Thiaminase enzymes

Raw fish toxicity—Various species of freshwater fish (such as carp) and crustaceans contain an enzyme (thiaminase) which destroys thiamin (vitamin B-1). Hence, persons eating these items raw may develop thiamin deficiencies, particularly if the rest of their diets are rich in carbohydrates and/or alcohol which raise the requirement for thiamin. However, thorough cooking destroys the enzyme, but leaves the thiamin intact. 

Haff disease appeared to be the human equivalent of Chastek paralysis, which affects mink and fox. Hence, it was concluded that it was caused by eating large quantities of inadequately cooked thiamineise-containing fish with the enzyme thiaminase inactivating the thiamin molecule, resulting in a thiamin deficiency. 

ANTITHIAMIN FACTORS IN FOOD. Certain raw fish and seafood—particularly carp, herring, clams, and shrimp— contain the enzyme thiaminase, which inactivates the thiamin molecule by splitting it into two parts. This effect has been seen in mink and fox fed 10 to 25% levels of certain raw fish, giving rise to a thiamin deficiency disease known as Chastek paralysis. This action can be prevented by cooking the fish prior to feeding, thereby destroying the thiaminase. Of course, humans seldom eat sufficient thiaminase-contain-ing raw fish or seafood to produce a thiamin deficiency. 

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. 

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. 

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... 

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 addition to those mentioned in Table II, there is a group of enzymes in foods which lower the nutritive content. This group includes thiaminase which acts on thiamine in flesh food, ascorbic acid oxidase in... 

Thiamine turnover is rapid because of the ubiquitous presence of thiaminase enzymes that hydrolyze thiamine into its pyrimidine and thiazole components. Thus, symptoms of thiamine deficiency can appear within 2 weeks of a diet depleted in thiamine. In Western societies, severe thiamine deficiency is most frequently found in alcoholics. Patients who chronically misuse alcohol are prone to thiamine deficiency arising from a number of factors including poor nutrition and poor absorption and storage, as well as an increased breakdown of TPP. Alcohol is known to inhibit the active absorption of thiamine. 

Besides hydrolyzing enzyme [30], oxidoreductase [39], RNase [ 13], transaminase [40], CD could be used to build up many analog enzymes, such as carbonic anhydrase, thiaminase [41], hydroxyl aldehyde condensation enzyme [42], biotin [29] and so on, and all of them had an excellent result. 

Thiaminase is present in bracken Pteridium aquilinum), and thiamin deficienq symptoms have been reported in horses consiuning this material. Raw fish also contains the enzyme, which destroys the thiamin in foods with which the fish is mixed. The activity of the thiaminase is, however, destroyed by cooking. 

Enzymes destroying thiamine have been encountered in some species in a number of fresh-water fish, in very few salt-water fish, in some shellfish and Crustacea, in bracken ferns and in three species of intestinal bacteria. They have all received the name of "thiaminase , but it is very probable that we have to do with several, different, enzymes >23 They all bring about a fission of the thiamine molecule, liberating the thiazole part, but only in one case, that of thiaminase of Bacillus aneurinolyticus Kimura el Aoyana, reasonable proof has been obtained of a hydrolytic fission, yielding both moieties of the vitamin molecule z-methyl-q-amino-S-hydroxymethylpyr-imidine and 4-methyl-5(/ -hydroxy)-ethylthiazole. 

The literature concerning thiaminases is still confused in some respects. Thus, according to some reports, bracken and other plants would contain heat-stable substances capable of destroying thiamine evidently not enzymes, but often mentioned together with these. Some have later been identified as flavonoids, phenols and tannins. In one report thiaminase has been claimed to occur in tissues of rabbits and chickens s, but this has never been confirmed. 

The seemingly erratic distribution of thiaminases in nature makes the function of such an enzyme rather enigmatic. However, many more tissues, hitherto negative, might appear to contain a thiaminase if re-examined with a suitable acceptor for the reaction. 

It has been repeatedly suggested that the main role of thiaminase in vivo would be not the decomposition, but the synthesis of thiamine. Indeed, thiamine has been formed by the enzyme, starting from a Pym-CH2 -Y+ conjugate and the thiazole moiety, but the equilibrium was far to the side of decomposition of the vitamin. 

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

Thiaminases are enzymes that are significant antivitamins, as are some low molecular weight substances from plant foods, such as phenolic compounds in blueberries, red currants, red cabbage and Brussel sprouts. Higher activity of thiaminase 1 is seen in raw meat and some fresh raw sea fish, molluscs, the raw offal of farm animals and some plants. Thiaminase II is an enzyme present in many bacteria. 

A second pyrimidine transferase has been found in clam tissue. This enzyme differs from thiaminase I in that it cannot use aromatic or heterocyclic amines as a pyrimidyl acceptor, but exchanges thiazole for hypo-taurine to form icthiamine (Si). 

A second thiaminase is known from BaciUiu anevrinolyHcus (SS) and from yeast (S4, S6). This enzyme hydrolyzes thiamine yielding pyrimidyl methanol and thiazole moieties. This reaction is therefore the reverse of thiamine thesis (see Section I), and in fact, thiaminase II is capable of catalyzing the synthesis of thiamine, if sufficient concentrations of the pyrimidyl and thiazole moieties are present in the reaction mixture. This enzyme differs from thiaminase I in that it is inhibited by aromatic and heterocyclic amines and does not decompose thiamine pyrophosphate (SO, SI). 

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