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J. Biol. Chem., Vol. 281, Issue 52, 39852-39859, December 29, 2006

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Human Erythrocyte Membranes Contain a Cytochrome b₅₆₁ That May Be Involved in Extracellular Ascorbate Recycling^*

Dan Su, James M. May, Mark J. Koury^¶, and Han Asard¹

From the Department of Biochemistry, University of Nebraska, Lincoln, Nebraska 68588 and the Department of Medicine, Vanderbilt University School of Medicine and ^¶Veterans Affairs Tennessee Valley Healthcare System, Nashville, Tennessee 37232

Human erythrocytes contain an unidentified plasma membrane redox system that can reduce extracellular monodehydroascorbate by using intracellular ascorbate (Asc) as an electron donor. Here we show that human erythrocyte membranes contain a cytochrome b₅₆₁ (Cyt b₅₆₁) and hypothesize that it may be responsible for this activity. Of three evolutionarily closely related Cyts b₅₆₁, immunoblots of human erythrocyte membranes showed only the duodenal cytochrome b₅₆₁ (DCytb) isoform. DCytb was also found in guinea pig erythrocyte membranes but not in erythrocyte membranes from the mouse or rat. Mouse erythrocytes lost a majority of the DCytb in the late erythroblast stage during erythropoiesis. Absorption spectroscopy showed that human erythrocyte membranes contain an Asc-reducible b-type Cyt having the same spectral characteristics as recombinant DCytb and biphasic reduction kinetics, similar to those of the chromaffin granule Cyt b₅₆₁. In contrast, mouse erythrocytes did not exhibit Asc-reducible b-type Cyt activity. Furthermore, in contrast to mouse erythrocytes, human erythrocytes much more effectively preserved extracellular Asc and transferred electrons from intracellular Asc to extracellular ferricyanide. These results suggest that the DCytb present in human erythrocytes may contribute to their ability to reduce extracellular monodehydroascorbate.

Received for publication, July 10, 2006 , and in revised form, September 27, 2006.

^* This work was supported by National Institutes of Health Grants 1P20RR17675 and DK050435, National Science Foundation Grant IBN-0416742, and a Merit Review (to M. J. K.) from the Department of Veterans Affairs. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed: Dept. of Biology, University of Antwerp, Groenenborgerlaan 171, B-2020 Belgium. Tel.: 32-3-2653638; Fax: 32-3-265-3417; E-mail: han.asard{at}ua.ac.be.

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