RP-HPLC-ESI-MS characterization of novel peptide fragments related to rat parotid secretory protein in parasympathetic induced saliva

Two peptides (MW 1211.7 and 928.5 Da) were detected by RP-HPLC-ESI-MS analysis of parotid saliva secreted upon continuous parasympathetic stimulation. The peptide with the higher mass (PSPFr-A) corresponded to the N-terminal dodecapeptide (Fragment 1-12) of rat parotid secretory protein (PSP), while the peptide with the lower mass (PSPFr-B) corresponded to the 4-12 fragment of the same protein. During stimulation, the PSPFr-A secretion increased, while the PSPFr-B secretion decreased (HPLC-ESI-MS). In the presence of cycloheximide, PSPFr-A was not demonstrated, while the PSPFr-B secretion decreased. In the presence of aprotinin, the PSPFr-B secretion was almost abolished, while the PSPFr-A secretion increased to higher levels than those observed in the absence of the inhibitor. In vitro perfusion, with artificial solution, of stimulated rat parotid glands excluded that the fragments were derived from the circulation. Neither peptide occurred in enriched granule preparations from unstimulated glands. The results suggest that at least two pathways - granular and vesicular - are responsible for the generation of the two peptides. PSPFr-A is the first cleavage product in both pathways. PRPFr-B is probably generated from granular PSPFr-A only and, at the end of the granule mediated pathway, by the action of an enzyme of the serine protease class.