Recent findings have demonstrated that the branchedchain amino acid leucine can activate the translational regulators, phosphorylated heat-and acid-stable protein regulated by insulin (PHAS-I) and p70 S6 kinase (p70 s6k ), in an insulin-independent and rapamycin-sensitive manner through mammalian target of rapamycin (mTOR), although the mechanism for this activation is undefined. It has been previously established that leucine-induced insulin secretion by ␤-cells involves increased mitochondrial metabolism by oxidative decarboxylation and allosteric activation of glutamate dehydrogenase (GDH). We now show that these same intramitochondrial events that generate signals for leucine-induced insulin exocytosis are required to activate the mTOR mitogenic signaling pathway by ␤-cells. Thus, a minimal model consisting of leucine and glutamine as substrates for oxidative decarboxylation and an activator of GDH, respectively, confirmed the requirement for these two metabolic components and mimicked closely the synergistic interactions achieved by a complete complement of amino acids to activate p70 s6k in a rapamycinsensitive manner. Studies using various leucine analogs also confirmed the close association of mitochondrial metabolism and the ability of leucine analogs to activate p70 s6k . Furthermore, selective inhibitors of mitochondrial function blocked this activation in a reversible manner, which was not associated with a global reduction in ATP levels. These findings indicate that leucine at physiological concentrations stimulates p70 s6k phosphorylation via the mTOR pathway, in part, by serving both as a mitochondrial fuel and an allosteric activator of GDH. Leucine-mediated activation of protein translation through mTOR may contribute to enhanced ␤-cell function by stimulating growth-related protein synthesis and proliferation associated with the maintenance of ␤-cell mass. Diabetes 50:353-360, 2001

Lipoprotein lipase (LpL) provides tissues with triglyceride-derived fatty acids. Fatty acids affect ␤-cell function, and LpL overexpression decreases insulin secretion in cell lines, but whether LpL is regulated in ␤-cells is unknown. To test the hypothesis that glucose and insulin regulate LpL activity in ␤-cells, we studied pancreatic islets and INS-1 cells. Acute exposure of ␤-cells to physiological concentrations of glucose stimulated both total cellular LpL activity and heparin-releasable LpL activity. Glucose had no effect on total LpL protein mass but instead promoted the appearance of LpL protein in a heparin-releasable fraction, suggesting that glucose stimulates the translocation of LpL from intracellular to extracellular sites in ␤-cells. The induction of heparinreleasable LpL activity was unaffected by treatment with diazoxide, an inhibitor of insulin exocytosis that does not alter glucose metabolism but was blocked by conditions that inhibit glucose metabolism. In vitro hyperinsulinemia had no effect on LpL activity in the presence of low concentrations of glucose but increased LpL activity in the presence of 20 mM glucose. Using dual-laser confocal microscopy, we detected intracellular LpL in vesicles distinct from those containing insulin. LpL was also detected at the cell surface and was displaced from this site by heparin in dispersed islets and INS-1 cells. These results show that glucose metabolism controls the trafficking of LpL activity in ␤-cells independent of insulin secretion. They suggest that hyperglycemia and hyperinsulinemia associated with insulin resistance may contribute to progressive ␤-cell dysfunction by increasing LpL-mediated delivery of lipid to islets.

Aldehyde dehydrogenases are versatile enzymes that serve a range of biochemical functions.  Although traditionally considered metabolic housekeeping enzymes because of their ability to detoxify reactive aldehydes, like those generated from lipid peroxidation damage, the contributions of these enzymes to other biological processes are widespread.  For example, the plant pathogen Pseudomonas syringae strain PtoDC3000 uses an indole-3-acetaldehyde dehydrogenase to synthesize the phytohormone indole-3-acetic acid to elude host responses. Here we investigate the biochemical function of AldC from PtoDC3000.  Analysis of the substrate profile of AldC suggests that this enzyme functions as a long-chain aliphatic aldehyde dehydrogenase.  The 2.5 Å resolution x-ray crystal of the AldC C291A mutant in a dead-end complex with octanal and NAD+ reveals an apolar binding site primed for aliphatic aldehyde substrate recognition.  Functional characterization of site-directed mutants targeting the substrate and NAD(H) binding sites identify key residues in the active site for ligand interactions, including those in the 'aromatic box' that define the aldehyde binding site.  Overall, this study provides molecular insight for understanding the evolution of the prokaryotic aldehyde dehydrogenase superfamily and their diversity of function.

The purpose of this study was to determine if the acute alveolar injury induced by subcutaneous injections of N-nitroso- N-methylurethane (NNMU) in rats is mediated by nitric oxide (NO ⋅). We show that intraperitoneal injections of the NO ⋅ synthase (NOS) inhibitor N ω-nitro-l-arginine methyl ester (l-NAME) or aminoguanidine significantly attenuate the NNMU-induced alveolar injury as assessed by 1) normalization of the alveolar-arterial O2difference, 2) attenuation of the lowered phospholipid-to-protein ratio in the crude surfactant pellet (CSP), 3) attenuation of the elevated minimal surface tension of the CSP, and 4) attenuation of polymorphonuclear neutrophilic infiltration into the alveolar space. Injections of N ω-nitro-d-arginine methyl ester, the inactive stereoisoform ofl-NAME, did not affect the acute lung injury. Western blot analysis of whole lung homogenates demonstrate an elevated expression of transcriptionally inducible, Ca2+-independent NOS (iNOS) in NNMU-injected rats compared with control saline-injected rats. NOS inhibitors did not affect NNMU-induced iNOS expression. These investigations demonstrate that the inhibition of NOS attenuates NNMU-induced acute lung injury, suggesting a role for NO ⋅ in the progression of acute respiratory distress syndrome.

Aldehyde dehydrogenases (ALDH) catalyze the conversion of various aliphatic and aromatic aldehydes to corresponding carboxylic acids.  Traditionally considered as housekeeping enzymes, new biochemical roles are being identified for members of ALDH family.  Recent work showed that AldA from the plant pathogen Pseudomonas syringae strain PtoDC3000 functions as an indole-3-acetaldehyde dehydrogenase for the synthesis of indole-3-acetic acid (IAA).  IAA produced by AldA allows the pathogen to suppress salicylic acid-mediated defenses in the model plant Arabidopsis thaliana.  Here we present a biochemical and structural analysis of the AldA indole-3-acetaldehyde dehydrogenase from PtoDC3000.  Site-directed mutants targeting the catalytic residues Cys302 and Glu267 resulted in a loss of enzymatic activity.  The x-ray crystal structure of the catalytically inactive AldA C302A mutant in complex with IAA and NAD+ showed the cofactor adopting a conformation that differs from the previously reported structure of AldA.  These structures suggest that NAD+ undergoes a conformational change during the AldA reaction mechanism similar to that reported for human ALDH.  Site-directed mutagenesis of the IAA binding site indicates that changes in the active site surface reduces AldA activity; however, substitution of Phe169 with a tryptophan altered the substrate selectivity of the mutant to prefer octanal.  This study highlights the inherent biochemical versatility of members of the ALDH enzyme superfamily in P. syringae.