Catabolism in vitro of 2-14C-Thiazole-labeled Thiamine by Rat Tissues

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Abstract
The catabolism in vitro of 2-14C-thiazole-labeled thiamine was studied by incubating it with rat liver homogenates. The criterion of thiamine breakdown was the production of 14CO2. The results show that the rat tissues contain an enzyme system capable of causing the release of carbon dioxide from the 2 position of the thiazole moiety of thiamine. The amount of thiamine degraded in this manner is very small, however. Of the total amount of thiamine broken down, about 85% was degraded during the first 4 hours of incubation. Heating the homogenates for varying periods of time caused a considerable decrease in the activity of the enzyme. The amount of thiamine broken down by the liver of stock-fed rats was significantly greater (P < 0.01) than that from the thiamine-deficient or stock-fed animals fasted for 46 hours. The kidney was found to be 90% as active as liver; other tissues had much less activity. Several compounds including ethanol, methanol, thiazole (4-methyl-5-β-hydroxyethyl thiazole), mercaptoethanol and folic acid were found to inhibit the enzyme activity. On an equivalent protein basis, mitochondrial and microsomal fractions had a greater enzyme activity than the nuclear and the supernatant fractions.