Enzyme thiaminase – enzyme that cause thiamin deficiency

Thiamin is one of the water soluble B vitamins and is also known as vitamin B1. Thiamin requirement can be greatly increased with the consumption of anti-thiamin substances in foods. Substances with an anti-thiamin activity are fairly common in nature and include structurally similar antagonists as well as structure-altering antagonist and thiamin degrading enzymes (thiaminases).

Thiaminase can cleave the vitamin B1 (by cleaving the pyrimidine from the thiazole ring) and render it inactive. People have acquired vitamin B1 or thiamin deficiency by eating raw clams and raw fish as a major part of their diet. This antagonist is easily destroyed by cooking. Since thiaminase is heat-labile, the problem can be avoided by cooking the fish before adding the other diet ingredients.

There are two forms of this enzyme, thiaminase I sand thiaminase II. The presence of thiaminase was discovered in clams by Fujita and Matsukawa in 1942 and in fresh-water fish by Green (1941, 1942). The enzyme was name ‘thiaminase’ by Sealock and White in 1949.

The presence of thiaminase was recorded in many freshwater and saltwater fish. Among fresh-water fish, carp and crusian carp exceed other species in the potency of this particular enzyme, which is most abundant in kidneys, intestines and gills.

Heat processing destroys thiaminase, which destroys thiamin in fish, shellfish, brussels sprouts, and red cabbage. In countries where large amounts of fish are eaten raw, human thiamin deficiency may occur.
Enzyme thiaminase – enzyme that cause thiamin deficiency

Absorption and Transport of Thiamin

The bioavailability of thiamin occurring naturally in foods is believed to be high. Dietary sources of thiamine include: yeasts, pork, sunflower seeds, legumes, chicken, fish, beef, wheat germ, cereal products, lentils, potatoes, rice polishing and nuts.

Occasionally, however, anti-thiamin factor may be present in the diet. For example, thiaminases present in the raw fish catalyze the cleavage of thiamin, thereby destroying its activity.

These thiaminases are thermolabile, however and cooking of fish rendered the enzymes inactive. Other anti-thiamin factors that are thermostable may be found in tea and certain fruits and vegetables such as blueberries, black currents, Brussels sprouts and red cabbage.

Dietary thiamine is absorbed readily from jejunum and proximal ileum and is transported to tissues where it is converted to the active form, thiamine pyrophosphate (TPP).

Absorption of thiamin can be both active and passive, depending upon the amount of the vitamin presented for absorption.

At low physiologic concentrations, thiamin absorption is an active process. This Na+ dependent, carrier mediated absorption occurs primarily in the jejunum but can occur in other portions of the small intestine as well.

When intakes of thiamin are high, the absorption route is predominantly passive. The rate of thiamin absorption is always quite high except in the case of ethanol ingestion and/or folate deficiency.

Thiamine is carried by the portal blood to the liver. In normal adults, 20-30% of plasma thiamine is protein-bound all of which appears to be TPP. The transport of thiamine into erythrocytes seems to be facilitated diffusion process, whereas it enters other cells by an active process.

Thiamine uptake by active transport is highest in the jejunum and ileum, with both passive diffusion and active carrier-mediated transport.

Ethanol ingestion interferes with active transport of thiamin, and folate deficiency prevents the normal duplication of enterocytes, thereby decreasing absorption, both active and passive.

Conversion to the active coenzyme form requires adenosine triphosphate (ATP) and thiamin pyrophosphokinase, an enzyme found in the liver and brain (and perhaps in other tissue as well).

Another form of thiamin (thiamin triphosphate, or ATP) is synthesized in the brain by action of a thiamin diphosphate (ADP) – ATP phosphoryl-transferase.
Absorption and Transport of Thiamin

Vitamins as antioxidants in processed foods

Oxidation, a series of chemical reactions yielding undesirable and products (off odors, colors, and flavors), may occur in many fruits and vegetables and foods high in fat and oil during exposure to air, light, heat, heavy metals, certain pigments or alkaline conditions.

Enzymatic browning may occur in some fruits and vegetables, particularly apples, banana, peaches, pear, and potatoes, which contain phenolase enzymes. Enzymatic browning is the rapid, enzyme-mediated conversion of plant phenolic compounds into dark polyphenolic polymers.

When these fruits and vegetables are cut or sliced and exposed to air, the phenolases catalyze oxidation of phenolics compounds to ortho-quinone compounds, which then polymerize, forming brown pigments.

Fruits and vegetables must be blanched and pretreated prior to pressing, in order to inactivate the enzyme. 

Oxidation in lipids (autoxidation) and in fat and oil containing foods, on the other hand, occurs as a result of the susceptibility of fatty acids (building blocks of fats and oils) to oxidations and subsequent formation of reactive compounds referred to as “free radicals”.

The free radicals promote the development of a series of chemical reactions which lead to the production of off-flavors, colors, odors, and rancidity.

Some oxidations have more than one function. For example, ascorbic acids may function as a free-radical chain terminator, and oxygen scavenger, or a metal chelator. Under certain conditions, it may act as a promoter for oxidation.

Due to increasing concern over the potential toxicity and safety of some phenolic antioxidants, the replacement of synthetic antioxidants by ‘safe, natural’ antioxidants such as vitamins E and C flavonoids and other plant phenolic has received considerable attention.

Vitamin E compounds are reported to effectively inhibit lipid oxidation in foods and biological systems.
Vitamins as antioxidants in processed foods