Horne DW, Holloway RS, Wagner C. Transport of S-adenosylmethionine in isolated rat liver mitochondria.
Arch Biochem Biophys 1997;
343:201-6. [PMID:
9224731 DOI:
10.1006/abbi.1997.0167]
[Citation(s) in RCA: 43] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
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.
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