Defensin

Defensins are roughly 29 to 45 residues folded into a triple-stranded beta-sheet locked by three disulfide bonds, and the class distinction is the pairing pattern of those bonds. Alpha-defensins pair cysteines 1 to 6, 2 to 4 and 3 to 5, and comprise the human neutrophil peptides HNP1 to HNP4 in azurophilic granules plus HD5 and HD6 from intestinal Paneth cells. Beta-defensins pair 1 to 5, 2 to 4 and 3 to 6, and are epithelial, with hBD-1 constitutive and hBD-2 and hBD-3 induced through NF-kB after Toll-like receptor engagement.

Where they act explains their apparent potency. Concentrations inside neutrophil granules reach the milligram per millilitre range, orders of magnitude above what kills in a test tube, while circulating levels are far lower, so the killing happens inside the phagolysosome rather than in plasma. Salt sensitivity varies by member: hBD-3 keeps activity at physiological ionic strength while hBD-1 and hBD-2 largely lose it.

The disulfide pattern is not decoration. Reduce or scramble the bonds and the fold, and usually the antimicrobial activity, is lost, which makes synthetic defensins a hard manufacturing problem: oxidative folding produces a mixture of connectivity isomers with identical mass and often similar retention time. Chromatographic purity and correct disulfide connectivity are separate questions, and a certificate reporting only the first has not addressed the second.

Salivary and serum defensin levels are marketed as immune-status readouts. They mostly track neutrophil turnover and degranulation, so a high HNP result shows that neutrophils were present and activated, not that mucosal defence is competent. Salt sensitivity produces the other standard error, where an impressive killing concentration measured in dilute buffer disappears in physiological saline and further in serum.