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Uric acid (1a) suppresses basal insulin in isolated rat pancreatic islets and inhibition of glucose-stimulated insulin secretion (GSIS) occurs right at hyperuricaemic levels (greater than or equal to 0.4mM). Conversely, 1mMquanidinium urate (2a) was completely ineffective, strongly suggesting that binding to an essential arginine residue triggers the inhibitory effect. A specific recognition of 2a molecule at the crucial beta-cell receptor is probably involved in the blocking glucose signal transduction.

Uric acid (1a) suppresses basal insulin release in isolated rat pancreatic islets, and inhibition of glucose-stimulated insulin secretion (GSIS) occurs right at hyperuricaemic levels (>0.4 mM). Conversely, 1 mM guanidinium urate (2a) was completely ineffective, strongly suggesting that binding to an essential arginine residue triggers the inhibitory effect. A specific recognition of 1a molecule at the crucial beta-cell receptor is probably involved in the blocking glucose signal transduction.

Diabetes mellitus results from either absolute or relative insulin deficiency. Despite well-known differences in the pathogenesis of type 1 and type 2 diabetes, the central event in the development of diabetes is a final destruction of the insulin-producing pancreatic b-cells via autoimmune (type 1) or toxic-degenerative (type 2) mechanisms. Biogenetic hypothesis of the aetiology of diabetes, postulating a possible production of highly selective b-cytotoxins, alloxan and/or alloxan-like compounds from uric acid, became apparent as a plausible experimental model in searching for the unique b-specific mechanisms, particularly with respect to the recently recognised antioxidant action of urate and production of reactive oxidation and/or nitrosation products. The aim of this study was to reinvestigate the biogenetic hypothesis by evaluating the influence of urate and a range of its derived chemical models on the glucose-stimulated insulin release in vitro. The results of the study on isolated rat islets of Langerhans revealed a pronounced, so far unknown inhibitory effect of urate and its NO-adducts, on the glucose stimulated insulin release.

Absolute (S)-configuration of the title compound was established by X-ray analysis.

C17H26N4O4, monoclinic, P12(1)1 (No. 4), a = 8.142(3) Angstrom, b = 6.611(2) Angstrom, c = 16.987(4) Angstrom, beta = 99.35(4)degrees, V = 902.2 Angstrom(3), Z = 2, rho(m) = 1.300 g.cm(-3), R-gt(F) = 0.043, R-w(F-2) = 0.086, T = 293 K.

The title compound is a rare example of the 1-acyl-1, 2-aminimide functionality. The planar ring (within 0.006 Angstrom) is best described as mesomeric betaine. The negative charge, formally located at C4, is stabilized by delocalization over a triskelion type array of C=X bonds (X=O, N). An additional recourse is open to the specific stabilization of the positive charge on tetrahedral N1, which involves the non-bonding electron pair of the carbonyl O1 antiperiplanar to the polar C5-N1 bond. The operation of n-->sigma* delocalization should give a partial sp hybride character to the carbonyl group, thus accounting for the most interesting structural features: a large opening of the angle [O1-C5-C4 140.7(2)degrees] opposite the long N1-C5 bond and the O1-C5 distance typical for a double bond. The structural parameters are comparable to those of 1, 1-dimethyl-3-phenyl-pyrazolium-5-oxide, a sole precedent of this unusual system ; differences in bond lengths (ca. 0.02 Angstrom) within the C3-C4-C5-O1 fragment (lengthening of C-C bonds adjacent to C4 and shortening of the C-O bond) are in conformity with a further delocalization stabilization of the negative charge by electron-withdrawing substituent in the 4-position. Selected bond lengths: N1-C5 1.577(2), N1-N2 1.465(2), N2-C3 1.308(2), C3-C4 1.432(2), C4-C5 1.386(3), O1-C5 1.212(2), C4-C7 1.447(3), O2-C7 1.204(2) Angstrom.

