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Antimicrobial Agents and Chemotherapy, December 2008, p. 4455-4462, Vol. 52, No. 12
0066-4804/08/$08.00+0     doi:10.1128/AAC.01103-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Identification of Potent Bactericidal Compounds Produced by Escapin, an L-Amino Acid Oxidase in the Ink of the Sea Hare Aplysia californica{triangledown} ,{dagger}

Ko-Chun Ko,1,2 Binghe Wang,3 Phang C. Tai,1,2 and Charles D. Derby1,2*

Department of Biology,1 Neuroscience Institute,2 Department of Chemistry, Georgia State University, Atlanta, Georgia3

Received 16 August 2008/ Returned for modification 23 September 2008/ Accepted 30 September 2008

The ink of sea hares (Aplysia californica) contains escapin, an L-amino acid oxidase that metabolizes L-lysine, thereby producing a mixture that kills microbes and deters attacking predators. This secretion contains H2O2, ammonia, and an equilibrium mixture of "escapin intermediate product" (EIP-K) that includes {alpha}-keto-{varepsilon}-aminocaproic acid and several other molecules. Components of the equilibrium mixture react nonenzymatically with H2O2 to form "escapin end product" (EEP-K), which contains {delta}-aminovaleric acid and {delta}-valerolactam. The proportions of the molecules in this equilibrium mixture change with pH, and this is biologically important because the secretion is pH 5 when released but becomes pH 8 when fully diluted in seawater. The goal of the current study was to identify which molecules in this equilibrium mixture are bactericidal. We show that a mixture of H2O2 and EIP-K, but not EEP-K, at low mM concentrations is synergistically responsible for most of the bactericidal activity of the secretion against Escherichia coli, Vibrio harveyi, Staphylococcus aureus, and Pseudomonas aeruginosa. Low pH enhances the bactericidal effect, and this does not result from stress associated with low pH itself. Sequential exposure to low mM concentrations of EIP-K and H2O2, in either order, does not kill E. coli. Reaction products formed when L-arginine is substituted for L-lysine have almost no bactericidal activity. Our results favor the idea that the bactericidal activity is due to unstable intermediates of the reaction of {alpha}-keto-{varepsilon}-aminocaproic acid with H2O2.

* Corresponding author. Mailing address: Department of Biology and Neuroscience Institute, Georgia State University, P.O. Box 5030, Atlanta, GA 30302-5030. Phone: (404) 413-5393. Fax: (404) 413-5117. E-mail: cderby{at}gsu.edu

{triangledown} Published ahead of print on 13 October 2008.

{dagger} Supplemental material for this article may be found at http://aac.asm.org/.

Antimicrobial Agents and Chemotherapy, December 2008, p. 4455-4462, Vol. 52, No. 12
0066-4804/08/$08.00+0     doi:10.1128/AAC.01103-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.





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