cDNA clones predicts that MBP is synthesized as a 23-kD precursor form (pro-MBP) which is subsequently cleaved to release the mature 14-kD MBP. The putative pro-MBP has a predicted pI of 6.0, but both the charged and the hydrophobic residues are asymmetrically distributed, creating a bipolar molecule. The NH2-terminal half has a predicted pI of 3.7 and is hydrophilic, while the COOH-terminal half (corresponding to mature MBP) has a predicted pI of 11.1 and is hydrophobic.

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