Fluorescence interference of polyphenols in assays screening for dipeptidyl peptidase IV inhibitory activity

Abstract Polyphenols are plant‐derived natural compounds, which are attributed to various beneficial effects on human health. Therefore, they are extensively investigated in different experimental setups. These compounds exhibit favorable antidiabetic effects in various clinical trials. However, their mode of action is not completely elucidated. A potential molecular target of certain polyphenols could be the dipeptidyl peptidase IV (DPP IV). This enzyme is abundant in human serum and represents a serine exopeptidase that cleaves and thus rapidly deactivates incretin hormones which are responsible for adequate postprandial insulin secretion from the pancreas. Multiple inhibitors of DPP IV have already been approved as antidiabetic drugs. In the course of screening assays exploring in vitro DPP IV inhibitory activities of polyphenols, some implausible observations prompted a detailed analysis of the interaction of polyphenols with fluorescence signals. The results revealed that quercetin quenched the fluorescence intensity, while 6‐methoxyflavone and urolithin B enhanced fluorescence signals depending on particular experimental conditions such as solvent composition. Thus, individual polyphenols might conflict with the experimental setup of fluorescence‐based DPP IV inhibitor screening assays. To avoid false positive or false negative results, polyphenols under investigations should be carefully controlled for potential assay interferences.