The activity of a key mitochondrial
enzyme, the
alpha-ketoglutarate dehydrogenase complex (KGDHC), declines in the brains of patients with
neurodegenerative diseases such as
Alzheimer's disease, as well as in
thiamine-deficient (TD) animals. The decreased activity often occurs without a reduction in
enzyme protein, which negates the use of immunocytochemistry to study cellular or regional changes in
enzyme activity within the brain. To overcome this limitation, an activity staining method using
nitroblue tetrazolium was developed. The histochemical activity staining was standardized in cultured cells. The assay was linear with time and was highly specific for KGDHC. The dark-blue reaction product (
formazan) formed a pattern that was consistent with mitochondrial localization. Treatment of the cultured cells with both reversible and irreversible inhibitors decreased
formazan production, whereas conventional
enzyme assays on cell lysates only revealed loss of KGDHC activity with irreversible inhibitors. The activity staining was also linear with time and highly specific for KGDHC activity in mouse brain sections. Staining occurred throughout the brain, and discrete neuronal populations exhibited particularly intense staining. The pattern of staining differed markedly from the distribution of KGDHC
protein by immunocytochemistry. Generalized decreases in the intensity of activity staining that occurred in the TD brains compared to controls were comparable with the loss of KGDHC activity by conventional
enzyme assay. Thus, the present study introduces a new histochemical method to measure KGDHC activity at the cellular and regional level, which will be useful to determine changes of in situ
enzyme activity.