Measurement and validation of the nature of salivary adiponectin

Adiponectin (Ad) is an adipocyte-derived hormone that plays an essential role in regulating insulin sensitivity, inflammation, and atherogenesis. Levels of some hormones in saliva change in a fashion similar to that in plasma in response to a disease or physiological condition. Since saliva is an easy to obtain biological fluid, measurements of salivary hormonal changes are preferred in diagnoses and treatments. Therefore, it was of interest to examine the nature of salivary Ad. While there have been two publications in the literature reporting presence of Ad in human saliva, the nature of salivary Ad has not been characterized. To this end, we investigated the effect of sample dilution on the measurement of Ad in saliva. To our surprise, we observed an increase in measurable level of Ad in saliva on sample dilution. One explanation for this paradoxical observation may be the presence of inhibitor(s) of Ad/anti-Ad binding in saliva that following dilution relieves the inhibitory effect. Working with this hypothesis, we were able to demonstrate the presence of an inhibitor in saliva that co-eluted with the dimeric form of Ad and was capable of inhibiting Ad assay. The presence of such inhibitor(s) may lead to underestimation of Ad in saliva.