Reduction of Fe(III) ions complexed to physiological ligands by lipoyl dehydrogenase and other flavoenzymes in vitro: implications for an enzymatic reduction of Fe(III) ions of the labile iron pool.
Enzymatic reduction of physiological Fe(III) complexes of the "labile iron pool" has not been studied so far. By use of spectrophotometric assays based on the oxidation of NAD(P)H and formation of [Fe(II) (1,10-phenanthroline)3]2+ as well as by utilizing electron paramagnetic resonance spectrometry, it was demonstrated that the NAD(P)H-dependent flavoenzyme lipoyl dehydrogenase (diaphorase, EC 1.8.1.4) effectively catalyzes the one-electron reduction of Fe(III) complexes of citrate, ATP, and ADP at the expense of the co-enzymes NAD(P)H. Deactivated or inhibited lipoyl dehydrogenase did not reduce the Fe(III) complexes. Likewise, in the absence of NAD(P)H or in the presence of NAD(P)+, Fe(III) reduction could not be detected. The fact that reduction also occurred in the absence of molecular oxygen as well as in the presence of superoxide dismutase proved that the Fe(III) reduction was directly linked to the enzymatic activity of lipoyl dehydrogenase and not mediated by O2. Kinetic studies revealed different affinities of lipoyl dehydrogenase for the reduction of the low molecular weight Fe(III) complexes in the relative order Fe(III)-citrate > Fe(III)-ATP > Fe(III)-ADP (half-maximal velocities at 346-485 microm). These Fe(III) complexes were enzymatically reduced also by other flavoenzymes, namely glutathione reductase (EC 1.6.4.2), cytochrome c reductase (EC 1.6.99.3), and cytochrome P450 reductase (EC 1.6.2.4) with somewhat lower efficacy. The present data suggest a (patho)physiological role for lipoyl dehydrogenase and other flavoenzymes in intracellular iron metabolism.
Petrat
Frank
F
Institut für Physiologische Chemie, Universitätsklinikum, Hufelandstrasse 55, D-45122 Essen, Germany.
Paluch
Sandra
S
Dogruöz
Elke
E
Dörfler
Petra
P
Kirsch
Michael
M
Korth
Hans-Gert
HG
Sustmann
Reiner
R
de Groot
Herbert
H
D000244:Adenosine Diphosphate / Q000031:analogs & derivatives* / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D003013:Clostridium / Q000201:enzymology; D008058:Dihydrolipoamide Dehydrogenase / Q000737:chemistry / Q000378:metabolism*; D004578:Electron Spin Resonance Spectroscopy; D044927:Electron-Transferring Flavoproteins / Q000737:chemistry / Q000378:metabolism*; D005290:Ferric Compounds / Q000378:metabolism*; D007700:Kinetics; D008024:Ligands; D009206:Myocardium / Q000201:enzymology; D013057:Spectrum Analysis; D013552:Swine
D044927:Electron-Transferring Flavoproteins; D005290:Ferric Compounds; D008024:Ligands; C029936:adenosine diphosphate-ferric chelate; D000244:Adenosine Diphosphate; C025314:ferric citrate; D000255:Adenosine Triphosphate; D008058:Dihydrolipoamide Dehydrogenase