Micelle-like structures in human saliva

Abstract

The acquired enamel pellicle can easily be visualized by scanning electron microscopy (SEM) at low magnifications, and some researchers have described the surface morphology of the acquired pellicle as being predominantly globular based on scanning and transmission electron microscopic examinations. This is consistent with the adsorption of salivary protein aggregates in the form of globular or micelle-like structures comprising amphiphilic salivary proteins. The aim of the present study was to demonstrate the presence of and to characterize possible globular structures of human saliva. Parotid saliva was collected from three healthy individuals and examined by transmission electron microscopy, photon correlation spectroscopy (PCS) and zeta potential determinations. TEM examinations (negative staining with a 2% ammonium molybdate solution) demonstrated globular structures in parotid saliva in the size range 150–200 nm. The globular structures appeared to be more clustered (comprising 6–10 globules) with increasing time after sampling. The PCS analyses likewise demonstrated particles in parotid saliva with an initial size in the range 100–150 nm. Unimodal analysis showed that the mean hydrodynamic diameter of the salivary particles increased to about 450 nm 50 min after sampling. Size distribution processor (SDP) analyses, furthermore, indicated the presence of three different size fractions of the salivary particles; one in the size range of about 10 nm, one in the size range of 40–110 nm and one of 240–500 nm. Initial addition of 0.5 ml of 50 mM pyrophosphate solution (PP) to the saliva samples seemed to inhibit the increase in size of the globular structures, which were then less than 50 nm. Addition of PP to the saliva samples 120 min after sampling, disintegrated the globular structures, as the unimodal mean of the particle size decreased from about 450 nm to 100–150 nm. The PCS intensity (counts s −1) also decreased. Zeta potential determinations indicated an overall net negative surface charge of the salivary globular structures of about −13 to −17 mV. The study clearly demonstrated the presence of negatively charged globular structures of human parotid saliva in the size range of about 100–500 nm, consistent with micelle-like structures consisting of amphiphilic salivary proteins. Calcium seems furthermore to be of importance in maintaining the structure of these salivary particles.