Glucuronides, vitamin

The plasma concentration of retinoyl glucuronide is between 5 and 14 mnol per L, and the activity of retinoic acid UDP-glucuronyltransferase increases in vitamin A deficiency, suggesting that glucuronidation may be important... 

There are few dietary sources of cholecalciferol. The richest sources are oUy fish (especially fish liver oUs), although eggs also contain a relatively large amount, and there is a modest amount in mUk fat and animal liver. In many countries, margarine is fortified with vitamin D. No common plant foods contain vitamin D, although some tropical plants contain calciferol glucuronides that are hydrolyzed in the intestinal lumen and are a source of the vitamin. Indeed, this can be a cause of hypervitaminosis and calcinosis in grazing animals. 

Most vitamin D is excreted in the bile less than 5% is excreted as water-soluble metabolites in urine. Some 2% to 3% of the vitamin D in bUe is cholecalciferol, calcidiol, and calcitriol, but most is a variety of polar metabolites and their glucuronide conjugates. In most tissues, the major pathway for inactivation of calcitriol is by way of 24-hydroxylation to calcitetrol, then onward oxidation byway of the 24-oxo-derivative, 23-hydroxylation, and oxidation to calcitroic acid (see Figure 3.3). In addition, a variety of hydroxylated and other polar metabolites have been identified in bile, and many of these onward oxidation products also undergo glucuronide conjugation in the liver (Reddy and Tserng, 1989). 

Barua AB (1997) Retinoyl beta-glucuronide a biologically active form of vitamin A Nutrition Reviews 55, 259-67. 

Besides synthetic derivatives useful in therapy there exists a natural derivative of retinol (98), and retinoic acid(s) e. g., all-fraws-retinyl 8-glucuronide (99) [183] and all-fraws-retinoyl P-glucuronide (100) [184]. These vitamin A glycosides were first identified as billiary mefabo-lites of vitamin A, and they are now prepared also chemically for their favorable biological effects [185,186]. 

Menadione in oil is 3 times more effective than a menadi-ine suspension in water. More of menadione than of vitamin K is ab.sorbed orally, but 38% of menadione is excreted by the kidney in 24 hours, whereas only very small amounts of vitamin K ate excreted by this route in 24 hours. In rats, menadione in pmt is reduced to the hydroquinone and excreted as the glucuronide (19%) and the. sulfate (9.3%). 

Vitamin E is absorbed in the gastrointestinal tract. Bile is necessary for absorption. Vitamin E is metabolized in the liver. The major metabolites of vitamin E are the glucuronides of tocopheronic acid. Vitamin E is distributed to all tissues. Lipid tissues store the vitamin for prolonged periods of time. Up to 30% is... 

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