An ichthyotoxic protein in the defensive skin secretion of the Red Sea trunkfish Ostracion cubicus

By the aid of acetone precipitation and RP-HPLC column chromatography coupl ed to various bioassays of ichthyotoxicity and cytolysis, a new ichthyotoxi n designated "boxin" was isolated from the defensive skin secretion of the Red Sea trunkfish Ostracion cubicus. Boxin is a stable, heat and proteolysi s resistant protein of 18 kDa. Its protein nature was assessed by spectral analysis, strong proteolysis, amino acid analysis and amino acid sequence d etermination. Similar to pahutoxin (PHN), an organic cationic surfactant de rived from the same secretion, boxin is not effective by injection, and its ichthyotoxicity is achieved only upon delivery to the surrounding water. T he latter suggests that fish lethality is mediated by externally allocated target sites (receptors). Boxin, however, differs from PHN by (1) possessin g a 30 times higher ichthyotoxicity (mole per mole) and (2) being devoid of PHN's capacity to permeabilize biological membranes. From an ecological po int of view, it is noteworthy that polypeptides are very useful in fulfilli ng allomonal functions in the marine environment due to the high informatio n content inherent in their structures and their solubility in seawater.