Thiamin-degrading enzymes

A similar cleavage is catalyzed by thiamin-degrading enzymes known as thiaminases which are found in a number of bacteria, marine organisms, and plants. 

There are only a few studies concerning the degradation and elimination of thiamin in animals. Most excess thiamin is eliminated as such by the kidneys. In a couple of older studies (Neal and Pearson 1964 Pearson et al. 1966), radioactive thiamin was administered in rats and the urines were analysed. Several radioactive degradation products of thiamin (2-methyl-4-amino-5-pyrimidine carboxylic acid and 4-methyl-thiazole-5-acetic acid), resulting from the cleavage between the thiazole and the pyrimidine moieties, were excreted in the urine. Other products were also detected but not identified. No enzymes specifically involved in thiamin degradation in mammals have been identified. 

Thiamine pyrophosphate is a coenzyme for several enzymes involved in carbohydrate metabolism. These enzymes either catalyze the decarboxylation of oi-keto acids or the rearrangement of the carbon skeletons of certain sugars. A particularly important example is provided by the conversion of pyruvic acid, an oi-keto acid, to acetic acid. The pyruvate dehydrogenase complex catalyzes this reaction. This is the key reaction that links the degradation of sugars to the citric acid cycle and fatty acid synthesis (chapters 16 and 18) ... 

The thiamine vitamers are relatively stable in the dried state at low temperature in the dark (67-69). In solution, they are generally unstable at elevated temperatures or under alkaline conditions. Thiamine is stable to heat, including autoclaving, and oxidation below pH 5.0. It is most stable at pH 2-4. In solution, TPP is stable at pH 2-6 if it is stored at low temperature. The thiamine vitamers are also susceptible to degradation by endogenous thiaminase enzymes and other thiamine... 

Beside the conversion of endogenic sulfur compounds the addition of S-compounds like sulfite, as an antimicrobial agent, antioxidant and enzyme inhibitor (75) or like thiamine (vitamin Bl), as a nutrient for yeasts, are allowed in the EEC (within defined maximum values). Furthermore chemical reactions like sulfite addition to aldehydes, Maillard reaction or Strecker degradation play an important role with regard to the sulfur chemistry of wines. 

Degradation of all three branched-chain amino acids begins with a transamination followed by an oxidative decarboxylation catalyzed by the branched-chain a-keto acid dehydrogenase complex. This enzyme, like a-ketoglutarate dehydrogenase, requires thiamine pyrophosphate, lipoic acid, coenzyme A, FAD, and NAD+ (Figure 7-11). 

Enzymes which degrade a-keto acids to COg and aldehydes contain thiamine pyrophosphate (D 10.4.5) as coenzyme. The keto acid is added in a reversible reaction to carbon atom 2 of the thiazole ring of thiamine pyrophosphate giving an oc-hydroxy acid derivative. This compound is decarboxylated and split to an aldehyde and thiamine pyrophosphate. Figure 25 shows this sequence of reactions for the enzyme pyruvate decarboxylase which splits pyruvate to acetaldehyde and COg. 

Thiamine stability in aqueous solution is relatively low. It is influenced by pH (Fig. 6.2), temperature (Table 6.8), ionic strength and metal ions. The enzyme-bound form is less stable than free thiamine (Fig. 6.2). Strong nucleophilic reagents, such as HS03 or OH , cause rapid decomposition by forming 5-(2-hydroxyethyl)-4-methylthiazole and 2-methyl-4-amino-5(methyl-sulfonic acid)-pyrimidine, or 2-methyl-4-amino-5-hydroxymethylpyrimidine (see Reactions 6.7). Thermal degradation of thiamine, which also initially yields the thiazole and pyrimidine... 

Certain enzymes convert some vitamins into inactive substances (e.g. lipoxygenase indirectly catalyses degradation of vitamin A and its provitamins, thiaminases decompose thiamine). 

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