The classic formula for theobromuric acid [1,2] was revised; the correct formula represents a virtually unknown acid aminal array [3]. The crystal structure of its derived methyl ester aminal, the methyl theobromurate, 8a-methoxy-1,7-dimethyl-dihydro-imidazo[1,5-a]-1,3,5-triazine-2,4,6,8-tetraone, corroborates the new assignment based on chemical and spectroscopic evidence. The angular substituent is oriented antiperiplanar to the ring carbonyl C8, torsion angle (C8-C8a-O1-C9)= -172.9(1)degrees. The imide fragment O2-C2-N3-H3 is involved in dimeric association via a pair of hydrogen bonds: H3...O2= 2.10(5) Angstrom, N3...O2= 2.895(2) Angstrom, N3-H3...O2= 154(4)degrees (2-x,-y,1-z).

Two independent molecules of the title compound adopt the ring-twisted conformation (b), defined by roughly orthogonal bridgehead torsion angles (N13-C14-C15-N17 = 95.1(3) and N23-C24-C25-N27 = 97.1(3)degrees). Stereoelectronically controlled ring-opening at the N3-C4 bond in the course of acidic hydrolysis thus occurs at the acid aminal stage of the multistep reaction [ref. 4] via a conformational change to the proper ring-twisted conformer (a).

The structure of 5-chloro and 5-alkoxy-5,7-dihydro-3//-purine-2,4,6-trio- nes (6 and 7-9) has been corroborated by their spectral properties and X-ray crystallography. The stereochemical model (fì)-10 of the key uricolytic intermediate was prepared using menthol as chiral auxiliary. In acidic solutions, depending on the N-substitution, the ring cleavage occurred either at the 4,9-bond 6b -» 13 (R1 = Me) or at the 3,4-bond 6c -» 14 (R1 = H). Opening of the 1,6-bond is the dominant process under alkaline conditions. Decarboxylative rearrangement into 1,3-dimethylallantoin (15) was specific for N(7)-unsubstituted derivatives. Evidence for intermediacy of the bicyclol tautomeric form was supplied by isolation of l-menthoxy-2,4-dimethyl-3,7-di- oxo-2,4,6,8-tetraazabicyclo[3.3.0]octane (16); fragmentation of 10 into 5- menthoxy-imidazolidin-2,4-dione (17) also takes place under forcing conditions. Conversely, no allantoin rearrangement was encountered in the R7*H cases; 7d underwent the pyrimidine ring fragmentation to give 5-methoxy-l- methyl-4-methylimino-imidazolidine-2-one (18) or 5-carboxamido-5-methoxy- l-methyl-4-methylimino-imidazolidine-2-one (19). A possible mechanism for these ring transformation reactions is discussed.

Evidence for the species trapped by trimethylsilylation in peroxidase mediated oxidation of urate leaving the tetrakis(trimethylsilyl)spirodihydantoin structure is provided by its hydrolysis, reaction with diazomethane, and actual isolation and identification of tetramethylspirodihydantoin.

The C-13-NMR spectra are characteristic and provide the pathway of uricase-catalysed oxidation of [5-C-13]urate to [4-C-13]allantoin ; 5-hydroxy[5-C-13]isouric acid, its ring-opened derivative 3 and [4-C-13]allantoin-5-carboxylate were identified as consecutive intermediates in D2O/phosphate buffer (pD 7.2).

Use of [8-13C]urate in combination with 13C-NMR method provided unequivocal evidence for the formation of [2-13C]allantoin in the course of chemical, uricase- or peroxidase- mediated oxidations.

The asymmetric unit contains two independent molecules. The title compound is the first example of a derivative of the bicyclol tautomer in the allantoin series and the key intermediate in the asymmetric synthesis of the X-ray structure determination confirm its diastereomeric identity and establish the absolute (1R, 5R) configuration.

Derivatives of isouric acid were synthetized and studied their degradations into allantoins.