Structural determination of a cyclic metabolite of NAD+ with intracellular Ca2+-mobilizing activity.

Incubation of NAD+ with extracts from sea urchin eggs resulted in production of a metabolite which could mobilize intracellular Ca2+ stores of the eggs. In this study we present structural evidence indicating that the metabolite is a cyclized ADP-ribose having an N-glycosyl linkage between the anomeric carbon of the terminal ribose unit and the N6-amino group of the adenine moiety. In view of this structure we propose cyclic ADP-ribose as the common name for the metabolite. The purification procedure for the metabolite consisted of deproteinizing the incubated egg extracts and sequentially chromatographing the extracts through three different high pressure liquid chromatography (HPLC) columns. The homogeneity of the purified metabolite was further verified by HPLC on a Partisil 5 SAX column. Using radioactive precursor NAD+ with label at various positions it was demonstrated that the metabolite was indeed derived from NAD+ and that the adenine ring as well as the adenylate alpha-phosphate were retained in the metabolite whereas the nicotinamide group was removed. This was confirmed by 1H NMR and two-dimensional COSY experiments, which also allowed the identification of all 12 protons on the two ribosyl units as well as the two protons on the adenine ring. From the chemical shifts of the two anomeric protons it was concluded that the C-1 carbons of both ribosyl units were still bonded to nitrogen. The positive and negative ion fast atom bombardment mass spectra showed (M + Na)+, (M - H + 2Na)+, (M - H)-, and (M - 2H + Na)- peaks at m/z 564, 586, 540, and 562, respectively. Exact mass measurements indicated a molecular weight of 540.0526 for (M - H)-. This together with the constraints imposed by the results from NMR, radioactive labeling, and total phosphate determination uniquely specified a molecular composition of C15H21N5O13P2. Analysis by 1H NMR and mass spectroscopy of the only major breakdown product of the metabolite after prolonged incubation at room temperature established that it was ADP-ribose, thus providing strong support for the cyclic structure.

Reaction of alkylcobalamins with thiols.

Carbon-13 NMR spectroscopy and phosphorus-31 NMR spectroscopy have been used to study the reaction of several alkylcobalamins with 2-mercaptoethanol. At alkaline pH, when the thiol is deprotonated, the alkyl-transfer reactions involve a nucleophilic attack of the thiolate anion on the Co-methylene carbon of the cobalamins, yielding alkyl thioethers and cob(II)alamin. In these nucleophilic displacement reactions cob(I)alamin is presumably formed as an intermediate. The higher alkylcobalamins react more slowly than methylcobalamin. The lower reactivity of ethyl- and propylcobalamin is probably the basis of the inhibition of the corrinoid-dependent methyl-transfer systems by propyl iodide. The transfer of the upper nucleoside ligand of adenosylcobalamin to 2-mercaptoethanol is a very slow process; S-adenosyl-mercaptoethanol and cob(II)alamin are the final products of the reaction. The dealkylation of (carboxymethyl)cobalamin is a much more facile reaction. At alkaline pH S-(carboxymethyl)mercaptoethanol and cob(II)alamin are produced, while at pH values below 8 the carbon-cobalt bond is cleaved reductively to acetate and cob(II)alamin. The reductive cleavage of the carbon-cobalt bond of (carboxymethyl)cobalamin by 2-mercaptoethanol is extremely fast when the cobalamin is in the "base-off" form. Because we have been unable to detect trans coordination of 2-mercaptoethanol, we favor a mechanism that involves a hydride attack on the Co-methylene carbon of (carboxymethyl)cobalamin rather than a trans attack of the thiol on the cobalt atom.

Methyl transfer from methylcobalamin to thiols. A reinvestigation.

The methyl transfer from methylcobalamin to thiols has been reinvestigated. By use of methylcobalamin selectively enriched with 13C in the methyl moiety, the methyl transfer to thiols was followed by 13C NMR. The methyl transfer occurs in aqueous mildly alkaline (pH 8-12) solution, even in the complete absence of oxygen. 31P NMR and EPR studies demonstrate that cob(II)alamin is the final corrinoid product. However, the pH dependence of the methyl-transfer reaction from methylcobalamin to beta-mercaptoethanol is consistent only with a nucleophilic displacement of the methyl group by a thiolate anion, resulting in the heterolytic cleavage of the carbon-cobalt bond. Difference visible spectroscopic measurements of the reaction mixture suggest that cob(I)alamin is formed as an intermediate.

Carbon-13 nuclear magnetic resonance studies of cobalamins.

The carbon-13 nuclear magnetic resonance spectra of aquocobalamin, adenosylcobalamin, methylcobalamin, and (carboxymethyl)cobalamin have been interpreted. The assignments were made by a comparison of the spectra with that of cyanocobalamin, by a study of the pH dependence of the chemical shifts, by an analysis of the effect of the axial ligands on the carbon atoms of the corrin ring, and by a study of the specific line broadening effect of the paramagnetic ions Mn2+ and Gd3+. The chemical shift changes that accompany the "base-on"----"base-off" conversion of the organocobalamins demonstrate that the conformation of the "western" half of the corrin ring and the conformations of the a, b, c, d, f, and g side chains are relatively constant. In contrast, the conformations of the "eastern" half of the corrin ring and the e propionamide side chain are highly variable.

Methyl transfer from methylcobalamin to diaquocobinamide.

The transfer of the methyl group from methylcobalamin to diaquocobinamide in aqueous solution has been demonstrated by proton, carbon-13, and phosphorus-31 nuclear magnetic resonance spectroscopy. The products of this reaction are aquocobalamin and the methylaquocobinamides. Dicyanocobinamide and the cyanoaquocobinamides do not serve as methyl acceptors, while ligands such as pyridine and histidine reduce the rate of the transfer reactions. The methyl transfer is not affected by oxidizing agents such as O2, N2O, and H2O2, suggesting that the reaction does not involve free Co(I) or Co(II) corrinoids. The pH dependence of the rate of the transfer reaction from methylcobalamin to diaquocobinamide demonstrates that methylcobalamin in the "base-on" form and diaquocobinamide are the most effective methyl donor and acceptor, respectively. The most plausible mechanism for the transfer reaction involves the one-electron oxidation of methylcobalamin by diaquocobinamide to a methylcobalamin radical cation and cob(II)inamide. The very unstable methylcobalamin radical cation releases a methyl radical, which reacts with cob(II)inamide to generate the methylaquocobinamides.