The protein composition of rat parotid saliva and secretory granules

Rat parotid saliva was collected by surgical cannulation of the ducts and stimulation with pilocarpine; The secreted salivary proteins were resolved on columns of DEAE-Sephadex into five major Fractions, I-V, which were characterized by polyacrylamide disc gel electrophoresis, amino acid analyses and enzymatic assay. Rat parotid secretory granules were isolated by density gradient centrifugation and lysed in hypotonic buffers. Granule content proteins were resolved and examined by the same techniques as for secreted proteins. In both experiments, Fraction I contained RNAase and a major unidentified protein, M1, Fraction II contained the isoenzymes of amylase; DNAase was present in Fraction III and, to a lesser degree, in Fraction IV. The proportions of the enzyme-containing peaks were the same in saliva and granule contents. Fractions IV and V contain proteins of unknown function; Fraction IV contains exceptionally high levels of glutamic acid, glycine and proline in its protein moieties and approx. 6-8% neutral sugars.