S-adenosylmethionine (SAM) is the sole methyl donor in the CNS where it is involved in a multitude of biochemical reactions. Peripherally administered SAM has been shown to increase SAM levels in cerebrospinal fluid and is reported to be effective in the treatment of numerous neurological disorders suggesting SAM crosses the blood-brain barrier (BBB). The mechanism of SAM entry into the brain remains unknown, but the presence of adenosyl and methionine residues in the molecule suggests probable entry via carrier mediated transport, We have investigated whether SAM utilises endogenous transport systems in cerebral endothelial Cells, using RBE4 cells, an in vitro model of the BBB. SAM did not influence the transport of [H-3]-methionine and only marginally reduced the uptake of [H-3]-leucine in RBE4 cells. The inhibition constant for the latter was 2.11+/-0.29 mM (mean +/- S.E.M.). However, increasing concentrations of SAM strongly inhibited the transport of [H-3]-adenosine in RBE4 cells in both the presence and the absence of sodium n the medium, with K, values of 199+/-32 and 139+/-8.4 muM, respectively. Lineweaver-Burk plots suggest a competitive mode of inhibition. The findings suggest that SAM is not recognised by the L-system transporter for large neutral amino acids at die brain endothelium. A significant interaction with the transport of adenosine, however, indicates that SAM has affinity for the nucleoside carrier systems; this is within the range of Km values of natural substrates and suggest that SAM may enter the CNS via the Na+-Independent nucleoside carrier systems at the brain capillary endothelium. (C) 2002 Elsevier Science B.V. All rights reserved.