Characterization of neuropeptide K processing in rat spinal cord S9 fractions using high-resolution quadrupole-Orbitrap mass spectrometry.

Tachykinins are a family of pronociceptive neuropeptides with a specific role in pain and inflammation. Several mechanisms regulate endogenous tachykinins levels, including the differential expression of protachykinin mRNA and the controlled secretion of tachykinin peptides from neurons. We suspect that proteolysis regulates extracellular neuropeptide K (NPK) and neurokinin A (NKA) concentrations and NPK is a precursor of NKA. Here, we provide evidence that proteolysis controls NPK and NKA levels in the spinal cord, leading to the formation of active C-terminal peptide fragments. Using high-resolution mass spectrometry, specific tachykinin fragments were identified and characterized. The metabolic stability in rat spinal cord fractions of NPK and NKA was very short, resulting in half-lives of 1.9 and 2.2 min respectively. Following the degradation of NPK, several C-terminal fragments were identified, including NPK1-26 , NKA, NKA2-10 , NKA3-10 , NKA5-10 and NKA6-10 , which conserve affinity for the neurokinin 2 receptor but also for the neurokinin 1 receptor. Interestingly, the same fragments were identified following the degradation of NKA. A specific proprotein convertases inhibitor was used and showed a significant reduction in the rate of formation of NKA, providing strong evidence that proprotein convertase is involved in C-terminal processing of NPK in the spinal cord, leading to the formation of NKA.