Thiaminases

Toxopyrimidine (pyramin lOlO) may be obtained from thiamine (vitamin Bi 1011) by acidic hydrolysis or by treatment with the thiaminase of Bacillus aneurinolyticus. Toxopyrimidine produces convulsions and death in rodents as do analogues, e.g. 2,4-dimethylpyrimidin-5-ylmethanol (1012) (65JMC750), but the effect is minimized or even... 

Some kinds of fish and Crustacea contain thiaminases. These enzymes cleave thiamin and thus inactivate the vitamin. Some plant phenols, e.g., chlorogenic acid, may possess antithiaminic properties, too, though their mechanism of action is so far not well understood. 

Evans, W. C. 1976. Bracken thiaminase-mediated neurotoxic syndromes. Pages 113-131 in E. H. Perring and B. G. Gardiner (eds.), The biology of bracken. Linnean Society of London-Academic Press, London. 

A peculiarity of thiamine is that the vitamin can easily become inactivated. An early instance was seen in 1941 when commercially reared mink became paralyzed (Chastek paralysis), a disorder which could be cured by giving the animals thiamine. The problem was traced to their having been fed fish that had partially decomposed. Later work showed that in decayed fish a microbial enzyme had been released, thiaminase, which destroyed the thiamine normally present in the food. A rather different process occurs when horses or cows are allowed to graze on bracken. This contains a protein which binds to thiamine, so reducing its availability. Once again the condition can be treated by administering the vitamin. 

The terms 3 and E, indicate hydrophobic and steric effects specific to m-substituents. Eq. 10 describes substituent effects on the rate of transfer of substituted anilines catalyzed by carp vicera thiaminase 11... 

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

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... 

Thiamine Blueberries, fish Foods containing thiaminases... 

Compounds naturally present in foods may have antivitamin activity. Many foods contain thiaminases and compounds that catalyze nonen-zymic cleavage of thiamin to biologically inactive products (Section 6.4.7). 

TWo analogs of thiamin, oxythiamin and pyrithiamin, are potent antimetabolites and have been widely used to induce thiamin deficiency in experimental animals. The mechanisms of action of pyrithiamin, oxythiamin, and thiaminases found in foods are discussed in Section 6.4.7. 

Thiaminolytic enzymes are found in a variety of microorganisms and foods, and a number of thermostable compounds present in foods (especially polyphenols) cause oxidative cleavage of thiamin, as does sulfite, which is widely used in food processing. The products of thiamin cleavage by sulfite and thiaminases are shown in Figure 6.1. 

In people whose thiamin intake is marginal, colonization of the gastrointestinal tract with thiaminolytic microorganisms may be a factor in the development of beriberi. The thiaminases present in raw fish can result in so-called Chastek paralysis of foxes and mink, as a result of destruction of thiamin, and may be important in parts of the world where much of the apparent thiamin intake is from fish that is eaten raw or fermented. The polyphenols and thiami-nase in bracken fern can cause thiamin deficiency (blind staggers) in horses, and tannic acid in tea and betel nut have been associated with human thiamin deficiency. 

Thiaminase II catalyzes a simple hydrolysis, releasing thiazole and meth-oxypyrimidine, which has some antivitamin Be antimetaboUc activity. It is relatively rare and is restricted to a small number of microorganisms. 

Edwin EE and Jackman R (1970) Thiaminase I in the development of cerebrocortical necrosis in sheep and cattle. Nature 228, 772-4. 

There are two classes of thiaminase. Thiaminase I catalyzes a base exchange reaction between the thiazole moiety of thiamin and a variety of bases, commonly primary, secondary, or tertiary amines, but also nicotincimide and other pyridine derivatives, and sometimes proline tmd sulfhydryl compounds. Thiaminase I is relatively widespread in a veuiety of microorgcmisms, plcmts, cmd fish. In addition to depleting thiamin, the products of base exchange catedyzed by thiaminase I eire structured emedogs of the viteimin and may have antagonistic effects (Edwin and Jackman, 1970). Similarly, the neurotoxic effects of the antibiotic metronidazole, which is a thiazole, may be fi om its activity as a substrate for thieiminase I, forming thiamin antimetabolites (Alston emd Abeles, 1987). 

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