Leucine and Protein Metabolism in Obese Zucker Rats

Abstract

Branched-chain amino acids (BCAAs) are circulating nutrient signals for protein accretion, however, they increase in obesity and elevations appear to be prognostic of diabetes. To understand the mechanisms whereby obesity affects BCAAs and protein metabolism, we employed metabolomics and measured rates of [1-14C]-leucine metabolism, tissue-specific protein synthesis and branched-chain keto-acid (BCKA) dehydrogenase complex (BCKDC) activities. Male obese Zucker rats (11-weeks old) had increased body weight (BW, 53%), liver (107%) and fat (∼300%), but lower plantaris and gastrocnemius masses (−21–24%). Plasma BCAAs and BCKAs were elevated 45–69% and ∼100%, respectively, in obese rats. Processes facilitating these rises appeared to include increased dietary intake (23%), leucine (Leu) turnover and proteolysis [35% per g fat free mass (FFM), urinary markers of proteolysis: 3-methylhistidine (183%) and 4-hydroxyproline (766%)] and decreased BCKDC per g kidney, heart, gastrocnemius and liver (−47–66%). A process disposing of circulating BCAAs, protein synthesis, was increased 23–29% by obesity in whole-body (FFM corrected), gastrocnemius and liver. Despite the observed decreases in BCKDC activities per gm tissue, rates of whole-body Leu oxidation in obese rats were 22% and 59% higher normalized to BW and FFM, respectively. Consistently, urinary concentrations of eight BCAA catabolism-derived acylcarnitines were also elevated. The unexpected increase in BCAA oxidation may be due to a substrate effect in liver. Supporting this idea, BCKAs were elevated more in liver (193–418%) than plasma or muscle, and per g losses of hepatic BCKDC activities were completely offset by increased liver mass, in contrast to other tissues. In summary, our results indicate that plasma BCKAs may represent a more sensitive metabolic signature for obesity than BCAAs. Processes supporting elevated BCAA]BCKAs in the obese Zucker rat include increased dietary intake, Leu and protein turnover along with impaired BCKDC activity. Elevated BCAAs/BCKAs may contribute to observed elevations in protein synthesis and BCAA oxidation.

Genetic evidence of a causal effect of insulin resistance on branched-chain amino acid levels

“…Insulin is the chief regulator of amino acid metabolism and changes in fasting BCAA levels may be regulated by the effect of insulin on the rate of appearance and clearance of BCAA together with a decreased activity of catabolic enzymes [2]. Loss-of-function mutations in PPM1K in humans [21], and disruption of key BCAA metabolism in obese mice and Zucker rats, exhibit a reduced expression of the mitochondrial isoform of branched chain-amino-acid transaminase (BCAT2; which catalyses the first and reversible step in BCAA catabolism), and mitochondrial branched chain α-keto acid dehydrogenase (BCKD) complex E1-α (which catalyses the rate-controlling and the first irreversible step), leading to increased plasma BCAA levels [22,23]. A decreased BCAA metabolism in fat tissue may contribute to higher BCAA levels in individuals with insulin-resistant obesity [2,[22][23][24].…”

General Control Nonderepressible 2 (GCN2) Kinase Protects Oligodendrocytes and White Matter during Branched-chain Amino Acid Deficiency in Mice

“…Concentrations of BCAA and total amino acids in whole body were measured using high performance liquid chromatography (HPLC) as described previously (14). Concentrations of amino acids in whole brain were analyzed by Kristine C. Olson under the supervision of Dr. Christopher J. Lynch (15). Free amino acids were pre-column-derivatized using Phenomenex EZ-fast reagent and analyzed by ultra performance liquid chromatography-MS using a Waters Synapt HDMS hybrid quantitative TOF with ion mobility, located at the Pennsylvania State College of Medicine Macromolecular Core Facility.…”

Time-course changes in circulating branched-chain amino acid levels and metabolism in obese Yucatan minipig

“…Both gene expression and protein concentration of enzymes involved in BCAA catabolism have been shown repeatedly to be blunted on VAT of obese individuals [21] and animal models [7,22], at least those genetically modified. In the present study, the BCAA transamination capacity of VAT and SCAT increased significantly with the obesity phenotype induced by the HFHS.…”