Purification of kinase activity from primate parotid glands

Salivary secretions contain phosphoproteins that contain phosphorylation sites composed of serine residues in acidic environments. The hypothesis of this study is that a protein kinase responsible for phosphorylating these proteins is similar to kinases that phosphorylate proteins in other glandular secretions. Homogenates and subfractions from macaque parotid glands were able to phosphorylate synthetic peptide substrates containing each of the phosphorylation sites in acidic proline-rich proteins, statherin, and histatin 1. Activity was purified from Golgi membranes to greater than 220-fold by extraction with Triton X-100 and affinity chromatography with the use of immobilized ATP. The enzyme preferred substrates containing serine residues in a specific acidic environment, particularly those containing the Ser-Xaa-acidic sequence, preferred ATP over GTP, and was sensitive to high concentrations of heparin. These characteristics are similar to those reported for Golgi casein kinase, which phosphorylates casein in vivo. Based on these observations, the parotid gland kinase may be related to other Golgi-localized serine kinases.