Flavin, generally functions

Enzymes containing amino acid radicals are generally associated with transition metal ions—typically of iron, manganese, cobalt, or copper. In some instances, the metal is absent it is apparently replaced by redox-active organic cofactors such as S -adenosylmethionine or flavins. Functionally, their role is analogous to that of the metal ion in metalloproteins. 

In the following the basic chemical and physical properties of free flavin will be described in some detail, because the knowledge of these properties is the key for a detailed understanding of the function of flavoproteins. In addition, some general and common properties of the classes of flavoproteins will be presented and discussed in relation to some new concepts. It will however not be possible to cover the whole literature. For the reader interested in more details, the recent review papers by Massey and Hemmerich Bruice , Walsh Simondson and Tollin , and Hemmerich (and references therein) should be consulted. An overview of the pertinent research on flavins and flavoproteins is easily accessible by the proceedings of the international symposia... 

Enzymatic cofactors, such as nicotinamide adenine dinucleotide (NADH), nicotinamide adenine dinucleotide phosphate (NADPH), flavin adenine dinucleotide (EAD), flavin mononucleotide (EMN), and pyridoxal phosphate, are fluorescent and commonly found associated with various proteins where they are responsible for electron transport (see Fig. lb and Table 1). NADH and NADPH in the oxidized form are nonfluorescent, whereas conversely the flavins, FAD and EMN, are fluorescent only in the oxidized form. Both NADH and FAD fluorescence is quenched by the adenine found within their cofactor structures, whereas NADH-based cofactors generally remain fluorescent when interacting with protein structures. The fluorescence of these cofactors is often used to study the cofactors interaction with proteins as well as with related enzymatic kinetics (1, 9-12). However, their complex fluorescent characteristics have not led to widespread applications beyond their own intrinsic function. 

Flavins are lost from the body as intael riboflavin, rather than as a breakdown product of riboflavin. Hence, vitamin status may be assessed by measuring the level of urinary riboflavin. Generally, the loss of 30 ig of riboflavin/g creatinine or less per day indicates a deficiency. This metht>d of assessment is not preferred because it is influenced by a number of factors unrelated to vitamin status. Another problem with this method is its great sensitivity to a short-term deficiency thus, it does not necessarily reflect the true concentrations of FAD and FMN in tissues. The most reliable way to assess riboflavin status is by a functional test. The test involves the assay of glutathione reductase, using red blood cells as the source of... 

The aromatic hydroxylases or mixed-function oxidases are no exception to the above generalization. For maximum activity they require a transition metal ion and an electron donor such as one of the pyridine or flavin nucleotide coenzymes further, they probably utilize molecular oxygen as the source of the hydroxylic oxygen an example is the liver microsomal hydroxylating system (27). As yet there is no comprehensive explanation to cover the mode of action of these enzymes, for on the one hand there are the specific hydroxylases which catalyze such conversions as L-phenylalanine to L-tyrosine (26) or tryptophan to 5-hydroxytrypto-... 

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