title: Transamination is required for {alpha}-ketoisocaproate but not leucine to stimulate insulin secretion.
abstract: It remains unclear how α-ketoisocaproate (KIC) and leucine are metabolized to stimulate insulin secretion. Mitochondrial BCATm (branched-chain aminotransferase) catalyzes reversible transamination of leucine and α-ketoglutarate to KIC and glutamate, the first step of leucine catabolism. We investigated the biochemical mechanisms of KIC and leucine-stimulated insulin secretion (KICSIS and LSIS, respectively) using BCATm(-/-) mice. In static incubation, BCATm disruption abolished insulin secretion by KIC, D,L-α-keto-β-methylvalerate, and α-ketocaproate without altering stimulation by glucose, leucine, or α-ketoglutarate. Similarly, during pancreas perfusions in BCATm(-/-) mice, glucose and arginine stimulated insulin release, whereas KICSIS was largely abolished. During islet perifusions, KIC and 2 mM glutamine caused robust dose-dependent insulin secretion in BCATm(+/+) not BCATm(-/-) islets, whereas LSIS was unaffected. Consistently, in contrast to BCATm(+/+) islets, the increases of the ATP concentration and NADPH/NADP(+) ratio in response to KIC were largely blunted in BCATm(-/-) islets. Compared with nontreated islets, the combination of KIC/glutamine (10/2 mM) did not influence α-ketoglutarate concentrations but caused 120 and 33% increases in malate in BCATm(+/+) and BCATm(-/-) islets, respectively. Although leucine oxidation and KIC transamination were blocked in BCATm(-/-) islets, KIC oxidation was unaltered. These data indicate that KICSIS requires transamination of KIC and glutamate to leucine and α-ketoglutarate, respectively. LSIS does not require leucine catabolism and may be through leucine activation of glutamate dehydrogenase. Thus, KICSIS and LSIS occur by enhancing the metabolism of glutamine/glutamate to α-ketoglutarate, which, in turn, is metabolized to produce the intracellular signals such as ATP and NADPH for insulin secretion.
authors.affiliation: Department of Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, Hershey, Pennsylvania 17033, USA.
authors.forename: Yingsheng
authors.initials: Y
authors.lastname: Zhou
authors.forename: Thomas L
authors.initials: TL
authors.lastname: Jetton
authors.forename: Stephanie
authors.initials: S
authors.lastname: Goshorn
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Lynch
authors.forename: Pengxiang
authors.initials: P
authors.lastname: She
mesh_terms: D000255:Adenosine Triphosphate / Q000737:chemistry; D000818:Animals; D005260:Female; D005947:Glucose / Q000737:chemistry / Q000378:metabolism; D005973:Glutamine / Q000737:chemistry; D007328:Insulin / Q000378:metabolism; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000166:cytology; D007651:Keto Acids / Q000737:chemistry*; D007656:Ketoglutaric Acids / Q000737:chemistry; D007930:Leucine / Q000737:chemistry*; D051379:Mice; D008822:Mice, Transgenic; D008928:Mitochondria / Q000201:enzymology*; D010100:Oxygen / Q000737:chemistry; D000637:Transaminases / Q000235:genetics* / Q000378:metabolism
chemical_list: D007328:Insulin; D007651:Keto Acids; D007656:Ketoglutaric Acids; D005973:Glutamine; C013082:alpha-ketoisocaproic acid; D000255:Adenosine Triphosphate; D000637:Transaminases; C019798:branched-chain-amino-acid transaminase; D007930:Leucine; D005947:Glucose; D010100:Oxygen