Does the N-terminal pyroglutamate residue have any physiological significance for crab hyperglycemic neuropeptides?

A characteristic feature of all crustacean hyperglycemic hormones (CHH) is that they are always present in the sinus gland as multiple forms or isoforms. The amino acid sequence of the minor form of CHH from the green shore crab, Carcinus maenas, was determined by automated microsequencing and MS, and was almost identical to that of the major form, except that the N-terminal residue was glutamine rather than pyroglutamate. Limited analysis (electrospray MS and amino acid composition) of the two corresponding forms of CHH from the edible crab, Cancer pagurus, suggested a similar phenomenon in this species. For C. maenas, both forms were indistinguishable in terms of their ability to cause sustained hyperglycemia in vivo and repression of ecdysteroid synthesis in vitro. Similarly, the two forms were immunologically identical in RIA, and exhibited similar binding characteristics in competitive-receptor-binding assays. CD studies showed only minor differences in presumed secondary structure. In vitro release experiments with isolated sinus glands demonstrated that both forms are probably released in a stoichiometric manner and that both peptides are present in the haemolymph at the same ratio as that in the sinus gland. Preliminary results suggest that the in vivo clearance/degradation rates of both peptides are similar. The unblocked (Gln) terminus is of particular significance, since the presence of this amino acid indicates that this peptide is derived from a precursor that does not possess the same structure of those of established preproCHH, or that N-terminal processing is slow, which results in the presence of unblocked CHH in sinus glands. The similar biological activity of the unblocked CHH to that of the blocked CHH suggests that the N-terminal pyroglutamate residue has no obvious biological significance (with respect to the known functions of CHH), an observation which is in contrast to the widely accepted paradigms concerning the stability and biological activity of N-terminally blocked and unblocked peptides.