title: Transport of S-adenosylmethionine in isolated rat liver mitochondria.
abstract: Mitochondria do not have the enzyme, methionine adenosyltransferase (ATP: L-methionine S-adenosyltransferase, EC 2.5.1.6), necessary for the biosynthesis of S-adenosylmethionine. Nevertheless, about 30% of total hepatic S-adenosylmethionine resides in the mitochondria and radiolabeled S-adenosylmethionine may be isolated from the mitochondria after administration of radiolabeled methionine. This leads to the hypothesis that a carrier-mediated system is responsible for S-adenosylmethionine transport from the cytosol into the mitochondria. We have characterized such a system in isolated rat liver mitochondria. Uptake of S-adenosylmethionine consisted of two components. One component was incorporation of the methyl group into phospholipids as shown by thin-layer chromatography. The second component represented uptake into the mitochondria since addition of excess unlabeled S-adenosylmethionine resulted in efflux of labeled substrate. This countertransport is characteristic of a carrier-mediated transport system. Uptake (corrected for incorporation into phospholipids) was saturable with an apparent Km = 8.9 microM and Vmax = 54.3 pmol x mg protein(-1) x min(-1). Uptake was not inhibited by methionine, adenosine, 5'-methylthioadenosine, carnitine, choline, betaine, quinine, or hemicholinium-3. Uptake was inhibited by sinefungin and by S-adenosylhomocysteine (Ki = 53.4 microM). Uptake of S-adenosylmethionine was not dependent on the electrical potential across the mitochondrial membrane. These results indicate that S-adenosylmethionine is taken up into mitochondria via a specific, carrier-mediated system.
authors.affiliation: Biochemistry Research Laboratory, Department of Veterans Affairs Medical Center, Nashville, Tennessee 37212, USA.
authors.forename: D W
authors.initials: DW
authors.lastname: Horne
authors.forename: R S
authors.initials: RS
authors.lastname: Holloway
authors.forename: C
authors.initials: C
authors.lastname: Wagner
mesh_terms: D000818:Animals; D001692:Biological Transport; D002352:Carrier Proteins / Q000378:metabolism; D007700:Kinetics; D008715:Methionine / Q000378:metabolism; D008930:Mitochondria, Liver / Q000378:metabolism*; D010743:Phospholipids / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D012435:S-Adenosylhomocysteine / Q000494:pharmacology; D012436:S-Adenosylmethionine / Q000378:metabolism*
chemical_list: D002352:Carrier Proteins; D010743:Phospholipids; D012436:S-Adenosylmethionine; D012435:S-Adenosylhomocysteine; D008715:Methionine