Phosphorylation of Salivary Proteins by Salivary Gland Protein Kinase

Abstract

Human saliva contains a number of phosphorylated acidic proline-rich proteins (APRP). Monkey parotid saliva contains a similar protein with the same phosphorylated sequences as the human proteins. A crude protein kinase was prepared from Macaca fascicularis parotid glands which phosphorylated human APRP. The enzyme was activated by Mg²⁺, it had a pH optimum between pH 7.0 and 7.5, the K_m for ATP was 78 μmol/L, and for APRP it was 85 μmol/L. Phosphorylation of APRP was independent of cAMP and calmodulin. Phosphate was incorporated as phosphoserine, and the kinase phosphorylated the same residues in dephosphorylated APRP which are phosphorylated in the secreted protein. In addition, the enzyme preparation also phosphorylated dephosphorylated and native APRP in a region which is not phosphorylated in the secreted protein. There was no difference in the rate of phosphorylation of APRPs and their tryptic peptides. The kinase also phosphorylated other dephosphorylated salivary phosphoproteins. An enzyme was demonstrated in the human salivary gland which gave the same pattern of phosphorylation of APRP as did the simian kinase. More than one kinase may be necessary for the observed phosphorylation.