Submitted on January 5, 2006
Accepted on March 24, 2006
Cationic amino acid transport across the blood-brain barrier is mediated exclusively by system y+
Robyn L O'Kane1, Juan R Vina2, Ian A Simpson3, Rosa Zaragoza2, Ashwini Mokashi4, and Richard A. Hawkins5*
1 Natural and Applied Sciences, LaGuardia Community College/CUNY, Long Island City, New York, United States
2 Bioquimica & Biologia Molecular, Facultad de Medicina, Universitat de Valencia, Valencia, Spain
3 Neural and Behavioral Sciences, Milton S. Hershey Medical Center, Penn State University College of Medicine, Hershey, United States
4 Physiology & Biophysics, The Chicago Medical School, Rosalind Franklin University of Medicine and Science, North Chicago, Illinois, Spain
5 Physiology & Biophysics, The Chicago Medical School, Rosalind Franklin University of Medicine and Science, North Chicago, Illinois, Spain; Bioquimica & Biologia Molecular, Facultad de Medicina, Universitat de Valencia, Valencia, Spain
* To whom correspondence should be addressed. E-mail: rah{at}post.harvard.edu.
Cationic amino acid (CAA) transport is brought about by two families of proteins found in various tissues: Cat (CAA transporter), referred to as the system y+ and Bat (broad scope AA transporter) which comprises systems b0,+, B0,+, and y+L. CAA traverse the blood-brain barrier (BBB), but experiments done in vivo have only been able to examine the BBB from the luminal (blood-facing) side. In our study plasma membranes isolated from bovine brain microvessels were used to identify and characterize the CAA transporter(s) on both sides of the BBB. We concluded that system y+ was the only transporter present, with most activity on the abluminal membrane. System y+ was voltage dependent, and had a Km of 470±106|µM (SE) for lysine, a Ki of 34 µM for arginine, and a ki290 µM for ornithine. In the presence of Na+, system y+ was inhibited by several essential neutral amino acids. The Ki values were 3-10 times the plasma concentrations suggesting that system y+ was not as important a point of access for these amino acids as system L1. Several small non-essential amino acids (serine, glutamine, alanine and glycine) inhibited system y+ with Ki values similar to their plasma concentrations, suggesting that system y+ may account for the permeability of the BBB to these amino acids. System y+ may be important in the provision of arginine for NO synthesis. Real-time PCR and Western blotting techniques established the presence of the three known nitric oxide synthases in cerebral endothelial cells: NOS-1 (neuronal), NOS-2 (inducible), and NOS-3 (endothelial).
