Vitamin tissue uptake

Because vitamin E is transported in lipoproteins secreted hy the liver, the plasma concentration depends to a great extent on total plasma lipids. Erythrocytes may also he important in transport, because there is a relatively large amount of the vitamin in erythrocyte membranes, and this is in rapid equilibrium with plasma vitamin E. There are two mechanisms for tissue uptake of the vitamin. Lipoprotein lipase releases the vitamin by hydrolyzing the tri-acylglycerol in chylomicrons and VLDL, whereas separately there is receptor-mediated uptake of LDL-bound vitamin E. Studies in knockout mice suggest that the main mechanism for tissue uptake of vitamin E from plasma lipoproteins is byway of the class B scavenger receptor (Mardones et al., 2002). 

Thiamin that is not bound to plasma proteins is rapidly filtered at the glomerulus. Diuresis increases the excretion of the vitamin, and patients who are treated with diuretics are potentially at risk of thiamin deficiency. Some of the diuretics used in the treatment of hypertension may also inhibit cardiac (and other tissue) uptake of thiamin, thus further impairing thiamin status, which may be a factor in the etiology of heart failure (Suter and Vetter, 2000). 

Intestinal bacteria synthesize riboflavin, and fecal losses of the vitamin may be five- to six-fold higher than intake. It is possible that bacterial synthesis makes a significant contribution to riboflavin intake, because there is carrier-mediated uptake of riboflavin into colonocytes in culture. The activity of the carrier is increased in riboflavin deficiency and decreased when the cells are cultured in the presence of high concentrations of riboflavin. The same carrier mechanism seems to be involved in tissue uptake of riboflavin (Said et al., 2000). 

Tissue uptake of vitamin Be is again by carrier-mediated diffusion of pyridoxal (and other unphosphorylated vitamers), followed by metabolic trapping by phosphorylation. Circulating pyridoxal and pyridoxamine phosphates are hydrolyzed by extracellular alkaline phosphatase. All tissues have pyridoxine kinase activity, but pyridoxine phosphate oxidase is found mainly in the liver, kidney, and brain. 

Plasma Vitamin B12 Binding Proteins and Tissue Uptake... 

Altbougb transcobalamin II is tbe metaboUcaUy important pool of plasma vitamin B12, it accounts for only 10% to 15% of tbe total circulating vitamin. Tbe majority is bound to baptocorrin (also known as transcobalamin I). Tbe function of baptocorrin is not well understood it bas a relatively long balf-life (7 to 10 days), and does not seem to be involved in tissue uptake or intertissue transport of tbe vitamin. Altbougb genetic lack of transcobalamin 11 results in severe (and fatal) vitamin B12 deficiency, genetic lack of baptocorrin seems to bave no adverse effects. 

D-lso-ascorbic acid (erythorbic acid see Figure 13.1) also has vitamin activity. in vivo and in cell culmre, it has only about 5% of the biological activity of ascorbate, but this seems to be from poor intestinal absorption and tissue uptake. In vitro with purified enzymes, it has the same cofactor activity as ascorbate. Although it is not a namrally occurring compound, erythorbic acid is widely used interchangeably with ascorbic acid, in cured meats and as an antioxidant in a variety of foods. 

In intestinal mucosal cells, all vitamers of vitamin E cue incorporated into chylomicrons, and tissues take up some vitamin E from chylomicrons. Most, however, goes to the liver in chylomicron remnants, a -Tocopherol, which binds to the liver a-tocopherol transfer protein, is then exported in very low-density lipoprotein (VLDL) and is available for tissue uptake (Traber and Aral, 1999 Stocker and Azzi, 2000). Later, it appears in low-density Upoprotein (LDL) and high-density lipoprotein, as a result of metabolism of VLDL in the circulation. The other vitamers, which do not bind well to the a-tocopherol transfer protein, are not incorporated into VLDL, but are metabolized in the Uver and excreted. This explains thelower biological potency of the othervitcimers,despitesimilar, or higher, in vitro antioxidant activity. 

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