Post-translation processing of neurohormonal peptide precursors in a human medullary thyroid carcinoma

Determination of the primary structures of peptides isolated from a human medullary thyroid carcinoma has provided insight into the pathways of post-translational processing of prohormones in the tumour cells. Cleavage of the signal peptide in preprocalcitonin occurs at the Ala²⁵-Ala²⁶ bond. The prohormone is further processed at the site of multiple basic residues to generate procalcitonin-(1-57)-peptide, procalcitonin-(60-91)-peptide (calcitonin), procalcitonin-(60-116)-peptide (a C-terminally extended form of calcitonin) and procalcitonin-(96-116)-peptide (Katacalcin). CGRP-I but not CGRP-II, was isolated from the tumour together with a high-M(r) form that may represent the unprocessed prohormone. Progastrin-releasing peptide was processed to a mixture of GRP-(1-27)-peptide and GRP-(18-27)-peptide, the latter component arising from proteolytic cleavage at the site of a single arginyl residue.