Thiamine, oxidation

Bi, thiamin oxidation-reduction potentials, hydroxylation reactions that require copper or iron Oxidative decarboxylation of amino acids,... 

Thermoplastics, MA-containing, 273, 292, 298 Thiamine, oxidation inhibitor, 156 Thiamine disulfide, MA-copolymer complexes, 431, 440... 

Certain bacterial strains convert propylene glycol to pymvic acid in the presence of thiamine (15) other strains do the conversion without thiamine (16). Propylene oxide is the principal product of the reaction of propylene glycol over a cesium impregnated siHca gel at 360°C in the presence of methyl ethyl ketone and xylene (17). 

The yellow form (11) on acidification is converted to the more stable thiol form (12). On oxidation, typically with alkaline ferhcyanide, yellow form (11) is irreversibly converted to thiochrome [299-35-4] (14), a yellow crystalline compound found naturally in yeast but with no thiamine activity. In solution, thiochrome exhibits an intense blue fluorescence, a property used for the quantitative determination of thiamine. 

The thiol form (12) is susceptible to oxidation (see Fig. 2). Iodine treatment regenerates thiamine in good yield. Heating an aqueous solution at pH 8 in air gives rise to thiamine disulfide [67-16-3] (21), thiochrome (14), and other products (22). The disulfide is readily reduced to thiamine in vivo and is as biologically active. Other mixed disulfides, of interest as fat-soluble forms, are formed from thiamine, possibly via oxidative coupling to the thiol form (12). 

Step 4 of Figure 29.12 Oxidative Decarboxylation The transformation of cr-ketoglutarate to succinyl CoA in step 4 is a multistep process just like the transformation of pyruvate to acetyl CoA that we saw in Figure 29.11. In both cases, an -keto acid loses C02 and is oxidized to a thioester in a series of steps catalyzed by a multienzynie dehydrogenase complex. As in the conversion of pyruvate to acetyl CoA, the reaction involves an initial nucleophilic addition reaction to a-ketoglutarate by thiamin diphosphate vlide, followed by decarboxylation, reaction with lipoamide, elimination of TPP vlide, and finally a transesterification of the dihydrolipoamide thioester with coenzyme A. 

Important organic applications are to the determination of quinine and the vitamins riboflavin (vitamin B2) and thiamine (vitamin Bj). Riboflavin fluoresces in aqueous solution thiamine must first be oxidised with alkaline hexacyanoferrate(III) solution to thiochrome, which gives a blue fluorescence in butanol solution. Under standard conditions, the net fluorescence of the thiochrome produced by oxidation of the vitamin Bj is directly proportional to its concentration over a given range. The fluorescence can be measured either by reference to a standard quinine solution in a null-point instrument or directly in a spectrofluorimeter.27... 

Fermenting baker s yeast also catalyzes the 1,4-addition of a formal trifluoroethanol-d1-synthon to a,/i-unsaturated aldehydes, to give optically active l,l,l-trifluoro-2-hydroxy-5-alka-nones52. Presumably, the mechanism involves oxidation of the alcohol to the corresponding aldehyde followed by an umpolung step with thiamine pyrophosphate and Michael addition to the a,/i-unsaturated aldehyde. For example, l,l,l-trifluoro-2-hydroxy-5-hexanone (yield 26%, ee 93%) is thus obtained from trifluoroethanol and l-bnten-3-one. 

Figure 17-5. Oxidative decarboxylation of pyruvate by the pyruvate dehydrogenase complex. Lipoic acid is joined by an amide link to a lysine residue of the transacetylase component of the enzyme complex. It forms a long flexible arm, allowing the lipoic acid prosthetic group to rotate sequentially between the active sites of each of the enzymes of the complex. (NAD nicotinamide adenine dinucleotide FAD, flavin adenine dinucleotide TDP, thiamin diphosphate.)...

Pyruvate is oxidized to acetyl-GoA by a multienzyme complex, pyruvate dehydrogenase, that is dependent on the vitamin cofactor thiamin diphosphate. 

Thiamin has a central role in energy-yielding metabo-hsm, and especially the metabohsm of carbohydrate (Figure 45-9). Thiamin diphosphate is the coenzyme for three multi-enzyme complexes that catalyze oxidative decarboxylation reactions pymvate dehydrogenase in carbohydrate metabolism a-ketoglutarate dehydro-... 

The water-soluble vitamins of the B complex act as enzyme cofactors. Thiamin is a cofactor in oxidative... 

Rice bran is the richest natural source of B-complex vitamins. Considerable amounts of thiamin (Bl), riboflavin (B2), niacin (B3), pantothenic acid (B5) and pyridoxin (B6) are available in rice bran (Table 17.1). Thiamin (Bl) is central to carbohydrate metabolism and kreb s cycle function. Niacin (B3) also plays a key role in carbohydrate metabolism for the synthesis of GTF (Glucose Tolerance Factor). As a pre-cursor to NAD (nicotinamide adenine dinucleotide-oxidized form), it is an important metabolite concerned with intracellular energy production. It prevents the depletion of NAD in the pancreatic beta cells. It also promotes healthy cholesterol levels not only by decreasing LDL-C but also by improving HDL-C. It is the safest nutritional approach to normalizing cholesterol levels. Pyridoxine (B6) helps to regulate blood glucose levels, prevents peripheral neuropathy in diabetics and improves the immune function. 

Oxidative stress contributes to selective neuronal cell death in thiamine-deficiency 600... 

In the 1930s, Peters and co-workers showed that thiamine deficiency in pigeons resulted in the accumulation of lactate in the brainstem [ 15]. Furthermore, they showed that the addition of small quantities of crystalline thiamine to the isolated brainstem tissue from thiamine-deficient birds in vitro resulted in normalization of lactate levels. These findings led to the formulation of the concept of the biochemical lesion in thiamine deficiency. Subsequent studies showed that the enzyme defect responsible for the biochemical lesion was a-KGDH rather than pyruvate dehydrogenase (PHDC), as had previously been presumed. a-KGDH and PHDC are major thiamine diphosphate (TDP)-dependent enzymes involved in brain glucose oxidation (Fig. 34-4). 

FIGURE 34-5 Induction of endothelial nitric oxide synthase (eNOS) in medial thalamus of thiamine-deficient rats. (A) Increased eNOS mRNA. (B) Increased eNOS immunolabeling of vascular endothelial cells (magnification x200). 

Gibson, G. E. and Zhang, H. Interactions of oxidative stress with thiamine homeostasis promote neurodegeneration. Neurochem. Int. 40 493-504, 2002. 

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