Occurrence of thiamin pyrophosphate‐dependent 2‐oxoglutarate decarboxylase in mitochondria of Euglena gracilis

2‐Oxoglutarate decarboxylase which catalyzes the conversion of 2‐oxoglutarate into succinate semialdehyde occurs in mitochondria of Euglena gracilis which lacks a 2‐oxoglutarate dehydrogenase complex. The enzyme reaction required thiamin pyrophosphate, MgCl2, 2‐mercaptoethanol and NADP+ for the maximum activity, and was not affected by pyruvate and oxalacetate. In the reaction, the enzyme consumed 2‐oxoglutarate, evolved CO2 and formed succinate semialdehyde in stoichiometric relationship. The maximum enzyme activity was found at pH 7.0 and 40° C, and K m values for 2‐oxoglutarate and thiamin pyrophosphate were 0.33 and 0.056 mM, respectively. These results indicate that the thiamin pyrophosphate‐dependent Euglena decarboxylase belongs to a new type of decarboxylase to be designated as 2‐oxoglutarate decarboxylase. The probable role of the new decarboxylase in Euglena mitochondria is discussed with regard to the tricarboxylic acid cycle.