Antimicrobial Peptides, Isolated from Horseshoe Crab Hemocytes, Tachyplesin II, and Polyphemusins I and II: Chemical Structures and Biological Activity1

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Abstract
Tachyplesin is an antimicrobial peptide recently found in the acid extract of hemocytes from the Japanese horseshoe crab (Tachypleus tridentatus) [Nakamura, T. et al. (1988) J. Biol Chem. 263, 16709–16713]. In our continuing studies on the peptide, we have found an isopeptide, tachyplesin II, and also polyphemusins I and II in hemocytes of the American horseshoe crab (Limulus polyphemus). The complete primary structures of these peptides, which are very similar to that of the previously isolated peptide, now named tachyplesin I, were determined to be as follows: Polyphemusin I NH2-R-R-W-C-F-R-V-C-Y-R-G-F-C-Y-R-K-C-R-CONH2 Polyphemusin II NH2-R-R-W-C-F-R-V-C-Y-K-G-F-C-Y-R-K-C-R-CONH2 Tachyplesin II NH2-R-W-C-F-R-V-C-Y-R-G-I- C-Y-R-K-C-R-CONH2 The isopeptide, tachyplesin II, consists of 17 residues with a COOH-terminal arginine a-amide. On the other hand, both polyphemusins I and II were found to contain 18 residues due to an additional Arg residue at the NH2-terminal end as well as a COOH-terminal arginine α-amide. The disulfide linkages for polyphemusin I consisted of two bridges between Cys-4 and Cys-17 and between Cys-8 and Cys-13, which was identical to in the case of tachyplesin I. Moreover, all of these peptides inhibited the growth of not only Gram-negative and -positive bacteria but also fungi, such as Candida albicans M9. Furthermore, complex formation between these peptides and bacterial lipopolysaccharides was also observed in a double diffusion test. These results suggest that tachyplesins and poly-phemusins are probably located in the hemocyte membrane, where they act on antimi-crobial peptides as a self-defense mechanism in the horseshoe crab against invading microorganisms.

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