DIACETYLTHIAMINEのアセチル転移反応について

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DAT (Diacetylthiamine) is characteristic in the property of transfering its S-acetyl group to -SH (such as cysteine or glutathione) or -NH_2 (such as aniline) radical whereas it turns into O-acetylthiamine. This transacetylation occurs very easily at the equimolecular basis if the concentration of DAT is high but it proceeds more and more slowly even in excess of cysteine or aniline if the concentration decreases to that comparable to thiamine contents of natural biological materials. Large excess of cysteine is required to complete transacetylation of DAT in its 0.1-1 mg % (0.0025-0.025mM) solution. For example, 0.026 mM DAT solution with 0.064 mM cysteine at pH 7 transfers only 11% of the total S-acetyl group after 24 hrs., while 2.6 mM DAT solution with 3.2 mM cysteine completes transacetylation after 24 hrs. Excess of cysteine such as 2.6 mM is necessary to complete transacetylation of 0.026 mM DAT solution within 24 hrs. The same is true in the transacetylation of DAT by aniline but aniline is less effective to accept the acetyl group of DAT : 2.5 mM DAT solution with 8.6 mM aniline at pH 6 transfers only 15% of the S-acetyl group after 18 days. Human dry plasma (in 2% solution with phosphate buffer) is very strong to convert DAT into O-acetylthiamine ; 100% conversion in 3 hrs. and in the case of 0.2% plasma solution 44-46% conversion in 3 hrs. for 0.0025-0.025 mM DAT solution. Esterase preparation from horse liver hydrolyses DAT into thiamine and in the presence of cysteine hydrolysis of DAT is greatly accelerated.
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DIACETYLTHIAMINEのアセチル転移反応について

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