Updated project metadata. Tet3 is an Fe2+-dependent enzyme that oxidizes genomic 5-methylcytosine to 5-hydroxymethylcytosine with the help of alpha-ketoglutarate and oxygen. It is the most abundant Tet enzyme in differentiated tissues including brain. Adult brain contains the highest 5-hydroxymethylcytosine levels. How alpha-ketoglutarate is made available for the oxidation of mC in brain cells and how the Tet activity is linked to neural activity are unsolved questions. Our experiments with full mouse brains show that Tet3 interacts in the nucleus directly with selected enzymes of the mitochondrial citric acid cycle. This leads to the formation of isocitrate. Tet3 also interacts with aspartate aminotransferase, which produces oxaloacetate. Although oxaloacetate and isocitrate are biosynthetic alpha-ketoglutarate precursors, they function as inhibitors of Tet3 and are needed to protect the reactive Fe2+ center from degrading DNA. The supply of Tet3 with alpha-ketoglutarate is established by a direct interaction of Tet3 with glutamate dehydrogenase (Glud1), which converts the neurotransmitter glutamate directly into alpha-ketoglutarate. This links Tet3 function to neural activity.