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Articles in PresS, published online ahead of print February 13, 2002
Am J Physiol Cell Physiol, 10.1152/ajpcell.00547.2001
Submitted on November 15, 2001
Accepted on February 8, 2002
1 Departments of Medicine and Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York, USA
* To whom correspondence should be addressed. E-mail: igoldman{at}aecom.yu.edu.
Although the reduced folate carrier (RFC1) and the thiamine transporters (THTR-1 and THTR-2) share ~40% identity in protein sequence, RFC1 does not transport thiamine and THTR-1 and THTR-2 do not transport folates. In this report we demonstrate that transport of thiamine monophosphate (TMP), an important thiamine metabolite present in plasma and cerebrospinal fluid, in L1210 murine leukemia cells is mediated by RFC1. Transport of TMP was augmented by a factor of five in cells that overexpresses RFC1, and was markedly inhibited by methotrexate, an RFC1 substrate, but not by thiamine. At a near-physiological concentration (50 nM), TMP influx mediated by RFC1 in wild-type L1210 cells was ~50 % of thiamine influx mediated by thiamine transporter(s). Within 1 min, the majority of TMP transported into R16 cells was hydrolyzed to thiamine with a component metabolized to thiamine pyrophosphate, the active enzyme cofactor. These data suggest that RFC1 may be one of the alternative transport routes available for TMP in some tissues when the thiamine transporter, THTR-1, is mutated in the autosomal recessive disorder- thiamine-responsive megaloblastic anemia.
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