Aquacobalamin

B 2 (lb)) and methylcobalamin [13422-55-4] (Ic). These, along with hydroxocobalamin [13422-51 -0] (vitamin B 2 (1 ))> die forms found in humans and other animals. Other forms of interest are iiitrocobalarnin (vitamin ( )) aquacobalamine chloride [13422-52-1] (vitamin The... 

Aquacobalamin (vitamin B12) does not bind NO in aqueous solution -nitrite impurities account for the reaction observed (71). 

Fig. 5. Volume profile for the reaction of aquacobalamine with 3-acetylpyridine.

Aquacobalamin reductase (NADH) [EC 1.6.99.8] catalyzes the reaction of two aquacob(III)alamin with NADH to form 2 cob(II)alamin and NAD+. Aquacobalamin reductase (NADPH) [EC 1.6.99.11] catalyzes the exact same reaction, albeit with NADPH. In addition, this enzyme can utilize hydroxycobalamin as a substrate, but not cyanocobalamin. 

AMINO ACID DEHYDROGENASE AQUACOBALAMIN REDUCTASES BENZOATE L,2-DIOXYGENASE BETAINE-ALDEHYDE DEHYDROGENASE CDP-GLUCOSE 4,6-DEHYDRATASE DEHYDROASCORBATE REDUCTASE... 

NICOTINAMIDE COENZYMES (SREC-TRAL PROPERTIES) NADH-dependent enzymes, AQUACOBALAMIN REDUCTASES BENZENE 1,2-DIOXYGENASE DEHYDROASCORBATE REDUCTASE GLUTAMATE SYNTHASE GLYOXYLATE REDUCTASE 3-HYDROXYBENZOATE 6-MONOOXY-GENASE... 

ACYL-CoA DEHYDROGENASES ADRENODOXIN ANTHRANILATE HYDROXYLASE AQUACOBALAMIN REDUCTASES... 

Nitroprusside forms both dinuclear and trinuclear complexes with aquacobalamin (Bj2a) [131]. Coordination involves a Fe-C=N-Co link, with linkage involving trans cyanides in the trinuclear complex. Such complexes are slower acting as hypotensive agents than nitroprusside itself [7]. 

The reaction between aquacobalamin, [Cbl(H20)], and NO at low pH was studied repeatedly and it is now accepted that imder such conditions nitrous acid impiuities are responsible for the reaction observed between vitamin B12 and NO (112-118). 

H. M. Marques, J. C. Bradley, and L. A. Campbell, J. Chem. Soc., Dalton Trans., 2019 (1992). Ligand Substitution Reactions of Aquacobalamin Evidence for a Dissociative Interchange Mechanism. 

Scheme 2 Structure of cobalamin, where X = CN for cyanocobalamin (vitamin Bi2), X = H20 for aquacobalamin (vitamin Bi2a) and X = 5 -deoxyadenosyl for coenzyme B12.

A perspective report emphasised the key role of the application of pressure in kinetic studies in bringing clarity to understanding the mechanism of substitution reactions of cobalamins.193 The effect of various alkyl substituents in the trans position on the kinetic, thermodynamic and ground-state properties has been studied. Cobalamins featuring in these studies were cyanocobalamin (vitamin Bi2), aquacobalamin and the complex formed when the cyano or water ligand is replaced... 

NO adds reversibly to reduced cobalamin, Cbl(II).156 It does not react directly with aquacobalamin(III), (0blni(H2O)), but it does add to Cbl,n(N02 ) and Cblm(NO).175 Acid hydrolysis of the dinitroso species releases nitrite, and binding of nitrite to Cblln(H20) generates Cbln,(. 02 ). This sequence thus affords a nitrite-catalyzed mechanism for NO substitution at Cblln(H20). The reaction of NO with Com porphyrins is quite complex.176 In the first step, NO displaces an axial water ligand to form a weakly bound mono NO complex this mono NO complex reacts with a second molecule of NO to form nitrite and a reduced Co-NO complex. This latter process is called reductive nitrosylation. Manganese(II) porphyrins bind NO very rapidly.177 Stability constants have been measured for the formation of mono and bis NO complexes of Cun(dithiocarbamate)2.157... 

The cyanide ligand may be replaced by water to give the so-called aquacobalamin, or vitamin Bi2a- In these compounds the cobalt is... 

Many antidotes have been developed to combat cyanide poisoning. Some of these are simple salts such as sodium thiosulfate and sodium nitrate, which demonstrated partial effect. Friedberg and Shukla (1975) reported the efficiency of aquacobalamin acetate [49552-79-6] as an antidote when given alone or combined with sodium thiosulfate. Aquacobalamin 100 and 200 mg/kg and thiosulfate 500 mg/kg injected separately with a 1-minute interval detoxified cyanide in guinea pigs and cats, respectively. 

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