Binding of citrus flavanones and their glucuronides and chalcones to human serum albumin

Naringenin and hesperetin glycosides are the major polyphenols (flavanones) of citrus fruits and juices and are thought to participate in the cardioprotective effects of diets rich in plant products. Naringenin and hesperetin glucuronides (resulting from conjugation at the A- or B-ring) are the main circulating metabolites in humans and their binding to human serum albumin (HSA) is expected to modulate their half-life in plasma and tissue distribution. In this work, the binding of flavanone glucuronides to HSA was investigated by fluorescence spectroscopy. Binding constants in the range of 3-9 × 10(4) M(-1) were estimated. The affinity of glucuronides for HSA is close to that of naringenin and hesperetin themselves. Competition experiments in the presence of the fluorescent probes dansylsarcosine and quercetin were used to gain information on the flavanone binding site. Naringenin and hesperetin chalcones were also included for comparison as their glucuronides too were detected in the general circulation. Naringenin and hesperetin chalcones spontaneously undergo cyclization back to the parent flavanones under neutral conditions. The cyclization was significantly slowed down by HSA but led to a racemic mixture of (2R) and (2S) flavanones in the absence or presence of HSA.