title: Homocysteine transport by human aortic endothelial cells: identification and properties of import systems.
abstract: Hyperhomocysteinemia is an independent risk factor for cardiovascular disease. Transport of L-homocysteine into and out of the human vascular endothelium is poorly understood. We hypothesized that cultured human aortic endothelial cells (HAEC) would import L-homocysteine on one or more of the L-cysteine transport systems. Inhibitors of the transporters were used to characterize the uptake of [35S]L-homocysteine, [35S]L-homocystine, and [35S]L-cysteine. We found that L-homocysteine uptake is mediated by the sodium-dependent cysteine transport systems X(AG), ASC, and A, and the sodium-independent transport system L. Thus, HAEC utilize multiple cysteine transporters (X(AG) > or = L > ASC > A) to import L-homocysteine. Kinetic analysis supported the uptake results. Michaelis-Menten constants (Km) for the four systems yielded values of 19.0, 27.1, 112, and 1000 microM for systems L, X(AG), ASC, and A, respectively. The binding and uptake of [35S]L-homocystine, the disulfide homodimer of L-homocysteine, was mediated by systems X(AG), L, and ASC but not by system A. In contrast to [35S]L-homocysteine, system x(c) was active for [35S]L-homocystine uptake. A similar pattern was observed for [35S]L-cysteine. Thus, L-homocysteine and L-homocystine found in hyperhomocysteinemic subjects can gain entry into the vascular endothelium by way of multiple L-cysteine transporters.
authors.affiliation: Department of Cell Biology, Lerner Research Institute, Cleveland Clinic Foundation, Cleveland, OH 44195, USA.
authors.forename: Beatrix
authors.initials: B
authors.lastname: Büdy
authors.forename: RoseMarie
authors.initials: R
authors.lastname: O'Neill
authors.forename: Patricia M
authors.initials: PM
authors.lastname: DiBello
authors.forename: Shantanu
authors.initials: S
authors.lastname: Sengupta
authors.forename: Donald W
authors.initials: DW
authors.lastname: Jacobsen
mesh_terms: D026924:Amino Acid Transport System A / Q000502:physiology; D026962:Amino Acid Transport System ASC / Q000502:physiology; D027062:Amino Acid Transport System L / Q000502:physiology; D027322:Amino Acid Transport System X-AG / Q000502:physiology; D001013:Aorta, Thoracic / Q000166:cytology; D001693:Biological Transport, Active; D002478:Cells, Cultured; D003545:Cysteine / Q000378:metabolism / Q000493:pharmacokinetics; D042783:Endothelial Cells / Q000378:metabolism*; D004730:Endothelium, Vascular / Q000166:cytology / Q000378:metabolism; D006710:Homocysteine / Q000378:metabolism* / Q000493:pharmacokinetics; D006801:Humans; D011485:Protein Binding; D013462:Sulfur Radioisotopes
chemical_list: D026924:Amino Acid Transport System A; D026962:Amino Acid Transport System ASC; D027062:Amino Acid Transport System L; D027322:Amino Acid Transport System X-AG; D013462:Sulfur Radioisotopes; D006710:Homocysteine; D003545:Cysteine