Transamination Is Required for α-Ketoisocaproate but Not Leucine to Stimulate Insulin Secretion*

Yingsheng Zhou,Thomas L Jetton,Stephanie Goshorn,Christopher J Lynch,Pengxiang She

doi:10.1074/jbc.m110.136846

Yingsheng Zhou, Thomas L Jetton + Show 3 more

Open Access

https://doi.org/10.1074/jbc.m110.136846

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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.

Highlights

Cose, amino acids, and free fatty acids as well as incretin hormones such as glucagon-like peptide-1
While glutamine by itself does not induce insulin secretion, it has been shown to be readily converted to glutamate by a phosphate-dependent glutaminase in islets, and it is often added to LSIS experiments (28)
We have found that disruption of the first step of branched-chain amino acid metabolism abolished insulin secretion stimulated by branched-chain keto acids (BCKAs) without affecting insulin secretion by glucose, arginine, glucose/arginine, leucine/ glutamine, ␣-KG/glutamine, or KCl

Summary

Introduction

Cose, amino acids, and free fatty acids as well as incretin hormones such as glucagon-like peptide-1. It was later found that KIC was not a very effective substrate for ATP production, and this led to a proposal that, in addition to generation of ␣-KG, metabolites of leucine/KIC such as acetoacetate may induce the insulinotropic effect of KIC (7, 8) Another mechanism involves the discovery in 1980, that leucine is an allosteric activator of glutamate dehydrogenase (GDH), the enzyme that oxidizes glutamate to ␣-KG with concomitant production of NADPH, a potential intracellular signal for insulin secretion (9). Fahien and MacDonald (16) posited that by interacting with succinate, mevalonate produced from leucine catabolism could be a signal for insulin release Both leucine catabolism and activation of GDH could be required for leucine/KIC to stimulate insulin secretion. The same group who proposed the catabolism mechanism later reported that intramitochondrial ␣-KG generation may regulate the insulin secretory potency of leucine and KIC (21)

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