Tyrosine-derived cross-linking amino acids in the sheath of Haemonchus contortus infective larvae

The sheath or second-molt cuticle (2M) was isolated from in vitro exsheathed Haemonchus contortus infective larvae (L3[2M]). Acid hydrolysates of 2-mercaptoethanol (2ME)-soluble and 2ME-insoluble cuticular proteins were analyzed by high performance liquid chromatography for tyrosine-derived cross-linking amino acids. Dityrosine and isotrityrosine were identified by their chromatographic behavior, absorbance spectra, and other chemical characteristics in both the 2ME-soluble and 2ME-insoluble fractions. Dityrosine and isotrityrosine were found in greater amounts in the 2ME-insoluble proteins. When intact 2M cuticles were labeled with 125I prior to acid hydrolysis, radiolabel was recovered in tyrosine but not dityrosine or isotrityrosine indicating that the tyrosine cross-links are not susceptible to iodination in the intact protein. The results are consistent with a hypothesis that tyrosine-derived cross-links are important components of H. contortus 2M cuticular proteins.