Thiamin monophosphate hydrolysis

Both free thiamin and thiamin monophosphate circulate in plasma about 60% of the total is the monophosphate. Under normal conditions, most is bound to albumin when the albumin binding capacity is saturated, the excess is rapidly filtered at the glomerulus and excreted in the urine. Although a significant amount of newly absorbed thiamin is phosphorylated in the Uver, aU tissues can take up both thiamin and thiamin monophosphate, and are able to phosphorylate them to thiamin diphosphate and thiamin triphosphate. In most tissues, it is free thiamin that is the immediate precursor of thiamin diphosphate, which is formed by a pyrophosphokinase both the p-and y-phosphates of ATP are incorporated. Thiamin monophosphate arises mainly as a result of sequential hydrolysis of thiamin triphosphate and thiamin diphosphate. 

Both thiamin monophosphate and free thiamin are found in cerebrospinal fluid. Uptake of thiamin monophosphate into cells in the central nervous system involves extracellular hydrolysis to free thiamin, probably catalyzed... 

The cofactor form of thiamin is thiamiri pyrophosphate (TPP). TPPis released from dietary proteins during hydrolysis in the gastrointestinal tract and then hydroly 2ed to thiamin- The thiamin is absorbed and transported into various tissues, where it Is converted back to TPP by the action of thiaminokinase (Figure 9.70). A small proportion of the body s thiamin occurs as thiamin monophosphate (TMP) and thiamin trisphosphate (TTP). 

Thiamine has a physiological function in the form of thiamine pyrophosphate. This compound is the coenzyme of a number of enzymes involved in carbohydrate metabolism carboxylase, pyruvic dehydrogenase, -ketoglutaric dehydrogenase, transketolase, phosphoketolase. Thiamine monophosphate and thiamine triphosphate cannot replace the pyrophosphate ester in this function a supposed activity of the triphosphate was later shown to be due to its partial hydrolysis to the pyrophosphate . 

TPP is inactivated by treatment with 1 N hydrochloric acid at 100° for 15 min. owing to the hydrolysis of the pyrophosphate linkage and yields orthophosphate and thiamine monophosphate. Treatment of TPP with alkali splits off pyrophosphate and gives free thiamine. TPP, like thiamine,... 

Vitamin Bi [1] exists in nature both in free (thiamin) and esterified form (thiamin monophosphate, diphosphate, and triphosphate), while thiamin hydrochloride is used as a supplement [4]. To evaluate the total content of vitamin Bi in a food, extraction usually consists of an acid hydrolysis (0.1 M HCl in a water bath at 100°C or in an autoclave at 121°C) followed by an enzymatic digestion (diastases possessing a phosphatase activity) [1,2,5,6]. The acid treatment frees protein-boimd forms and converts starch into soluble sugars. The enzymatic treatment may require several hours (on average 3 hr) of incubation for complete dephosphorylation of the thiamin esters. 

Thiamine is relatively stable in acidic solutions (pH < 5). Thiamine diphosphate is unstable in weakly acidic and neutral solutions, and its hydrolysis yields thiamine monophosphate and thiamine. In neutral and alkaline solutions, thiamine exists as the fi ee base, which is very unstable. It is hydrolysed to 4-amino-5-hydroxymethyl-2-methylpyrimidine and 5-(2-hydroxyethyl)-... 

Thiamine can be considered to be the product of the quatemization of 4-methyl-5-(2-hydroxymethyl)thiazole (5) by an active derivative of 4-amino-5-(hydroxymethyl)-2-methyl pyrimidine (4) (Scheme 2). In living cells, pyramine can be activated by conversion into the diphosphate 7, via monophosphate 6, and the substrate of the enzyme responsible for the quatemization is not the thiamine thiazole, but its phosphate 8. The product of the condensation, thiamine phosphate (9), is finally converted into diphosphate 2—the biochemically active derivative—by hydrolysis to free thiamine, followed by diphosphorylation, or more directly, in some cases. Enzymes are known for all of the steps depicted in Scheme 2, and adenosine triphosphate (ATP) is, as usual, the phosphate donor. 

Lipid oxidation products and their reaction products with amino acids (proteins) have a considerable influence on the typical odour and taste of meat. Particularly significant aminocarboxylic acids include glutamic acid, alanine, threonine and lysine, guanidine compounds (creatine and creatinine), quaternary ammonium compounds (choline and carnitine), peptides (P-alanylhistidine peptides and some products of proteolysis), free nucleotides, nucleosides and their bases (especially inosine 5 -monophosphate, IMP), proteins, carboxylic acids (especially lactic acid), sugars (mainly glucose, fructose and their phosphates, ribose formed by hydrolysis of free nucleotides) and some vitamins (especially thiamine). Some of these compounds, such as glutamic acid and IMP, are additionally used as food additives, namely as flavour enhancers. 

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