The effect of thiamin on thiamin pyrophosphate-dependent 2-oxoglutarate (2-OG) decarboxylase activity in Euglena gracilis was investigated. The total activity of 2-OG decarboxylase in thiamin-sufficient cells in 3 times that in thiamin-deficient cells. The addition of thiamin to thiamin-deficient cells causes the total enzyme and holoenzyme activities to increase and reach similar levels to that in thiamin-sufficient cells. Cycloheximide and chloramphenicol, inhibitors of protein synthesis, have no effect on the total enzyme activity. Immunochemical titration and determination of 2-OG decarboxylase mRNA by using an antibody directed against Euglena 2-OG decarboxylase indicate that the increase in the holoenzyme activity of 2-OG decarboxylase is due to activation of pre-existing protein and does not require synthesis of new proteins in thiamin-deficient cells. During the period of the increase in the total activity, the apoenzyme increases and reaches a temporary peak in 2 h. Immunoblot analysis demonstrates that the precursor form (a 65 kDa subunit) of 2-OG decarboxylase in thiamin-deficient cells is more abundant than that in thiamin-sufficient cells and the increase in the apoenzyme by addition of thiamin results from the conversion of the precursor form into the mature form (a 62 kDa subunit).

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