Thiamin sulfite cleavage

Figure 5.3 Alkali-induced transformations of thiamin and cleavage by sulfite. (A) In alkaline medium, thiamin (1) can be slowly transformed to the so-called yellow form (4). This transformation involves the formation of a pseudobase (2) and opening of the thiazolium ring to form a thiolate (3).

The point of attachment on the pyrimidine fragment was of course already marked by the position of the sulfonic group introduced during the sulfite cleavage of the vitamin. Thus, in 1936, the structure I could be assigned to thiamine, ten years after the crystalline vitamin was first obtained. 

Indeed both -lactylthiamine pyrophosphate (XX) and a-hydroxyethyl-thiamine pyrophosphate (XXI) have been isolated and identified as products after incubation of pyruvate with a purified carboxylase preparation " . When [2- - C]pyruvate is used, the radioactivity is found in the thiazole part of the molecule after sulfite cleavage of XXL Acetaldehyde is formed from pyruvic acid by yeast carboxylase by enzymic cleavage of intermediate XXI, Uberating thiamine pyTophosphate . XXI has also been identified as intermediate in the formation of acetyl-coenzyme A from pyruvic acid by p3u uvic oxidase . The transketolase reaction has been shown to proceed via a gly-colaldehyde-enzyme intermediate here one may expect to find dihydroxy-ethylthiamine pyrophosphate as active glycol-aldehyde . Such experiments strongly support Breslow s concept of the reaction mechanism. 

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. 

An undesirable reaction of sulfite in food is the cleavage of thiamin by means of an attack at the pyrimidin moiety (Zoltewicz et al., 1984). This was one of the reasons for a ban, in many countries, on the use of sulfite in meat. Another reason is the preserving effect on the meat color, which makes stale meat look as if it were fresh. Sulfite, however, is unable to reduce metmyoglobin back to myoglobin (Wedzicha and Mountfort, 1991). 

Sulfite reacts with a series of food constituents, e. g., proteins with cleavage of disulfide bonds (cf. 1.4.4.4), with various cofactors like NAD , folic acid, pyridoxal, and thiamine (cf. 6.3.1.3) and with ubiquinone ... 

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