In vitro systems for studying thiamin transport in mammals.

This chapter discusses in vitro systems employed for studying thiamin transport in mammals. In mammals the transport of low (physiological) concentrations of thiamin is a carrier-mediated, mainly energy-dependent process that, in vitro, can be studied in preparations with different levels of anatomical complexity (intact tissues, isolated cells, and membrane vesicles). Each of these allows different aspects of the transport mechanism to be investigated. Thus intact tissue preparations (everted jejunal sacs or rings, human small intestinal biopsies, and brain slices) can be used to study biotransformations of thiamin related to transport. Isolated cell preparations both from normal (hepatocytes, enterocytes, and erythrocytes) and pathological tissues (Ehrlich ascites tumor and neuroblastoma cells) can also be used, according to the particular problem to be assessed. Choroid plexus and placenta preparations, as well as blood–brain barrier, can give insight into the modalities of thiamin transfer from the plasma through complex biological structures. By contrast, membrane preparations (small intestinal brush border or basolateral membrane vesicles, brain microsacs, and erythrocyte ghosts) are well suited for characterizing pure membrane thiamin transport mechanism without the interference of intracellular thiamin metabolism.