Pro‐Thyrotropin‐Releasing Hormone Processing by Recombinant PC1

: Pro‐thyrotropin‐releasing hormone (proTRH) is the precursor to thyrotropin‐releasing hormone (TRH; pGlu‐His‐Pro‐NH2), the hypothalamic releasing factor that stimulates synthesis and release of thyrotropin from the pituitary gland. Five copies of the TRH progenitor sequence (Gln‐His‐Pro‐Gly) and seven cryptic peptides are formed following posttranslational proteolytic cleavage of the 26‐kDa rat proTRH precursor. The endopeptidase(s) responsible for the physiological conversion of proTRH to the TRH progenitor form is currently unknown. We examined the in vitro processing of [3H]leucine‐labeled or unlabeled proTRH by partially purified recombinant PC1. Recombinant PC1 processed the 26‐kDa TRH precursor by initially cleaving the prohormone after the basic amino acid at either position 153 or 159. Based on the use of our well‐established antibodies, we propose that the initial cleavage gave rise to the formation of a 15‐kDa N‐terminal peptide (preproTRH25–152 or preproTRH25–158) and a 10‐kDa C‐terminal peptide (preproTRH154–255 or preproTRH160–255). Some initial cleavage occurred after amino acid 108 to generate a 16.5‐kDa C‐terminal peptide. The 15‐kDa N‐terminal intermediate was further processed to a 6‐kDa peptide (preproTRH25–76 or preproTRH25–82) and a 3.8‐kDa peptide (preproTRH83–108), whereas the 10‐kDa C‐terminal intermediate was processed to a 5.4‐kDa peptide (preproTRH206–255). The optimal pH for these cleavages was 5.5. ZnCl2, EDTA, EGTA, and the omission of Ca2+ inhibited the formation of pYE27 (preproTRH25–50), one of the proTRH N‐terminal products, by 48, 82, 72, and 45%, respectively. This study provides evidence, for the first time, that recombinant PC 1 enzyme can process proTRH to its predicted peptide intermediates.