Homocysteine transport by human aortic endothelial cells: Identification and properties of import systems

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 XAG, ASC, and A, and the sodium-independent transport system L. Thus, HAEC utilize multiple cysteine transporters (XAG ⩾ 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 μM for systems L, XAG, ASC, and A, respectively. The binding and uptake of [35S]l-homocystine, the disulfide homodimer of l-homocysteine, was mediated by systems XAG, L, and ASC but not by system A. In contrast to [35S]l-homocysteine, system xc 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.