This is the rat form of urotensin-II, and it differs from the other variants in two respects visible in the supplied sequence: it carries a pyroglutamate at the N-terminus and terminates in isoleucine where the human, frog and goby forms end in valine. At fourteen residues it is the longest of the variants offered. The N-terminal pyroglutamate is a cyclised glutamine residue and is chemically consequential rather than cosmetic. It blocks the free alpha-amino group, which confers resistance to aminopeptidase degradation and also means the peptide will not sequence by standard Edman chemistry without prior treatment, a practical detail worth knowing before sending material for N-terminal analysis. Despite these differences the disulfide-bridged cyclic core is conserved, as it is across every species form, and that ring carries the receptor-binding activity. This variant is the appropriate choice for studies in rats, where matching the peptide to the species avoids confounds from differing clearance and potential immune recognition in repeated dosing, and for cross-species pharmacology asking how the variable N-terminal extension modulates potency at the UT receptor. | 