Enzymes ascorbate free radical reductase

Arrigoni, O., Dipierro, S., and Borraccino, G., Ascorbate free radical reductase, a key enzyme of the ascorbic acid system, FEBS Lett., 125, 242-244, 1981. 

It is possible to reduce enzymatically both AFR and DHA, regenerating ascorbate and thus decreasing nutritional requirements in animals unable to carry out the synthesis de novo. Enzymatic systems that reduce AFR usually utilize NADH as an electron donor (one-electron reduction pathway or NADH-ascorbate free radical reductase). Reduction of DHA to ascorbate involves two electrons and has been attributed to thioltransferase (glutaredoxin) and protein disulfide isomerase (Wells et aL, 1990). However, taking into consideration the reported values for DHA (millimolar range) it is unlikely that the latter enzymes are involved in the maintenance of ascorbate in its reduced state in vivo and that function is carried out by NADH-AFR reductase (Minetti et aL, 1992). 

The effects of hydroxyurea in the purified ribonucleotide reductase systems of E. coU, phage T4, calf thymus and mouse cells have been described above (p. 36, 42). Inhibition of substrate reduction in vitro (I50 = 2 - 3 10 ) is accompanied by loss of the tyrosyl radical, but not iron from E. coli subunit B2. Studies with substituted hydroxyl-amines and hydroxamates showed good correlation between their ability to undergo one-electron oxidation and enzyme inhibition, unless branched substituents prevented interaction with the protein (Table 8) Thus the mode of inhibition of E. coli ribonucleotide reductase is essentially solved Within steric restrictions of accessibility to the active site the compounds donate an electron to the enzyme s free radical, producing an inactive protein with still intact binuclear iron complex (Eq. VI). This process is irreversible in vitro until iron is removed, and then reintroduced with Fe(II)ascorbate in the presence of oxygen, whereupon radical and enzyme activity reappear. No other enzyme of E. coli has been found to be inactivated by hydroxyurea. 

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