Elevations in
branched-chain amino acids (BCAAs) in human
obesity were first reported in the 1960s. Such reports are of interest because of the emerging role of BCAAs as potential regulators of satiety,
leptin,
glucose, cell signaling, adiposity, and
body weight (mTOR and PKC). To explore loss of catabolic capacity as a potential contributor to the
obesity-related rises in BCAAs, we assessed the first two enzymatic steps, catalyzed by mitochondrial
branched chain amino acid aminotransferase (BCATm) or the branched chain
alpha-keto acid dehydrogenase (BCKD E1alpha subunit) complex, in two rodent models of
obesity (ob/ob mice and Zucker rats) and after surgical
weight loss intervention in humans. Obese rodents exhibited hyperaminoacidemia including BCAAs. Whereas no
obesity-related changes were observed in rodent skeletal muscle BCATm, pS293, or total BCKD E1alpha or BCKD
kinase, in liver BCKD E1alpha was either unaltered or diminished by
obesity, and pS293 (associated with the inactive state of BCKD) increased, along with BCKD
kinase. In epididymal fat,
obesity-related declines were observed in BCATm and BCKD E1alpha. Plasma BCAAs were diminished by an overnight fast coinciding with dissipation of the changes in adipose tissue but not in liver. BCAAs also were reduced by surgical
weight loss intervention (
Roux-en-Y gastric bypass) in human subjects studied longitudinally. These changes coincided with increased BCATm and BCKD E1alpha in omental and subcutaneous fat. Our results are consistent with the idea that tissue-specific alterations in BCAA metabolism, in liver and adipose tissue but not in muscle, may contribute to the rise in plasma BCAAs in
obesity.