title: Rhein as an electron acceptor for various flavoproteins and for electron transport particles.
abstract: Rhein (4,5-dihydroxyanthraquinone-2-carboxylic acid) which has been previously employed as an inhibitor for electron transport particles, NADH dehydrogenase, and other flavoproteins is reducible under physiological conditions. Soluble hydrogenase from Alcaligenes eutrophus H 16, several flavoproteins, and electron transport particles from baker's yeast and from beef heart were found to catalyse NADH oxidation with 9 micrometers to 2mM rhein as the electron acceptor. Dithionite or enzymatically reduced rhein (lambda max = 408 nm) is immediately reoxidized to rhein lambda max = 437 nm) by oxygen. Cyclovoltagrams reveal the midpoint redox potentials --0.240 V, -0.270 V, -0.280 V, -0.335 V at pH 6.0, 7.0, 7.7, 9.2, respectively. Due to its redox behaviour, caution should be exercised using rhein as a flavin-site-directed inhibitor for biological electron transfer systems.
mesh_terms: D000421:Alcaligenes / Q000201:enzymology; D000818:Animals; D000880:Anthraquinones / Q000494:pharmacology; D002417:Cattle; D004579:Electron Transport; D005420:Flavoproteins / Q000378:metabolism*; D006864:Hydrogenase; D007700:Kinetics; D009206:Myocardium / Q000378:metabolism; D010088:Oxidoreductases / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000378:metabolism; D013053:Spectrophotometry
chemical_list: D000880:Anthraquinones; D005420:Flavoproteins; D010088:Oxidoreductases; D006864:Hydrogenase; C020491:rhein