O. Wiss

Absorption and Distribution of Vitamin E in the Tissues

Publisher Summary The mechanism of the absorption of vitamin E is undoubtedly very similar to that of the other fat soluble vitamins, but compared to vitamin A, for example, its efficiency appears to be much lower. The specific site of absorption is not too well established, but it is probably linked with fat and oil absorption and is facilitated by bile. It has also been noted that the difference in the efficiency of absorption of an oil solution and an aqueous emulsion of vitamin E is not a great as the difference encountered with vitamin A. It is apparent from the work of several investigators that a high percentage of a test dose of tocopherol is excreted in the feces. The data from balance studies and the considerable data collected on blood and liver levels of tocopherol, however, strongly indicate that the absorption of vitamin E is incomplete. With the establishment of the quantitative relationship between tocopherol intake and liver storage together with the availability of precise methods of measurement of the tocopherol in the liver, a method for the bioassay of tocopherols based on liver storage was quick to develop. Although there is agreement concerning a linear relationship between tocopherol intake and liver tocopherol storage, there is some controversy over the relationship between tocopherol intake and plasma tocopherol.

Chemical modification of tryptophan at the binding site of thiamine-pyrophosphate in transketolase from Baker's yeast.

Abstract Apotransketolase from bakers yeast has been modified by the tryptophan reagent dimethyl-(2-hydroxy-5-nitrobenzyl)-sulfonium chloride(HNB-(CH 3 ) 2 SCl). The modification of apotransketolase resulted in the loss of enzymatic activity. No change in enzymatic activity was found when coenzyme thiamine pyrophosphate (TPP) had been added to apotransketolase prior to the addition of the tryptophan reagent. Evidence for the modification of tryptophan came from the change in intensity of the tryptophan magneto circular dichroism band as well as from 2-hydroxy-5-nitrobenzyl incorporation. Three of the 18 tryptophan residues of apotransketolase have been modified by HNB-(CH 3 ) 2 SCl. The results are interpreted as showing that tryptophan is involved in the TPP binding site of transketolase.

Metabolism of Vitamin A and the Determination of Vitamin A Status

Publisher Summary This chapter describes the metabolism of vitamin A and the determination of vitamin A status. Estimations of the vitamin A status of humans and animals usually are based upon determinations of the vitamin A concentration in plasma. The plasma vitamin A level is correlated to the amount in the liver, the main site of storage of the vitamin in the body, only in cases of extreme hypo- and hypervitaminosis A but is constant over a wide range of intermediate vitamin A levels in the liver. The rate of urinary excretion of radioactive metabolites after a loading dose of labeled vitamin A, was studied in rats possessing different vitamin A reserves. It is possible to calculate the amount of vitamin A metabolites excreted in the urine in terms of vitamin A equivalents on the bases of the urinary radioactivity and of the specific radioactivity of the vitamin A contained in the liver when excretion of urinary vitamin A-derived radioactivity had reached a constant rate about 7 days after treatment with labeled vitamin A. Positive results for a reliable determination of the vitamin A status of rats were obtained by an isotopic dilution technique consisting of an intravenous administration of a physiological dose of radioactive vitamin A acetate and the determination of the specific radioactivity of the plasma vitamin A after a certain equilibration period.

Absorption, Distribution, Storage, and Metabolites of Vitamins K and Related Quinones

Publisher Summary This chapter describes the absorption, distribution, storage, and metabolites of vitamins K and related quinones. About 10% of vitamin K, dl -α-tocopherylquinone, and dl -α-tocopherol, administered in emulsified form, are absorbed. Vitamin K and dl -α-tocopherylquinone differ from dl -α-tocopherol as these two substances are absorbed and excreted faster. Ubiquinone-9 is absorbed much more slowly than dl -α-tocopherol and to the extent of only about 4% of the administered dose. It is, however, even better retained than dl -α-tocopherol. Maximum tissue levels are obtained only after about 24 hours. The excretion in the urine within that period amounts to only about 10% of the absorbed quantity. The distribution pattern of vitamin K, dl - α -tocopherylquinone, and dl -α-tocopherol in the body of rats is similar. Only dl -α-tocopherol is stored highly selectively in the adrenals. The liver is the preferred storage place for ubiquinone-9. The small amounts of radioactivity originally present in other organs move to the liver so that after 24 hours 97% of the total amount can be found there. All quinones —namely vitamin K, vitamin MK-4, dl -α-tocopherylquinone, phytylplastoquinorie, and ubiquinone-9, are metabolized in the same way, yielding y-lactones that are excreted in the urine as conjugates having side chains of seven carbon atoms, which correspond to the metabolite of vitamin E identified previously.

Chemistry and Biochemistry of the K Vitamins

Publisher Summary The elucidation of the chemical basis of the K vitamins is because of the schools of Dam, Doisy, and Karrer, who succeeded in 1939 in isolating two pure compounds with vitamin K activity, namely vitamin K 1 (I) from alfalfa and vitamin K 2 (II) from putrefied fish meal. The structure of vitamin K 1 was proved by oxidative degradation and by synthesis to be 2-methyl-3-phytyl-1, 4-naphthoquinone, the side chain consisting of four isoprene units (4 ×5 = 20 C-atoms). It had been established that vitamin K 2 contains the same ring system with a longer and more unsaturated side chain. This chapter summarizes the progress in chemistry and biochemistry of the naturally occurring K vitamins and their close analogs. In addition, the chapter reviews the observed differences in action between K vitamins and menadione and an attempt is made to explain them on biogenetic considerations.

