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Altered metabolism of the inositol sugars myo-inositol (MI) and D-chiro-inositol is implicated in diabetic complications. In animals, catabolism of MI and D-chiro-inositol depends on the enzyme MI oxygenase (MIOX), which catalyzes the first committed step of the glucuronate-xylulose pathway, and is found almost exclusively in the kidneys. The crystal structure of MIOX, in complex with MI, has been determined by multiwavelength anomalous diffraction methods and refined at 2.0-Å resolution (R ‫؍‬ 0.206, Rfree ‫؍‬ 0.253). The structure reveals a monomeric, single-domain protein with a mostly helical fold that is distantly related to the diverse HD domain superfamily. Five helices form the structural core and provide six ligands (four His and two Asp) for the di-iron center, in which the two iron atoms are bridged by a putative hydroxide ion and one of the Asp ligands, Asp-124. A key loop forms a lid over the MI substrate, which is coordinated in bidentate mode to one iron atom. It is proposed that this mode of iron coordination, and interaction with a key Lys residue, activate MI for bond cleavage. The structure also reveals the basis of substrate specificity and suggests routes for the development of specific MIOX inhibitors.D iabetes mellitus is a chronic disease characterized by hyperglycemia caused by defective action and͞or secretion of insulin. Hyperglycemia leads to complications such as nephropathy, neuropathy, retinopathy, and cataract. There is considerable evidence that both type-1 and type-2 diabetes are associated with altered inositol metabolism, particularly of myo-inositol (MI) and its less abundant epimeric form, D-chiro-inositol (DCI) (1-3). MI is an osmoregulator and a precursor for inositol-based second messengers, and both MI and DCI are known components of endogenous inositol phosphoglycans, which act as insulin mediators (4, 5), leading to the hypothesis that control of inositol levels may have therapeutic potential. Consistent with this hypothesis, administration of inositols, especially DCI and derivatives, lowers blood glucose in diabetes and enhances insulin action (5-7).The enzyme MI oxygenase (MIOX; EC 1.13.99.1) is a key regulator of inositol levels, catalyzing the first committed step in the glucuronate-xylulose pathway (8, 9), the only known pathway for MI catabolism (10). MIOX is almost exclusively expressed in the kidneys, where it is localized to the proximal tubular epithelial cells (11). Importantly, MIOX also acts on DCI (12) and potentially mediates its catabolism as well. MIOX expression is up-regulated in conditions of hyperosmotic stress (13-14), and in db͞db mice, a model for type-2 diabetes, increased MIOX activity correlates with increased hyperglycemia (13).MIOX is a 33-kDa nonheme iron protein that catalyzes the oxidative conversion of MI to D-glucuronic acid (15). This reaction (Fig. 1), in which the bond between C6 and C1 of MI is cleaved, involves a dioxygen-dependent four-electron oxidation that appears to be unique in biological systems (9). Recent reports indicat...

BACKGROUND AND PURPOSE Amylin (Amy) is an important glucoregulatory peptide and AMY receptors are clinical targets for diabetes and obesity. Human (h) AMY receptor subtypes are complexes of the calcitonin (CT) receptor with receptor activity‐modifying proteins (RAMPs); their rodent counterparts have not been characterized. To allow identification of the most clinically relevant receptor subtype, the elucidation of rat (r) AMY receptor pharmacology is necessary. EXPERIMENTAL APPROACH Receptors were transiently transfected into COS‐7 cells and cAMP responses measured in response to different agonists, with or without antagonists. Competition binding experiments were performed to determine rAmy affinity. KEY RESULTS rCT was the most potent agonist of rCT(a) receptors, whereas rAmy was most potent at rAMY1(a) and rAMY3(a) receptors. rAmy bound to these receptors with high affinity. Rat α‐calcitonin gene‐related peptide (CGRP) was equipotent to rAmy at both AMY receptors. Rat adrenomedullin (AM) and rAM2/intermedin activated all three receptors but were most effective at rAMY3(a). AC187, AC413 and sCT8‐32 were potent antagonists at all three receptors. rαCGRP8‐37 displayed selectivity for rAMY receptors over rCT(a) receptors. rAMY8‐37 was a weak antagonist but was more effective at rAMY1(a) than rAMY3(a). CONCLUSIONS AND IMPLICATIONS AMY receptors were generated by co‐expression of rCT(a) with rRAMP1 or 3, forming rAMY1(a) and rAMY3(a) receptors, respectively. CGRP was more potent at rAMY than at hAMY receptors. No antagonist tested was able to differentiate the rAMY receptor subtypes. The data emphasize the need for and provide a useful resource for developing new CT or AMY receptor ligands as pharmacological tools or potential clinical candidates. LINKED ARTICLES This article is part of a themed section on Secretin Family (Class B) G Protein‐Coupled Receptors. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.166.issue-1

The Zucker diabetic fatty (ZDF) rat is a commonly used animal model of type 2 diabetes yet complete descriptions of insulin resistance in this model are limited. We present a full characterisation of in vivo insulin resistance in obese (fa/fa) animals compared to lean (+/?) littermates. Anaesthetised, ten-week old, obese ZDF rats and their lean littermates underwent a hyperinsulinaemic euglycaemic glucose clamp. Compared with lean littermates, obese ZDF rats required an 89% lower glucose infusion rate to maintain euglycaemia and showed a 35% decrease in peripheral glucose disposal. Insulin-stimulated glucose uptake (R(g')) in obese animals was also significantly less in all skeletal muscles studied. R(g') in cardiac and white adipose tissue was not different between the two groups. Total glycogen content in skeletal and cardiac muscle was significantly less in obese animals, while total glycogen content in the liver was significantly greater than in lean littermates. Glycogen synthesis was also decreased in skeletal muscle of obese animals. Compared with lean animals, total triglyceride content was significantly greater in skeletal muscle, heart and liver of obese ZDF rats. Obese animals also showed significantly increased glucose incorporation into lipid in all of these tissues, indicating an increase in lipogenesis. Collectively, these results provide an integrated characterisation of in vivo insulin resistance in obese ZDF rats and a direct comparison with lean littermates.