Schematic of whole-body BCAA metabolism.

Ketoacids are formed by reversible transamination catalyzed by the mitochondrial or cytosolic isoforms of branched chain amino acid transaminase (BCAT). The action of the branched chain keto acid dehydrogenase complex (BCKDC) in the mitochondrial matrix leads to the evolution of CO₂ from the 1-carbon of the keto acids including KIC, which was ¹⁴C labeled and measured from the expired air in these studies. Subsequent intramitochondrial metabolism leads to the formation of various acyl-coenzyme A (R-CoA) esters that can reversibly form acylcarnitines (not displayed). Neither FAD and NAD Cofactors nor CO₂ and H₂O substrates are displayed. Bold font indicates metabolites or corresponding acylcarnitines that were detected and measured quantitatively in the 24 h urines (Table 4–5). AA, amino acids.