Two novel muramidases from skin mucosa of rainbow trout ( Oncorhynchus mykiss)

Abstract

Two novel antibacterial muramidases were purified to homogeneity from skin exudates of rainbow trout ( Oncorhynchus mykiss). Unusually, one has an acidic isoelectric point and it is the first anionic muramidase to be reported for fish. Its molecular mass is 14 268 Da, as determined by mass spectrometry. The other muramidase is cationic with a mass of 14 252 Da. Partial N-terminal amino acid sequencing and peptide mapping strongly point to it being a c-type lysozyme, the first to be purified and characterised from skin of a salmonid. Its optimum pH ranges from 4.5 to 5.5 and its optimum temperature, at pH 5.0, is 33–49 °C, although it still exhibits activity at 5 °C. It is strongly bactericidal to the Gram-(+) bacterium Planococcus citreus, with a minimum bactericidal concentration of 100 U ml −1, but is neither chitinolytic nor haemolytic. These two muramidases probably contribute to epithelial defence of the fish against microbes, either alone or in synergism with antibacterial peptides.