Riboflavin equilibrium

Mitochondria are surrounded by two lipoprotein membranes, together about 180 A thick. The inner membrane is folded into the cell as a series of invaginations known as cristae. About one-quarter of the protein part of the cristae consists of oxysomes (respiratory assemblies), i.e. ordered arrangements of riboflavine-protein, coenzyme Q, cytochromes b, c, c, a, and a (in that sequence) together with their specific proteins. Ferredoxins (Section 11.0) also play an important part. The tricarboxylic acid cycle ensures the reduction of the first two members of the above chain, and each member is oxidized by the member on its right (in the above list), and so on to the end of the chain at cytochrome a which is in equilibrium with atmospheric oxygen. 

A second remark concerns the rapid isomerization of riboflavin phosphates in acid solution (pH < 2), especially at elevated temperature. The thermodynamic equilibrium is characterized by the presence of about 65% 5 -FMN, 11 % 4 -FMN, 8% 3 -FMN, and 15% 2 -FMN. The rate constants for the various isomerization reactions under a variety of experimental conditions have been determined by reversed-phase HPLC (83). According to the official method of the American Association of Analytical Chemists, protein-bound flavins are extracted from biological samples by treatment with 0.1 M HCl at 121°C for 15 to 30 min. It is known that this treatment leads to hydrolysis of FAD. As shown in Figure 9, it should also be noted that a substantial fraction of 5 -FMN is converted to other isomeric phosphates under these conditions. 

The reduced and oxidized forms of riboflavin form a couple with = —0.21 V and the acetate/acetaldehyde couple has = -0.60 V imder the same conditions. What is the equilibrium constant for the reduction of riboflavin (Rib) by acetaldehyde (ethanal) in neutral solution at 25°C The reaction is... 

Figure 13 Chemical structures of xanthosinate 5 -monophosphate, which exists at the physiological pH of about 7.5 as (XMP - H) ", with a tautomeric equilibrium between (N1) /(N3)H and (N1)H/(N3) sites [96,97,162], and flavin mononucleotide (FMN = riboflavin 5 -phosphate) as well as for comparison of glycerol 1-phosphate (GIP "). (XMP - H) is shown in its dominating anti conformation [11-13].

The fact however, that this group cannot be cleaved by nucleophiles such as RS or NH2OH remains very puzzling. Edmondson (24) has shown that 8a-oxoriboflavin, in analogy to 8a-oxo-lumiflavin mentioned above, is in slow equilibrium with its 5 -hemiacetal. This in turn is identical with the mixture of riboflavin derivatives obtained from acid hydrolysis of cyt C552 flavin. 

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