Novel functions of the alpha-ketoglutarate dehydrogenase complex may mediate diverse oxidant-induced changes in mitochondrial enzymes associated with Alzheimer's disease.

Measures in autopsied brains from Alzheimer's Disease (AD) patients reveal a decrease in the activity of alpha-ketoglutarate dehydrogenase complex (KGDHC) and an increase in malate dehydrogenase (MDH) activity. The present experiments tested whether both changes could be caused by the common oxidant H(2)O(2) and to probe the mechanism underlying these changes. Since the response to H(2)O(2) is modified by the level of the E2k subunit of KGDHC, the interaction of MDH and KGDHC was studied in cells with varying levels of E2k. In cells with only 23% of normal E2k protein levels, one-hour treatment with H(2)O(2) decreased KGDHC and increased MDH activity as well as the mRNA level for both cytosolic and mitochondrial MDH. The increase in MDH did not occur in cells with 100% or 46% of normal E2k. Longer treatments with H(2)O(2) inhibited the activity of both enzymes. Glutathione is a major regulator of cellular redox state and can modify enzyme activities. H(2)O(2) converts reduced glutathione (GSH) to oxidized glutathione (GSSG), which reacts with protein thiols. Treatment of purified KGDHC with GSSG leads to glutathionylation of all three KGDHC subunits. Thus, cellular glutathione level was manipulated by two means to determine the effect on KGDHC and MDH activities. Both buthionine sulfoximine (BSO), which inhibits glutathione synthesis without altering redox state, and H(2)O(2) diminished glutathione to a similar level after 24 h. However, H(2)O(2), but not BSO, reduced KGDHC and MDH activities, and the reduction was greater in the E2k-23 line. These findings suggest that the E2k may mediate diverse responses of KGDHC and MDH to oxidants. In addition, the differential response of activities to BSO and H(2)O(2) together with the in vitro interaction of KGDHC with GSSG suggests that glutathionylation is one possible mechanism underlying oxidative stress-induced inhibition of the TCA cycle enzymes.

Protein glutathiolation in human blood.

Glutathione (GSH) exists in both free and protein-bound (glutathiolated) forms (GSSP). Protein glutathiolation may represent an important post-translational regulatory mechanism for proteins. However, there are little data regarding the regulation of glutathiolation in blood. Our objectives were to examine GSSP levels of human blood by determining the distribution and variability of blood GSSP, as well as its relationship to free GSH and hemoglobin in healthy adults. To this end, we used a newly modified method allowing for rapid analysis of both GSH and GSSP in blood. GSSP was found in red cells with levels ranging from 4 to 27% of total (free+bound) GSH (mean+/-SD: 12.1+/-4.5%) with a concentration of 0.13+/-0.05 microEq GSH/mL (mean+/-SD). No correlations were observed between GSSP and either GSH (r=-0.085) or hemoglobin (r=0.086). Together these results suggest that the extent of protein glutathiolation in blood is substantial ( approximately 0.1 mmol/L). While the interindividual variation in GSSP is large (34%), its levels are apparently not regulated by GSH content.