To test whether previously demonstrated reductions in the activity of the alpha-ketoglutarate dehydrogenase complex (KGDHC) in Alzheimer's disease (AD) brain also occur in morphologically normal AD tissues, we examined KGDHC in cultured skin fibroblasts from patients with familial AD (FAD). KGDHC activity was reduced by 44% in the FAD cells (p < 0.002) from the 4 families studied, including a within-kindred comparison of affected and escapee subjects in the chromosome 14q24.3-linked Nova Scotia kindred. The activities of several other glutamate- and glutamine-metabolizing enzymes were normal in the FAD cells, as was the activity of another mitochondrial multienzyme dehydrogenase complex, that for pyruvate. Mixing experiments indicated that the abnormality of KGDHC activity in FAD fibroblasts was not due to an inhibitor or to excess protease activity. KGDHC is a complex of three proteins. Immunoblots for the E2k component under conditions of optimal protease inhibition revealed the expected 46-kd species in both AD and non-AD fibroblasts, but the patient cells also regularly contained an additional 29-kd species that was absent or present in minimal amounts in the controls. Immunoblotting demonstrated no abnormalities in the E1k and E3 components. Other studies indicate that the human gene for E2k residues on chromosome 14q24.3, in a region associated with FAD in a number of families including the KGDHC-deficient Nova Scotia kindred. The persistence of abnormalities in KGDHC and particularly in its E2k component in FAD fibroblasts indicates that abnormalities of this autosomally coded nuclear component are an intrinsic part of the AD process, and the possible role of this abnormality in the pathogenesis of AD is discussed.