Short-chain fructooligosaccharides (FOS) are prebiotics, which escape digestion in the small intestine and are fermented by the colonic microflora into short-chain fatty acids. Recently, we found that the daily consumption of 20 g FOS decreased basal hepatic glucose production in healthy subjects without any effect on insulin-stimulated glucose metabolism. In this study, we evaluated the effects of the chronic ingestion of FOS on plasma lipid and glucose concentrations, hepatic glucose production and insulin resistance in type 2 diabetics. Type 2 diabetic volunteers (n = 10; 6 men, 4 women) received either 20 g/d FOS or sucrose for 4 wk in a double-blind crossover design. FOS did not modify fasting plasma glucose and insulin concentrations or basal hepatic glucose production. The plasma glucose response to a fixed exogenous insulin bolus did not differ at the end of the two periods. Erythrocyte insulin binding also did not differ. Serum triacylglycerol, total and HDL cholesterol, free fatty acid, apolipoproteins A1 and B and lipoprotein (a) concentrations were not modified by the chronic ingestion of FOS. We conclude that 4 wk of 20 g/d of FOS had no effect on glucose and lipid metabolism in type 2 diabetics.

The kynurenine pathway has long been regarded as a valuable target for the treatment of several neurological disorders accompanied by unbalanced levels of metabolites along the catabolic cascade, kynurenic acid among them. The irreversible transamination of kynurenine is the sole source of kynurenic acid, and it is catalyzed by different isoforms of the 5-pyridoxal phosphate-dependent kynurenine aminotransferase (KAT). The KAT-I isozyme has also been reported to possess ␤-lyase activity toward several sulfur-and selenium-conjugated molecules, leading to the proposal of a role of the enzyme in carcinogenesis associated with environmental pollutants. We solved the structure of human KAT-I in its 5-pyridoxal phosphate and pyridoxamine phosphate forms and in complex with the competing substrate L-Phe. The enzyme active site revealed a striking crown of aromatic residues decorating the ligand binding pocket, which we propose as a major molecular determinant for substrate recognition. Ligand-induced conformational changes affecting Tyr 101 and the Trp 18 -bearing ␣-helix H1 appear to play a central role in catalysis. Our data reveal a key structural role of Glu 27 , providing a molecular basis for the reported loss of enzymatic activity displayed by the equivalent Glu 3 Gly mutation in KAT-I of spontaneously hypertensive rats.In mammals, the kynurenine pathway is the main route for the degradation of tryptophan exceeding anabolic needs and represents the source for de novo NAD biosynthesis. Several metabolites along this pathway, collectively indicated as kynurenines, act as potent neuroactive compounds (1) exerting their function by either inducing free radicals generation (2) or engaging ionotropic excitatory amino acids receptors in the central nervous system (3). The amino acid L-kynurenine (L-Kyn) 1 is a key metabolite along this pathway, standing at the central branching point of the metabolic cascade (4). Indeed, L-Kyn undergoes different fates: i) it can be transformed to either 3-hydroxy-DL-kynurenine or anthranilic acid and conveyed into the flux of freshly synthesized NAD, or ii) it can be used for the synthesis of kynurenic acid (KA). A major role in the central nervous system is ascribed to KA. In fact, it represents the only known endogenous antagonist of the excitatory action of excitatory amino acids, showing the highest affinities for the glycine modulatory site of the Nmethyl-D-aspartate subtype of glutamate receptor (5-7) and the ␣7-nicotinic acetylcholine receptor (8 -10). Its inhibitory action underlies its neuroprotective and neurolectic properties; indeed, low endogenous brain KA level profoundly influences vulnerability to excitotoxic attacks (11)(12)(13)(14). On the other hand, a KA increase significantly correlates with schizophrenia (15) and cognitive impairment (16 -18) suggesting an additional role of KA in the pathophysiology of psychiatric disorders and mental retardation. The KA requirement in the central nervous system is satisfied by the in situ irreversible transamination of L-kynu...

Kynurenic acid is an endogenous neuroactive compound whose unbalancing is involved in the pathogenesis and progression of several neurological diseases. Kynurenic acid synthesis in the human brain is sustained by the catalytic activity of two kynurenine aminotransferases, hKAT I and hKAT II. A wealth of pharmacological data highlight hKAT II as a sensible target for the treatment of neuropathological conditions characterized by a kynurenic acid excess, such as schizophrenia and cognitive impairment. We have solved the structure of human KAT II by means of the single-wavelength anomalous dispersion method at 2.3-Å resolution. Although closely resembling the classical aminotransferase fold, the hKAT II architecture displays unique features. Structural comparison with a prototypical aspartate aminotransferase reveals a novel antiparallel strand-loopstrand motif that forms an unprecedented intersubunit ␤-sheet in the functional hKAT II dimer. Moreover, the N-terminal regions of hKAT II and aspartate aminotransferase appear to have converged to highly similar although 2-fold symmetry-related conformations, which fulfill the same functional role. A detailed structural comparison of hKAT I and hKAT II reveals a larger and more aliphatic character to the active site of hKAT II due to the absence of the aromatic cage involved in ligand binding in hKAT I. The observed structural differences could be exploited for the rational design of highly selective hKAT II inhibitors.Kynurenic acid (KYNA) 3 is one of the neuroactive metabolites of the kynurenine pathway, the main route of oxidative tryptophan degradation in most living organisms (1). At concentrations recorded in the mammalian brain, KYNA antagonizes both the ␣7 nicotinic acetylcholine receptor (␣7-nAChR) and the glycine co-agonist site of N-methyl-D-aspartate (NMDA) receptor, suggesting possible functions in brain physiology (2-5). Notably, given the critical role played by ␣7-nAChR and NMDA receptors in the brain, abnormal KYNA disposition may contribute to the pathogenesis and progression of neurological or psychiatric diseases that are associated with impaired cholinergic and/or glutamatergic neurotransmission (6). Indeed, reductions in endogenous brain KYNA lead to augmented neuronal vulnerability to NMDA receptor-mediated excitotoxic insults (7), whereas pharmacologically induced increases in KYNA provide neuronal protection against ischemic damage and have anticonvulsant effects (8, 9). Neurochemical studies show that KYNA-induced inhibition of ␣7-nAChRs causes a reduction in glutamate release and, secondarily, a decrease in extracellular dopamine levels (10). Inhibition of KYNA formation, on the other hand, results in an elevation in striatal dopamine levels, indicating a bi-directional modulation of dopaminergic neurotransmission by KYNA (11,12). Taken together, these and other supportive data from animals and humans (13-16) suggest that KYNA may play a pathophysiologically significant role in the onset and progression of catastrophic brain diseases that ar...

Alkylated DNA-protein alkyltransferases repair alkylated DNA bases, which are among the most common DNA lesions, and are evolutionary conserved, from prokaryotes to higher eukaryotes. The human ortholog, hAGT, is involved in resistance to alkylating chemotherapy drugs. We report here on the alkylated DNA-protein alkyltransferase, SsOGT, from an archaeal species living at high temperature, a condition that enhances the harmful effect of DNA alkylation. The exceptionally high stability of SsOGT gave us the unique opportunity to perform structural and biochemical analysis of a protein of this class in its post-reaction form. This analysis, along with those performed on SsOGT in its ligand-free and DNA-bound forms, provides insights in the structure-function relationships of the protein before, during and after DNA repair, suggesting a molecular basis for DNA recognition, catalytic activity and protein post-reaction fate, and giving hints on the mechanism of alkylation-induced inactivation of this class of proteins.