Thiamin thiol

A small amount of thiamin is excreted in the urine unchanged, accounting for about 3% of a test dose, together with small amounts of thiamin monophosphate and thiamin diphosphate. As discussed in Section 6.5.1, this can be used to assess thiamin nutritional status. One of the major excretory products is thiochrome cyclization to thiochrome is the basis of the normal method of determining thiamin so, most reports of thiamin excretion are actually of thiamin plus thiochrome. In addition, small amounts of thiamin disulfide, formed by the oxidation of thiamin thiol, are also excreted. 

Figure 6.1. Thiamin and thiamin analogs, products of tliiaminolysis, and experimental antimetabolites. Relative molecular masses (M,) 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.

Figure 5.12 Formation of thiamine thiol, thiamine disulfide and thiochrome...

Figure 5.15 Reaction of thiamine with proteins P = protein, T-SH = thiamine thiol, T-S-S-T = thiamine disulfide.

The yellow form (11) on acidification is converted to the more stable thiol form (12). On oxidation, typically with alkaline ferhcyanide, yellow form (11) is irreversibly converted to thiochrome [299-35-4] (14), a yellow crystalline compound found naturally in yeast but with no thiamine activity. In solution, thiochrome exhibits an intense blue fluorescence, a property used for the quantitative determination of thiamine. 

The thiol form (12) is susceptible to oxidation (see Fig. 2). Iodine treatment regenerates thiamine in good yield. Heating an aqueous solution at pH 8 in air gives rise to thiamine disulfide [67-16-3] (21), thiochrome (14), and other products (22). The disulfide is readily reduced to thiamine in vivo and is as biologically active. Other mixed disulfides, of interest as fat-soluble forms, are formed from thiamine, possibly via oxidative coupling to the thiol form (12). 

The weakly basic portion of thiamin or of its coenzyme forms is protonated at low pH, largely on N-l of the pyrimidine ring. 86 88 The pKa value is 4.9. In basic solution, thiamin reacts in two steps with an opening of the thiazole ring (Eq. 14-15) to give the anion of a thiol form which may be crystallized as the sodium salt.79 84 This reaction, like the competing reaction described in Eq. 7-19, and which leads to a yellow... 

Thiamin is unstable at high pH90 91 and is destroyed by the cooking of foods under mildly basic conditions. The thiol form undergoes hydrolysis and oxidation by air to a disulfide. The tricyclic form (Eq. 7-19) is oxidized to thiochrome, a fluorescent compound... 

T4 lysozyme 33,497 helix stability of 528, 529 hydrophobic core stability of 533, 544 Tanford j8 value 544, 555, 578, 582-Temperature jump 137, 138, 541 protein folding 593 Terminal transferase 408,410 Ternary complex 120 Tertiary structure 22 Theorell-Chance mechanism 120 Thermodynamic cycles 125-131 acid denaturation 516,517 alchemical steps 129 double mutant cycles 129-131, 594 mutant cycles 129 specificity 381, 383 Thermolysin 22, 30,483-486 Thiamine pyrophosphate 62, 83 - 84 Thionesters 478 Thiol proteases 473,482 TNfn3 domain O-value analysis 594 folding kinetics 552 Torsion angle 16-18 Tbs-L-phenylalanine chloromethyl ketone (TPCK) 278, 475 Transaldolase 79 Tyransducin-o 315-317 Transit time 123-125 Transition state 47-49 definition 55... 

Tandon, S.K. and Prasad, S., Effect of thiamine on the cadmium-chelating capacity of thiol compounds, Hum. Exp. Toxicol., 19, 523-528, 2000. 

As far as 2-methyl-3-furanthiol is concerned, it can be formed from cysteine, but thiamine constitutes its more important precursor by far.256 When the reaction is carried out in the presence of thiamine in an aqueous medium at 120 °C for 1 h, only about 8% of the thiol is derived from cysteine, and, in the absence of thiamine from the mix, no thiol was detected. The probable mechanism of formation from thiamine is shown in Scheme 5.16.257... 

At the centre is enzyme 2 which binds the acetyl group through a lipoic acid-lysine amide. On the one side this acetyl group is delivered from pyruvate by the ministrations of thiamine pyrophosphate and enzyme T and on the other it is delivered to CoA as the free thiol ester. Enzyme 3 recycles... 

But this thiol ester is not formed by the expected mechanism in the enzymatic reaction. Thiamine delivers a nucleophilic acetyl group to an c/ectrophilic sulfur atom—the reverse polarity to normal ester formation. 

The thiol form (12) undergoes reactions mainly via its /V-formyl or ene—thiol groups. Heating an aqueous solution of the thiol form (Fig. 2) effects hydrolysis to the diamine 4-amino-5-aminomethyl-2-methyl-pyrimidine [95-02-3] (16), 5-hydroxy-3-mercaptopentan-2-one [15678-01 -0] (17), and formic acid (20). Neutralization of a solution of the thiolate anion with carbon dioxide gives a fat-soluble basic material [21682-72-4, 35922-43-1] (20), presumably via dihydrothiochrome (10) (21). Acylation of the thiolate occurs on both sulfur and oxygen to give mono- or diacyl thiamines, some of which are interesting fat-soluble depot forms of thiamine. 

---