Table 1 Categories of flavoenzymes
Pyridine nucleotide dehydrogenases or reductases Nonpyridine nucleotide dehydrogenases Pyridine nucleotide-disulfide oxidoreductases Dehydrogenase-oxygen reductases Flavoprotein oxidases [O2 ! H2O2] Flavoprotein monooxygenases (1/2 O2 ! H2O) Internal («-hydroxy fatty acid ! fatty acid (n-1) + CO2) External (RH ! ROH)
Mitochondrial electron-transfer flavoprotein
Mitochondrial NADH dehydrogenase Mitochondrial succinate dehydrogenase Glutathione reductase
Bacterial lactate monooxygenase Microsomal FAD-containing monooxygenase
Source: Merrill AH, Lambeth JD, Edmondson DE, and McCormick DB (1981) Formation and mode of action of flavoproteins. Annual Review of Nutrition 1: 281-317.
FAD while others specifically require FMN; it is difficult to account for this dichotomy. Table 1 lists the broad categories of flavoenzymes found in living tissues: the range of reaction types is considerable, but all of them clearly center around redox processes involving hydrogen transfer. This fact reflects the central biochemical reaction of the flavin coenzymes, which is the interconversion of the reduced, dihydro form of the flavin ring and the more stable oxidized form. One of the most important sites of action of flavoenzymes within higher animals is that of the electron transport chain in the mitochondria. The flavins, which form part of succinic dehydrogenase and NADH dehydro-genase, form an essential redox link between the oxi-dizable energy-rich substrates of aerobic metabolism, and the cytochrome chain leading to molecular oxygen, which can generate around 38 moles of energy-rich ATP per mole of glucose oxidized.
Hormone status can affect riboflavin economy in a number of important ways, and there is also some evidence that riboflavin status can affect hormone production. One important control valve for ribofla-vin economy is thyroid hormone status: hypothyroid-ism leads to reduced tissue levels of flavin coenzymes, and hence to inactivation of certain flavoenzymes, thus resembling the effects of dietary riboflavin deficiency. Both flavokinase (ATP: riboflavin 5'-phospho-transferase EC 126.96.36.199) and FAD pyrophosphorylase (ATP: FMN adenyltransferase EC 188.8.131.52) are sensitive to thyroid hormone status. In the kidney, synthesis of flavokinase and hence of flavin coenzymes is controlled by aldosterone in a similar manner.
The amount of absorbed riboflavin that can remain within the body and the circulation (in blood plasma) is strictly regulated by glomerular and tubular filtration and tubular reabsorption in the kidneys. The latter is an active, saturable, sodium-dependent transport process, with characteristics similar to those of active transport in the gastrointestinal tract. It is responsible for the very sharp and characteristic transition between minimal urinary excretion of ribofla-vin at low intakes, and a much higher level of excretion, proportional to intake, at higher intakes. This transition point has been extensively used to define and to measure riboflavin status and requirements (see below), and to permit studies of intestinal absorption in vivo (see above). Excretion of riboflavin is affected by some chemicals (such as boric acid, which complexes with it), and by certain diseases and hormone imbalances.
In addition to the excretion of unchanged ribo-flavin, there are also small amounts of hydroxylated breakdown products of the vitamin, which arise through normal turnover, either within the tissues of the body, or in the gastrointestinal tract from bacterial action, before absorption. The rate of destruction of riboflavin by this turnover pathway is very low in all species examined to date, and riboflavin within the mammalian body seems to be remarkably efficiently conserved, apparently throughout many cycles of cell turnover.
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