Flavins are very versatile coenzymes, functioning with considerable efficiency in a wide variety of enzymic reactions involving either two-electron or one-electron transfers, with both functions often catalysed by the same enzyme. They are responsible for catalysing the dehydrogenation of many different types of compounds, including dithiols, reduced nicotinamide nuclwtides, alcohols and a-hydroxy acids, amines and a-amino acids, and even saturated C-C bonds, provided that a suitable activating group such as a carbonyl residue is situated a/% to the bond to be oxidized. In the process of catalysing these dehydrogenation reactions, the flavin is itself reduced, and, in order to function catalytically, the oxidized form must be regenerated at the expense of reduction of some acceptor. The acceptor may be in some cases the oxidized form of the same type of compound that serves as reducing substrate, e.g. it might be a disulphide, an oxidized nicotinamide nucleotide or an unsaturated compound such as fumarate or crotonylCoA. In such a case the enzyme might conveniently be classified as a transhydrogenase and subclassified as a C-C, C-S, C-N or N-N transhydrogenase, depending on the nature of the atoms acting as hydrogen donor and hydrogen acceptor (Hemmerich & Massey, 1979). In most cases, however, the acceptor molecule will be molecular 0, or another redox protein, such as an iron-sulphur protein or a cytochrome. In the latter cases the flavoprotein necessarily acts as a mediator between two-electron and one-electron transfers. Flavoproteins fill a unique spot in biochemistry with this capacity. An equal richness of possibilities exists in the reactions of different flavoproteins with molecular 0,. In some cases one-electron transfer is carried out, with superoxide (02-) and flavin semiquinone as the immediate products of the reaction. In other cases a direct two-electron reduction of 0, to H,O, appears to occur, and in another class of flavoproteins, the mono-oxygenases, one atom of the 0, molecule is incorporated into H20 and the other is incorporated into another substrate of the enzyme, to form an oxygenated product.