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 coupled to various bioassays of ichthyotoxicity and cytolysis, a new ichthyotoxin designated “boxin” was isolated from the defensive skin secretion of the Red Sea trunkfish Ostracion cubicus. Boxin is a stable, heat and proteolysis resistant protein of 18 kDa. Its protein nature was assessed by spectral analysis, strong proteolysis, amino acid analysis and amino acid sequence determination. Similar to pahutoxin (PHN), an organic cationic surfactant derived from the same secretion, boxin is not effective by injection, and its ichthyotoxicity is achieved only upon delivery to the surrounding water. The latter suggests that fish lethality is mediated by externally allocated target sites (receptors). Boxin, however, differs from PHN by (1) possessing a 30 times higher ichthyotoxicity (mole per mole) and (2) being devoid of PHN's capacity to permeabilize biological membranes. From an ecological point of view, it is noteworthy that polypeptides are very useful in fulfilling allomonal functions in the marine environment due to the high information content inherent in their structures and their solubility in seawater. Received: 22 September 1999 / Accepted: 28 January 2000