A circular dichroism study of transketolase from baker's yeast

Abstract The circular dichroism of transketolase from bakers yeast was examined in the 400 to 200 nm region in the presence and absence of its coenzyme and substrates. Coenzyme binding caused an extrinsic Cotton effect v pyrophosphate and a tryptophan moiety of the enzyme protein. Addition of donor substrates as D-fructose-6-phosphate and hydroxy-pyruvate abolished the ellipticity band at 235 nm. Steric hindrance might be responsible for the fact that 2-(1,2-dihydroxyethyl) thiamine pyrophosphate (“active glycolaldehyde”) can no longer form a charge transfer complex. On the other hand, acceptor substrates in the transketolase reaction, such as ribose-5-phosphate, did not influence the broad negative dichroism at 325 nm.

Biochemical Pathology of Vitamin B6 Deficiency

Publisher Summary Various pyridoxal phosphate enzymes are affected differently by vitamin B 6 depletion. Enzymes involved in the degradation of tryptophan and of the sulfur amino acids are impaired at an early stage of the deficiency. Therefore, alterations of the metabolism of these amino acids are the first signs of the vitamin B 6 deficiency state. The apoenzyme synthesis of pyridoxal phosphate-containing enzymes seems to be induced and regulated by pyridoxal phosphate. Consequently, organs with a high protein turnover lose their pyridoxal phosphate enzymes more rapidly by vitamin B 6 depletion than those with a low protein turnover. Correlations exist between cerebral symptoms produced by vitamin B 6 depletion, the reduction of the pyridoxal phosphate content in the brain and the activity of its enzymes dependent on pyridoxal phosphate.

Uptake and release of [1-14C]ascorbic acid and [1-14C]dehydroascorbic acid by erythrocytes of guinea pigs

Abstract The uptake and release of [1- 14 C] ascorbic acid and [1- 14 C]dehydroascorbic acid by erythrocytes of guinea pigs kept on a normal diet and of those on an ascorbic acid-deficient diet was studied in vitro . Results strongly suggest that the erythrocyte membrane is permeable to both ascorbic acid and dehydroascorbic acid in either direction. Mean ratios of the uptake of dehydroascorbic acid to ascorbic acid by the erythrocytes of normally fed guinea pigs (2.03 ± 0.69) and of vitamin C-deficient animals (2.49 ± 0.44) are given. After incubation with [1- 14 C]ascorbic acid, only ascorbic acid was found in the erythrocytes and only ascorbic acid was released during reincubation. In the case of [1- 14 C]dehydroascorbic acid, mainly dehydroascorbic acid was found to be present in the erythrocyte, but some ascorbic acid was always detectable. Both dehydroascorbic acid and ascorbic acid were released, mainly, however, dehydroascorbic acid.

Site of intestinal absorption of ascorbic acid in guinea pigs and rats

Abstract The site of absorption of ascorbic acid by the small intestine was studied in vivo in guinea pigs, normal and hypophysectomized rats after oral application of 14C-ascorbic acid. A species-specific difference was revealed. The site of absorption in the guinea pig was located in the duodenal and proximal small intestinal wall, whereas the rat showed highest absorption in the ileum. Hypophysectomy in rats caused a shift of the absorption site from the ileum to the jejunum. No absorption was observed in the duodenum and ileum. A regulatory role of the pituitary gland in the absorption of ascorbic acid by the small intestine is discussed.

Studies on the uptake of [1-14C]ascorbic acid and [1-14C]-dehydroascorbic acid by platelets of guinea pigs

Abstract Ascorbic acid as well as dehydroascorbic acid were shown to penetrate the platelet membrane in vitro. The percentage uptake by platelets of control guinea pigs (ascorbic acid 0.22 ± 0.014; dehydroascorbic acid 0.54 ± 0.033; p = 0.001) as well as of vitamin C-deficient animals (ascorbic acid 0.40 ± 0.026; dehydroascorbic acid 0.88 ± 0.060; p = 0.001 SEM given) were found to be significantly different. The uptake of ascorbic acid as well as of dehydroascorbic acid were significantly enhanced compared to control animals (p = 0.001). However, the ratios of uptake of dehydroascorbic acid to ascorbic acid were unaffected (control animals 2.30 ± 0.24; vitamin C-deficient animals (2.30 ± 0.17). The uptake of ascorbic acid was found to be fairly dependent on the concentration and was not influenced by ouabain. On the contrary, the uptake of dehydroascorbic acid did not show a linear dependency on the concentration and was inhibited by 10−3 M ouabain. These findings suggest an active transport mechanism for the uptake of dehydroascorbic acid whereas in case of ascorbic acid the facilitated diffusion mechanism is discussed. After incubation of platelets of control and vitamin C-deficient animals with [1-14C]ascorbic acid as well as with [1-14C]dehydroascorbic acid only [1-14C] ascorbic acid was detectable in the platelets and only [1-14C]ascorbic acid was released by the platelets.