Purification and properties of the calcium-precipitable protein in submaxillary saliva of normal and cystic fibrosis subjects.

A calcium-precipitable protein (CaPP), previously shown to cause turbidity of submaxilliary saliva 
(SMS) in patients with cystic fibrosis (CF), was purified from normal 
and CF-SMS by (NH4)2SO4 precipitation, gel filtration, and preparative polyacrylamide gel 
electrophoresis. This protein has a molecular weight of 12,000. It contains 
large numbers of serine, glycine, and glutamic acid residues, 0.85% phosphorus 
as phosphate monoester, and small amounts of carbohydrate. Precipitation (aggregation) of 
CaPP occurs at basic pH values in the presence of ionic 
calcium and is enhanced by increasing calcium concentration, CaPP concentration, or 
pH. Aggregated CaPP binds calcium in molar quantities equal to the 
number of phosphate prosthetic groups. Alkaline phosphatase removes all phosphate from 
CaPP and eliminates calcium-induced precipitation. Studies of CaPP from CF saliva 
have demonstrated no electrophoretic or compositional differences from normal CaPP. Higher 
than normal pH values or elevated CaPP concentrations are not present 
in SMS of patients with CF and do not contribute to 
the formation of turbid SMS. Aggregation and precipitation of CaPP in 
saliva of patients with CF seems to be induced primarily by 
elevated levels of calcium in saliva. This precipitation may be entirely, 
or in part, responsible for focal obstruction of submaxillary gland ducts 
in patients with CF. However, little or no CaPP is secreted 
in the lower gastrointestinal or tracheobronchial tracts. Therefore, CaPP does not 
play a more general role in the obstructive processes which characterize 
CF. 