Branched-chain 2-oxoacid transamination increases GABA-shunt metabolism and insulin secretion in isolated islets.
We have previously shown that oxo-4-methylpentanoate promotes islet GABA (gamma-aminobutyric acid) metabolism and stimulates insulin secretion. The main aim of this work was to explore the participation of the transamination of branched-chain 2-oxoacids in these processes with the aid of several inhibitors of this enzyme activity. No correlation was found between the transamination of branched-chain 2-oxoacids in islet homogenates and insulin secretion. However, in vivo transamination rates correlated better with the secretion capacity of the different branched-chain 2-oxoacids. Gabapentin, a specific inhibitor of the cytosolic isoenzyme, showed greater potential to decrease the in vitro transamination rates of oxo-3-methylbutyrate and oxo-3-methylpentanoate than those of oxo-4-methylpentanoate and oxohexanoate; this correlated with its capacity to decrease insulin secretion. 4-Methylvaleric acid very strongly inhibited the transamination of all the branched-chain 2-oxoacids and blocked their capacity to decrease islet GABA and to stimulate insulin secretion. KCl at 70 mM at stimulated islet GABA release, subsequently decreasing its tissue concentration. This 'non-metabolic' decrease of GABA suppressed the second phase of insulin secretion triggered by oxo-4-methylpentanoate and oxohexanoate. Oxo-4-methylpentanoate and oxo-3-methylpentanoate suppressed dose-dependent 2-oxoglutarate dehydrogenase activity in islet homogenates. In conclusion, the transamination of branchedchain 2-oxoacids is more important to the stimulation of insulin secretion than their catabolism, and transamination decreases islet GABA concentrations by promoting GABA metabolism. Also, inhibition of 2-oxoglutarate dehydrogenase by branched-chain 2-oxoacids may increase metabolic flux in the 'GABA-shunt' at the expense of reduced tricarboxylic acid cycle flux.
Pizarro-Delgado
Javier
J
Department of Biochemistry, Medical School, Complutense University, Madrid 28040, Spain.
Hernández-Fisac
Inés
I
Martín-Del-Río
Rafael
R
Tamarit-Rodriguez
Jorge
J
D000612:4-Aminobutyrate Transaminase / Q000037:antagonists & inhibitors / Q000378:metabolism; D000588:Amines / Q000494:pharmacology; D000818:Animals; D003509:Cyclohexanecarboxylic Acids / Q000494:pharmacology; D000077206:Gabapentin; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000187:drug effects / Q000378:metabolism; D008297:Male; D008954:Models, Biological; D011189:Potassium Chloride / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D005680:gamma-Aminobutyric Acid / Q000378:metabolism* / Q000494:pharmacology
D000588:Amines; D003509:Cyclohexanecarboxylic Acids; D007328:Insulin; D005680:gamma-Aminobutyric Acid; D011189:Potassium Chloride; D000077206:Gabapentin; D000612:4-Aminobutyrate Transaminase