Vitamins bound forms

Liver tissue B12 is assayed by suspending 50 mg of lyophilized human liver in 100 ml of aconitate buffer to which 100 mg sodium metabisulfite is added. The suspension is autoclaved for 30 minutes at 16 psi to liberate the bound forms of the vitamin. Aliquots are diluted 1 10 after autoclaving to reduce metabisulfite toxicity on microorganism growth. Each milliliter now contains 50 pg of liver (w/v) 1.0, 1.5, and 2.0 ml of this hydrolyzate is added. After dilution to a volume of 5 ml, this represents an assay of 10, 15, and 20 ig of liver per milliliter of final solution. Normal liver contains between 2-14 mpg of vitamin B /mg liver powder. 

H. Mitsuda. Isolation from rice bran of OS088 a bound form of vitamin B-6 and its identification as 5 -0-(beta-D-gluco-pyranosyl) pyridoxine. Agr Biol Chem 1977 41 1061. 

An interesting and fimctionally important aspect of transcriptional activators is that one and the same protein can act as both an activator and a repressor. The alternative functionality is determined by the sequence environment, the presence of other transcriptional activators (steroid receptors, see ch. 4), by specific repressors or by low molecular weight effectors. Examples are the receptors for vitamin A acid, which, in the absence of its ligand, represses the genes with cognate DNA elements. TTie repression is exerted in the DNA-bound form. In the presence of its hgand, vitamin A acid, the same receptor acts as a transcriptional activator (see ch. 4). 

To replace losses, oxaloacetate can be synthesized from pyruvate and C02 in a reaction that uses ATP as an energy source. This is indicated by the heavy gray line leading downward to the right from pyruvate in Fig. 10-1 and at the top center of Fig. 10-6. This reaction depends upon yet another coenzyme, a bound form of the vitamin biotin. Pyruvate is formed from breakdown of carbohydrates such as glucose, and the need for oxaloacetate in the citric acid cycle makes the oxidation of fats in the human body dependent on the concurrent metabolism of carbohydrates. 

Bioavailability of Niacin. Factors which cause a decrease in macm availability include (1) Cooking losses (2) bound form in corn (maize), greens, and seeds is only partially available (3) presence of oral antibiotics (4) diseases which may cause decreased absorption (5) decrease in tiyptophan conveision as in a vitamin B deficiency. Fac.tois that increase availability include (1) alkali treatment of cereals (2) storage in bver and possibly in muscle and kidney tissue and (3) increased intestinal synthesis. 

Maize is low in vitamins but contains useful amounts of biotin and carotenoids. Niacin is present in a bound form and together with a low level of tryptophan, a niacin precursor, leads to this vitamin being particularly limiting in maize-based diets unless supplemented. 

The terms cofactor, coenzymes, and prosthetic group are used to describe the nonprotein moieties of the enzyme active center. The distinction between these terms is not sharp. Some of the cofactors are derivatives of vitamins that form either covalent or noncovalent linkages at or near the active site of the enzyme, and some are metal ions. If a cofactor (coenzyme) is tightly bound to the protein moiety (the apoenzyme), it is often referred to as a prosthetic group. A coenzyme that is easily removed from the holoenzyme, leaving behind the apoenzyme, is often regarded as a second substrate. 

Q7 Calcium is present in both intracellular fluid (ICF) and ECF, but the concentration in the ECF is twice as high as that in the ICF. Calcium is found in both ionized and bound forms, and Ca2+ homeostasis is mainly controlled by parathyroid hormone, which increases absorption of calcium in the intestine and reabsorption in the nephron. Calcitonin also affects ECF calcium concentration by promoting renal excretion when there is an excess of calcium in the body. The normal kidney filters and reabsorbs most of the filtered calcium however, in renal disease this is reduced and blood calcium decreases. Calcium and phosphate imbalance can occur in patients with renal failure, leading to osteomalacia (defective mineralization of bone). Osteomalacia is mainly due to reduced production of 1,25-dihydroxycholecalciferol, an active form of vitamin D metabolized in the kidney. Deficiency of 1,25-dihydroxycholecalciferol reduces the absorption of calcium salts by the intestine. 

That nongrowing animals require niacin implies that it is lost from the body either as intact niacin or as a modified or breakdown product of the vitamin. An amount of niacin equivalent to nearly 90% of our daily intake is excreted in the forms of N-methyl-2-p)nidone-5urinary metabolites can be used to assess niacin status. Loss of the normal quantity in the urine each day indicates that the supply in the diet is adequate. In humans, the healthy adult excretes 4 to 6 mg of N-methyl-nicoti-namide per day. An abnormally low level indicates that the dietary intake is not adequate. Measurement of urinary niacin metabolites has proven useful in determining the amoimt of niacin available in a variety of foods. The body s ability to use niacin in different foods may vary even if the foods contain identical quantities of the vitamin. One contributing factor to the low availability of niacin is the occurrence of the vitamin in the "bound form," as mentioned earlier. Excretion of normal levels of pyridone, for example, depends not only on normal absorption of the vitamin from the diet, but also on its conversion to NAD or NADP, followed by catabolism to the metabolite. 

Diets that are low in protein, zinc, selenium, vitamin Bg, and fat may collectively depress immune function. This type of diet may be associated with either a low-caloric-intake diet or low-fat, low-meat-products diet. Zinc is an essential trace element for many biological functions, including immune functions. Indeed, zinc is required for the biological activity of a thymic hormone, called thymulin in its zinc-bound form, and is important for the maturation and differentiation of T-cells. With advancing age, zinc, thymic functions, and peripheral immune efficiency show a progressive decline. Supplementing zinc in old age restores immune efficiency. 

