Mitochondrial enzyme dysfunction in Alzheimer's disease: a systematic review of human metabolic evidence.

Alzheimer's disease (AD) is characterized by progressive cognitive decline accompanied by profound disturbances in cerebral energy metabolism. Mitochondrial dysfunction has long been implicated in AD pathophysiology; however, the specific contribution of mitochondrial enzymes in human disease remains fragmented across heterogeneous studies. Enzymes regulating carbon entry into the tricarboxylic acid cycle, oxidative phosphorylation, and redox balance represent key metabolic control points whose dysfunction may contribute to neuronal vulnerability. To systematically synthesize human evidence on mitochondrial enzyme alterations in Alzheimer's disease and to evaluate the feasibility of quantitative meta-analysis based on current reporting practices. A systematic literature search was conducted in PubMed, Scopus, and Web of Science from database inception through January 2026 in accordance with PRISMA 2020 guidelines. Studies were included if they investigated mitochondrial enzymes in human postmortem brain tissue, human-derived cellular models, or peripheral biospecimens. Risk of bias was assessed using the ROBINS-I tool. The feasibility of meta-analysis was evaluated based on the availability and comparability of group-level summary statistics. Fifteen studies met the eligibility criteria and were included in the final synthesis. Mitochondrial enzymes involved in carbon entry into the tricarboxylic acid cycle, oxidative phosphorylation, redox regulation, and neurotransmitter-linked mitochondrial metabolism were the most frequently investigated targets. Direct enzyme-activity evidence most consistently implicated selected metabolic control points, particularly PDHC and αKGDHC, whereas additional studies supported mitochondrial impairment through protein or post-translational modification changes, respiratory dysfunction, redox alterations, or RNA-regulatory mechanisms. Quantitative meta-analysis was not feasible due to heterogeneous assay methodologies, variable normalization strategies, and inconsistent reporting of group-level summary statistics. Human evidence consistently implicates mitochondrial enzyme dysfunction as a central metabolic feature of Alzheimer's disease. However, progress toward cumulative quantitative synthesis remains limited by methodological heterogeneity and incomplete reporting of enzyme activity outcomes. Standardized measurement and reporting of mitochondrial enzyme alterations will be essential to advance mechanistic understanding and enable future meta-analytic integration.