Cellular uptake of biotin: mechanisms and regulation.

This review describes our knowledge of biotin transport in the small intestine of humans and other mammals and presents recent findings in the area. Previous studies have shown that biotin transport across the brush border membrane of the small intestinal absorptive cells occurs via a carrier-mediated, Na+ gradient-dependent, electroneutral mechanism. Exit of biotin out of the enterocyte, i.e., transport across the basolateral membrane, also occurs via a carrier-mediated process, but the process is Na+ independent and electrogenic. Recent studies from our laboratory have shown that the uptake process of biotin in Caco-2 cells, a human-derived cultured intestinal epithelial cell line, are under the cellular regulation of both a protein kinase C- and a Ca/calmodulin-mediated pathway. In addition, the uptake process is shared by another water-soluble vitamin, pantothenic acid. For the first time, other recent studies have detected the existence of a Na+-dependent, carrier-mediated mechanism for biotin uptake at the apical membrane of colonocytes, which could theoretically mediate absorption of the biotin synthesized by colonic microflora. This system was again found to be shared by pantothenic acid, which is also synthesized by the normal microflora of the large intestine.