S12). A second serum binder for folate has been described (W9, R6). Althou this binder has a high affinity for folate, its capacity is low moreover it binds oxidized folate well but 5-methyltetrahydrofolate poorly. Folate attached to this protein is not removed by charcoal, indicating that the bond is firm. Vitamin bound to tbis protein is not given up to cells and possibly represents a storage form. 

The possible existence of bound forms of ascorbic add in animal and plant tissues forms one of the most contradictory subjects in the biochemical literature. The very high concentrations of ascorbic acid within both plant and animal cells provide reasons to look for such combined forms. The realization that most of the water-soluble vitamins function... 

The importance of vitamins to plants themselves is often overlooked, but they play the essential roles in a plant metabolism too (Smith et ah, 2007). Some algal species require different combinations of certain vitamins such as vitamins B12 and Bi. Because the concentration of these vitamins in the natural environment is quite low, their absorption is insufficient (Croft et ah, 2006). According to Yamada et ah (1996a), red algae Porphyra tenera can take up the free (not protein-bounded) form of vitamin B12 from the incubation medium by concentration- and temperature-dependent processes. The amoimt of uptake increases with the time of incubation. 

Sources. Niacin and substances that are convertible to niacin are found naturally in meat (especially red meat), poultry, fish, legumes, and yeast. In addition to preformed niacin, some L-tryptophan found in the proteins of these foods is metabolized to niacin. Niacin is also present in cereal grains, such as corn and wheat. However, consumption of corn-rich diets has resulted in niacin deficiency in certain populations. The reason for this is that niacin exists in cereal grains in bound forms, such as the glycoside niacytin, which exhibit little or no nutritional availability. Interestingly, niacin deficiency is not common in Mexico and Central America even though the diets of those in these countries are based on com. Alkaline treatment, such as soaking corn in a lime solution—the process used by the populations of Mexico and Central America in the production of com tortillas—yields release of bound niacin and increased availability of the vitamin. 

Thiamine (vitamin Bj) occurs in foods in free and bound forms, the free form predominates in cereals and plants, whereas the pyrophosphate ester is the main form in animal products. Acid hydrolysis is required to release thiamine from the food matrix. Enzymatic hydrolysis is then needed to convert phosphate esters to thiamine. Prior to CE analysis it is necessary to clean up samples by using ethanol to precipitate protein and by passing through an ion-exchange resin. Thiamine has been determined in meat and milk samples using MEKC with ultraviolet (UV) detection at 254 nm, obtaining comparable sensitivity to that achieved by HPLC using an ion-pair reversed-phase column with postcolumn derivat-ization and fluorescence detection. 

Vitamin B Vitamin Bg is the generic name for six naturally occurring vitamers which are derivatives of 2-methyl-3-hydroxy pyridone. All forms of vitamin Bg have been detected in human plasma with the exception of pyridoxine phosphate. Both free and protein bound forms can be extracted with trichloroacetic acid or perchloric acid, the excess of which should be removed prior to chromatography. Cation-exchange chromatography with linear gradients of hydrochloric acid and phosphate buffers produces a profile of the vitamin Bg compounds in 50 min. 

In 1939, Giorgy isolated from liver a vitamin called vitamin H, which prevented the development of the deleterious effects observed after the administration of avidin. After several years, the biochemists succeeded in relating these isolated findings and demonstrating that bios, vitamin R, and vitamin H are all the same compound, the official name of which became biotin. Du Vigneaud established that biotin is the A-cis-hexahydro-2-keto-l H-thieno (3,4) imidazole-4-valeric acid, an optically active monocarboxylic acid. Biotin is found in a bound form in many animal and plant tissues [72]. 

We have developed a method for simultaneous analysis of thiamine hydrochloride, pyridoxine hydrochloride and cyanocobalamin in pharmaceuticals and dietary supplements (Marszall et al. 2005) and in fortified food (Lebiedzinska and MarszaH 2006) using HPLC-ED. Vitamins were determined in their free forms, so an extraction step from fortified fruit juice was performed prior to the chromatographic isolations. The extraction procedure was based on a study by Ndaw el al. (2000). The enzymatic digestion prior to the separation and quantification step made it possible to release the vitamins bound to proteins or sugars and converted vitamin esters to free forms thus we were able to obtain the total vitamin contents of the fruit juices. The supernatants were adjusted to pH 4.5 with 2.5 M sodium acetate and a single extraction procedure for all vitamins was carried out using mixture of the enzymes, papain and diastase (Lebiedzinska and MarszaH 2006). 

Serum cobalamin consists of picomolar amounts of several forms of the vitamin bound to transcobalamin or haptocorrin. 

Riboflavin needs to be present in the human typical diet, as animals, unlike many plants, fungi and bacteria, are unable to synthesize this molecule. Dietary intake of this vitamin includes free riboflavin and also its protein bound form, as FAD and FMN in flavoproteins (Figure 37.1 A). In the latter case, flavins need to be first released from carrier proteins during digestion and then hydrolysed to riboflavin by alkaline phosphatases and FMN/FAD pyrophosphatase in order to be absorbed at the small intestine. 

Natural antioxidante are present in foods as their endogenous constituents or are added to them to preserve their quality. Antioxidants that occur naturally in plant foods belong to e phenolic group of compounds and include phenolic acids, phenylpropanoids, flavonoids and isofiavones, including catechins and anthocyanins/anthocyanidins, as well as phytates, sterols and carotenoids, among others. In addition, vitamins C and E as well as phospholipids may act as antioxidants in foods. In oilseeds, phenolic antioxidmits are present in die fiee, esterified, glycosidic or bound form and may be simple or complex in their chemical stractures. 

---