Vitamin transporters

The water-soluble vitamins generally function as cofactors for metabolism enzymes such as those involved in the production of energy from carbohydrates and fats. Their members consist of vitamin C and vitamin B complex which include thiamine, riboflavin (vitamin B2), nicotinic acid, pyridoxine, pantothenic acid, folic acid, cobalamin (vitamin B12), inositol, and biotin. A number of recent publications have demonstrated that vitamin carriers can transport various types of water-soluble vitamins, but the carrier-mediated systems seem negligible for the membrane transport of fat-soluble vitamins such as vitamin A, D, E, and K. 

Another important vitamin is folate, which is required for purine and pyrimidine nucleotide synthesis. Since folate and its derivatives are generally lipo-phobic anions, they do not traverse biological membranes via simple diffusion but rather have to be taken up into the cells by specific transport processes 

Recently, Prasad et al. cloned a mammalian Na+-dependent multivitamin transporter (SMVT) from rat placenta [305], This transporter is very highly expressed in intestine and transports pantothenate, biotin, and lipoate [305, 306]. Additionally, it has been suggested that there are other specific transport systems for more water-soluble vitamins. Takanaga et al. [307] demonstrated that nicotinic acid is absorbed by two independent active transport mechanisms from small intestine one is a proton cotransporter and the other an anion antiporter. These nicotinic acid related transporters are capable of taking up monocarboxylic acid-like drugs such as valproic acid, salicylic acid, and penicillins [5], Also, more water-soluble transporters were discovered as Huang and Swann [308] reported the possible occurrence of high-affinity riboflavin transporter(s) on the microvillous membrane. 

Bile acids secreted into the small intestine facilitate absorption of fat-soluble vitamins and cholesterol. The majority of bile acids are reabsorbed from the intes- 

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Prasad, P. D., et al. Cloning and functional expression of a cDNA encoding a mammalian sodium-dependent vitamin transporter mediating the uptake of pantothenate, biotin, and lipoate. J. Biol. Chem. 1998, 273, 7501-7506. 

R2. Rachmilewitz, B., Rachmilewitz, M., Chaouat, M., and Schlesinger, M., The production of TC-ll-—The vitamin transport protein—by mouse mononuclear phagocytes. Blood 52, 1089-1097 (1978). 

Know the mechanisms of fat-soluble vitamin transport throughout the organism. 

Transport. Transcobalamin II dehvers the absorbed vitamin B 2 to cells and is the primary plasma vitamin transport protein. It is... 

Spector, R., and Eells, J. Deoxynucleotide and vitamin transport into the central nervous system. FedProc 43 196-200, 1984. 

Most, if not all, of the tissues and organs in the body are adversely affected by chronic ingestion of excessive amounts of alcohol, including the liver, pancreas, heart, reproductive organs, central nervous system, and the fetus. Some of the effects of alcohol ingestion, such as the psychotropic effects on the brain or inhibition of vitamin transport, are direct effects caused by ethanol itself. However, many of the acute and chronic pathophysiologic effects of alcohol relate to the pathways of ethanol metabolism (see Chapter 25). 

Because most vitamins function as coenzymes, the symptoms of vitamin deficiencies reflect the loss of specific enzyme activities dependent on the coenzyme form of the vitamin. Thus, drugs and toxins that inhibit proteins required for coenzyme synthesis (e.g., vitamin transport proteins or biosynthetic enzymes) can cause the symptoms of a vitamin deficiency. This type of deficiency is called a functional deficiency, whereas an inadequate intake is called a dietary deficiency. 

Some recent studies on vitamin transport using membrane vesicles include those of vitamin B6 by rat kidney brush border membranes (Bowman et al, 1990), ascorbic acid by teleost intestinal brush border membranes (Mafha et ai, 1993), biotin by human kidney brush border membranes (Baur and Baumgartner, 1992), pantothenate by human placental brush border membranes (Grassl, 1992), folate and riboflavin by rabbit intestinal brush border membranes (Said and Mohammadkhani, 1993a,b Said et al, 1993), and thiamine by rat small intestine basolateral membranes (Laforenza et al, 1993). Bile acid transport in human placental, rat ileal, and rabbit small intestinal brush border membrane vesicles (Dumaswala et al, 1993 Gong et al, 1991 Kramer et al, 1993) and the effect of vitamin D status... 

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