Amino acid differences in cat adrenocorticotropin account for the inability of a human-based immunoradiometric assay to detect the molecule in cat plasma.

A commercial immunoradiometric assay kit designed for the measurement of endogenous adrenocorticotropin (ACTH) concentrations in human plasma does not detect the molecule in plasma samples from cats. It was hypothesized that the inability of the assay to detect the molecule was related to variation(s) in the amino acid sequence of cat ACTH, compared with human ACTH. Cat ACTH complementary DNA was cloned from pituitary tissue and sequenced. The deduced structure showed amino acid differences from the human molecule with cat ACTH having a valine instead of alanine at amino acid 32 and a threonine instead of alanine at amino acid 34. Cat and human ACTH were synthesized along with 2 modified peptides containing alanine substitutions at cat ACTH 32 and 34. Only the human ACTH was detected using the commercial kit, indicating that an epitope recognized by one of the antibodies in the assay requires the presence of 2 alanines near the C-terminus of the molecule.