Thiamin Vitamers And Antagonists

As shown in Figure 6.1, thiamin consists ofpyrimidine and thiazole rings, linked by a methylene bridge; the alcohol groupof the side chain can be esterified with one, two, or three phosphates, yielding thiamin monophosphate, thiamin diphosphate (also known as thiamin pyrophosphate, the metabolically active coenzyme), and thiamin triphosphate. The vitamin was originally named aneurine, the antineuritic vitamin, because of its function in preventing or

Figure 6.1. Thiamin and thiamin analogs, products of thiaminolysis, and experimental antimetabolites. Relative molecular masses (Mr): thiamin, 266.4 (chloride-hydrochloride, 337.3); thiamin monophosphate, 345.3; thiamin diphosphate, 425.3; thiamin triphosphate, 505.3; thiochrome, 262.3; thiamin thiol, 282.4 (oxidizes to thiamin disulfide, 562.7); oxythiamin, 301.8; and pyrithiamin, 420.2.

pyrithiamin

Figure 6.1. Thiamin and thiamin analogs, products of thiaminolysis, and experimental antimetabolites. Relative molecular masses (Mr): thiamin, 266.4 (chloride-hydrochloride, 337.3); thiamin monophosphate, 345.3; thiamin diphosphate, 425.3; thiamin triphosphate, 505.3; thiochrome, 262.3; thiamin thiol, 282.4 (oxidizes to thiamin disulfide, 562.7); oxythiamin, 301.8; and pyrithiamin, 420.2.

curing polyneuritis in deficient animals. When its chemistry was discovered, it was called thiamine, because of the presence of both sulfur and an amino group in the molecule. The final -e has been dropped; the amino group is not involved in the metabolic role of the vitamin.

The free base is unstable, and two derivatives of thiamin are commonly used in food enrichment and pharmaceutical preparations: thiamin chloride hydrochloride (generally known simply as thiaminhydrochloride) and thiamin mononitrate. The mononitrate is less hygroscopic than the chloride hydrochloride and is the preferred form for food enrichment. There is considerable difficulty in interpreting much of the literature on thiamin requirements, because many authors quote mg of thiamin, without specifying whether it was as the free base, the chloride hydrochloride, or the equivalent amount of free base. Because the Mr of free thiamin is 266.4, and that of the chloride hydrochloride is 337.3, this confusion can result in errors of the order of 26%.

Oxidative cleavage of the thiazole ring occurs in alkaline solution, forming a reactive sulfhydryl group (thiamin thiol) that can react with other thiols, forming thiamin alkyl disulfides - allithiamins. A number of allithiamins occur in plants (especially members of the genus Allium). They are biologically active; on reductive cleavage of the disulfide bridge, they spontaneously dehydrate to yield thiamin. However, they do not undergo alkali-catalyzed ring closure to thiochrome, which is the basis of the most commonly used method for determining thiamin chemically, so may be overlooked in chemical analysis.

Synthetic allithiamin derivatives, such as thiamin propyl and tetrahydro-furfuryl disulfides, have been used for the prevention and treatment of thiamin deficiency. Because they are lipid soluble, and are not subject to the normal control of thiamin absorption by saturation of the intestinal transport system, they have potential benefits in the treatment of thiamin-deficient alcoholics, whose absorption of thiamin is impaired.

Thiamin is labile to sulfite, which cleaves the methylene bridge. The reaction is slow at acid pH, but rapid above pH 6. Sulfite treatment of dried fruit and other foods results in more or less complete loss of thiamin.

Two analogs of thiamin, oxythiamin and pyrithiamin, are potent an-timetabolites 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.

Beat The Battle With The Bottle

Beat The Battle With The Bottle

Alcoholism is something that can't be formed in easy terms. Alcoholism as a whole refers to the circumstance whereby there's an obsession in man to keep ingesting beverages with alcohol content which is injurious to health. The circumstance of alcoholism doesn't let the person addicted have any command over ingestion despite being cognizant of the damaging consequences ensuing from it.

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