Use of fluorescein isothiocyanate isomer I to study the mechanism of intestinal absorption of fucoidan sulfate in vivo and in vitro

Abstract A new method to label fucoidan sulfate was established with tyramine and fluorescein isothiocyanate isomer I (FITC). Fluorescence spectrophotometry and high performance liquid chromatography verified the successful labelling of fucoidan by FITC. The results of the single‐pass intestinal perfusion indicated that the jejunum and ileum are the main absorption sites, and there was carrier saturation. In addition, fucoidan sulfate at 1 mg/ml had no inhibitory effect on Caco‐2 cell proliferation. Studies on the transmembrane transport mechanism showed that fucoidan can be absorbed because the apparent permeability coefficient of the drugs (P app ) A → B was 3.78 + 0.03 ×10 −6 and that of B → A was 1.42 + 0.19 ×10 −6 . The peak absorption of fucoidan occurred at 120 min after administration; moreover, the higher the concentration used, the worse the absorption was, suggesting the saturation of transport carriers. The absorption was temperature dependent: the absorption at 37°C was much better than that at 4°C. Further, the absorption of fucoidan sulfate might rely on clathrin endocytosis as chlorpromazine (10 μg/ml) significantly inhibited it.