A novel biologically active eel calcitonin analogue with carboxyl terminal Hse32-amide: carboxyl terminal Pro32-amide in calcitonin is not essential for biological activity.

Eel calcitonin (CT) analogues having C-terminal homoserine (Hse)-amide at position 31, 32, or 33 were synthesized, and in vivo hypocalcemic activity of the analogues were determined. The present study showed that: (i) An eel CT analogue having Hse-amide at position 32 was more active than native eel CT, and the duration of hypocalcemic action of the analogue was equivalent to that of native eel CT. (ii) Either curtailment or elongation of [Hse32-amide]-eel CT peptide chain by one amino acid resulted in a great loss of hypocalcemic activity. The results of the present study indicate that Pro-amide at the C-terminus of CT is not essential for its biological activity in vivo. Replacement of C-terminal Pro32-amide by Hse32-amide in eel CT molecule produced an analogue with a hypocalcemic activity greater than that of native eel CT.