The Role of Surface Properties in the Transport of Folates Across the Intestine

Abstract

The importance of the intestinal epithelial cell glycocalyx in the transport of folates in the intestine has been demonstrated. It was proposed that the glycocalyx acts as a retaining layer for the acid microclimate on the jejunal mucosal surface. Histological investigation of human jejunal biopsies from healthy volunteers and patients with gastrointestinal disease showed an abnormality in the glycocalyx of subjects with conditions associated with folate deficiency. The abnormal appearance, which presented the glycocalyx as knobbed or broken up, may result from abnormal synthesis of the structural components of the glycocalyx, thereby resulting in an incomplete structure similar to that observed on immature cells. The absorptive cells in these conditions may therefore be underdeveloped for their normal function. The transport of folic acid into everted sacs of rat jejunum was investigated using radiolabelled folic acid. A correlation was observed between the total folic acid transfer and the acidity of the microclimate. An association of fluid transfer in the movement of folic acid from the tissue into the serosal compartment was also demonstrated. The administration of compounds and the application of treatments to rats had varied effects on folic acid transfer which could be explained largely in terms of the existence of an acid microclimate. Conditions have been discussed which in humans, could induce folate deficiency. The immediate uptake of folates was investigated using isolated rat jejunal epithelial cells, obtained by exposing tissue for different lengths of time to hyaluronidase. There was a marked association between the histological appearance of the glycocalyx and the uptake characteristics of folic acid. The present investigation has provided definitive evidence to support an important role of surface properties in the transport of folates across the intestine. The mechanism of absorption of folic acid and 5-methyltetrahydrofolic acid involves passive diffusion following conversion to the neutral species within the acid microclimate.