The discovery of vitamins resulted from the searches for the cures of diseases caused by diets deficient in such micronutrients. The further investigation of specific roles has clarified most of the metabolic alterations that are necessary to convert vitamins to functional forms, which include conversions of water-soluble B complex vitamins to coenzymes. More recent work is providing information on how these metabolic conversions are regulated. Importantly different cells have somewhat different means for facilitating uptake, often with relative specificity for any one of the different structural groups of vitamins. Closure of the knowledge gap as to how vitamins are transported into the cell and how processing of vitamins within the cell are connectedly related is the thrust of ongoing studies. Our own work in this area will be the primary base for this Minireview.

Since such vitamins as riboflavin, B₆, biotin, and C have been the focus of considerable investigation in our laboratory, they will serve as primary examples to make both detailed statements as well as comparisons that often apply broadly. In particular, we have studied the importing of these vitamins into the parenchymal liver cells and proximal tubular kidney cells from the rat as a convenient animal model, because less was known about vitamin uptake in these tissues than in the small intestine. One interesting possibility for future application of some of the knowledge gained may be to piggyback a bioactive compound, e.g., drug or signal transducer, onto a transported solute, e.g., vitamin or other nutrient, that gains facilitated entry into a cell and is, thereafter, metabolized to release the active compound.

In general, the subject of transport through plasma membranes of cells has progressed from a classic physiologic description to the identification of mechanisms and components of specific transporters at the biophysical and biochemical level (1).