Effect of Leucine on the Pyridine Nucleotide Contents of Islets and on the Insulin Released—Interactions In Vitro with Methylene Blue, Thiol Oxidants, and p-Chloromercuribenzoate

Abstract

In the presence of glucose (2 mg/ml), leucine (10 mM) noticeably increased islets' NADPH contents as well as the NADPH:NADP ratio; the changes occurred as soon as 1 min after its addition. NADH concentrations were also increased by leucine. The NADPH:NADP ratio as well as insulin release stimulated by glucose plus leucine were markedly decreased by methylene blue. The thiol oxidants diamide and tert-butyl hydroperoxide also inhibited insulin secretion in response to glucose plus leucine. Employing the perfused pancreas technique, the insulin-releasing action of p-chloromercuribenzoate was further enhanced by leucine. The combined effects were inhibited by tert-butyl hydroperoxide, however. Our data suggest that the insulin-releasing action of leucine depends on the islets' NADPH and reduced glutathione (GSH); in addition, leucine may contribute to insulin secretion by increasing the islet NADPH:NADP ratio and the NADH:NAD ratio. From the data, we assume that the observed increase of NADPH may lead via GSH to an increase in the number of such thiol groups in the beta-cell membrane, which are believed to be related to stimulation of insulin release and, thus, to increase the sensitivity of the beta-cell to stimulation by glucose and/or leucine.

Overcoming the spatial barriers of the stimulus secretion cascade in pancreatic β-cells

“…The involvement of GRX1 in the stimulus-secretion cascade was demonstrated by a 40% increase in glucosestimulated insulin output after GRX1 overexpression in INS-1 and rat islets, whereas siRNA-mediated knockdown of GRX1 renders INS-1 832/13 cells insensitive to the stimulatory actions of NADPH. 103,104 Cytosolic NADPH levels rise rapidly in response to several metabolic fuels, such as glucose or leucine 106,107 and when the cytosolic rise in the NADPH/NADP + ratio is blunted by the addition of NADP + or menadione, a poor glucose responsive state is attained. 104,108,109 Most of the glucose-induced rise in NADPH levels originates from the cytosolic isoforms of malic enzyme (MEc) and isocitrate dehydrogenase (ICDCc), which are operative in the pyruvate-malate, pyruvate-citrate and pyruvate-isocitrate pathway 14 (Fig.…”

Effect of Leucine on GSH Levels of Rat Pancreatic Islets

“…Two previous observations suggested that the insulin releasing effect of leucine not only depends on the level of NADPH but rather on a shift of the redox state of the NADPH/NADP couple towards the reduced side. These are: a) a decrease of the NADPH/NADP ratio by methylene blue, an oxidant of NADPH, is associated with inhibition of the leucine-induced insulin release {Ammon, Hoppe, Akhtar and Niklas 1979) and b) an increase of the NADPH/NADP ratio by leucine in the presence of 5.6 mM and 11.1 mM of glucose {Ammon, Hoppe, Akhtar and Niklas 1979;Hoppe 1978) is associated with stimulation of insulin release.…”

“…From the data shown in Table 1 it further appears that the effect of leucine depends on the presence of glucose. Employing thiol oxidants, including diamide and t-butyl hydroperoxide, which decrease islet GSH/GSSG ratios, the insulin releasing effect of leucine was shown to be inhibited (Ammon, Hoppe, Akhtar and Niklas 1979) suggesting that the insulin releasing capacity of leucine depends on the GSH/GSSG ratio. According to the data, presented in this study, it might be possible, that leucine at least in part contributes to the stimulation of insulin release by increasing islet levels of GSH via elevation of the NADPH/NADP ratio.…”

Branched‐Chain Amino Acid–Enriched Diet: Effects on Insulin Secretion and Cellular Immune Aggression

“…The addition of leucine in long‐term isolated islet cultures was reported to exert a beneficial effect on cell function, especially insulin biosynthesis (40) and to increase NADPH:NADP and NADH:NAD ratios in islets, augmenting the defense mechanism against oxidative damage (41).…”