The amino acid composition of the various types of cuticle of Limulus polyphemus

Abstract

Amino acid analyses are presented for various fractions of the carapace and separately for epicuticle and procuticle of arthrodial membrane of the horseshoe crab, Limulus polyphemus L. The carapace is characterized by high percentages of glycine, alanine, and tyrosine; the epicuticle is high in glycine and especially tyrosine; the procuticle of soft membrane is notably high in aspartic and glutamic acids. Of the common amino acids, tryptophan and cystine are absent. In constrast to other structural proteins but in general agreement with other reports on arthropod cuticle, the proteins contain high percentages of phenyl, dicarboxylic, diamino, and other acids which make bulky side groups on the peptide chains. In the carapace there are no striking differences in amino acid composition between outer, middle, and inner fractions although both dicarboxylic and diamino acids are somewhat higher in the dark outer portion. It follows that the events of sclerotization in this species are not reflected by amino acid composition. The data also give no support to the idea that tyrosine contributes to make a ‘self-tanning protein’ in cuticle. The data support previous suggestions that the epicuticle over sclerites is different from that over intersegmental membranes. Histological preparations show that the brown portion of the sclerotized carapace of Limulus is different from darkened sclerites of insects in that it can be stained with both acid fuchsin and orange G.