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J. Biol. Chem., Vol. 282, Issue 35, 25189-25198, August 31, 2007

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Discovery and Characterization of a Second Mammalian Thiol Dioxygenase, Cysteamine Dioxygenase^*

From the Department of Nutritional Sciences, Cornell University, Ithaca, New York 14853

There are only two known thiol dioxygenase activities in mammals, and they are ascribed to the enzymes cysteine dioxygenase (CDO) and cysteamine (2-aminoethanethiol) dioxygenase (ADO). Although many studies have been dedicated to CDO, resulting in the identification of its gene and even characterization of the tertiary structure of the protein, relatively little is known about cysteamine dioxygenase. The failure to identify the gene for this protein has significantly hampered our understanding of the metabolism of cysteamine, a product of the constitutive degradation of coenzyme A, and the synthesis of taurine, the final product of cysteamine oxidation and the second most abundant amino acid in mammalian tissues. In this study we identified a hypothetical murine protein homolog of CDO (hereafter called ADO) that is encoded by the gene Gm237 and belongs to the DUF1637 protein family. When expressed as a recombinant protein, ADO exhibited significant cysteamine dioxygenase activity in vitro. The reaction was highly specific for cysteamine; cysteine was not oxidized by the enzyme, and structurally related compounds were not competitive inhibitors of the reaction. When overexpressed in HepG2/C3A cells, ADO increased the production of hypotaurine from cysteamine. Similarly, when endogenous expression of the human ADO ortholog C10orf22 in HepG2/C3A cells was reduced by RNA-mediated interference, hypotaurine production decreased. Western blots of murine tissues with an antibody developed against ADO showed that the protein is ubiquitously expressed with the highest levels in brain, heart, and skeletal muscle. Overall, these data suggest that ADO is responsible for endogenous cysteamine dioxygenase activity.

Received for publication, April 12, 2007 , and in revised form, June 19, 2007.

^* This research was supported in part by NIDDK, National Institutes of Health Public Health Service Grant DK056649 (to M. H. S.). 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 Supported by a National Science Foundation graduate student fellowship award.

² To whom correspondence should be addressed: Division of Nutritional Sciences, 227 Savage Hall, Cornell University, Ithaca, NY 14853. Tel.: 607-255-2638; Fax: 607-255-1033; E-mail: mhs6{at}cornell.edu.

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