Baslow Morris H, Guilfoyle David N
Center for Neurochemistry, Nathan S. Kline Institute for Psychiatric Research, 140 Old Orangeburg Road, Orangeburg, NY 10962, USA.
Anal Biochem. 2002 Dec 15;311(2):133-8. doi: 10.1016/s0003-2697(02)00403-7.
N-acetyl-L-aspartic acid (NAA) is an amino acid present in the vertebrate brain that is synthesized and stored primarily in neurons, although it cannot be hydrolyzed in these cells. Nonetheless, neuronal NAA is dynamic and turns over more than once each day by cycling, via extracellular fluids (ECF), between neurons and catabolic compartments in oligodendrocytes. One important role of the NAA intercompartmental cycle appears to be osmoregulatory, and in this role it may be the primary mechanism for the removal of metabolic water, against a water gradient, from myelinated neurons. However, the number of water molecules that might be cotransported to ECF per NAA molecule released is as yet unclear. In this investigation, using a proton nuclear magnetic resonance method and diffusion measurements at two magnetic field strengths on water and NAA phantoms in vitro, the effect of NAA on the diffusion coefficient of water has been measured, and a ratio (K) of obligated water molecules per molecule of NAA has been determined. For NAA measured at 100mM and 3 Tesla K=24 and at 7 Tesla K=14. Based on these results, apparent K(NAA) varies inversely with field strength, and with a computed field strength factor of 2.55mmol water/unit Tesla, K(NAA) in the absence of any applied magnetic field strength would be 32.
N-乙酰-L-天冬氨酸（NAA）是脊椎动物大脑中存在的一种氨基酸，主要在神经元中合成和储存，尽管它在这些细胞中不能被水解。尽管如此，神经元中的NAA是动态的，每天通过细胞外液（ECF）在神经元和少突胶质细胞的分解代谢区室之间循环周转不止一次。NAA在不同区室间循环的一个重要作用似乎是渗透调节，在这个作用中，它可能是有髓神经元逆水梯度去除代谢水的主要机制。然而，每释放一个NAA分子可能共转运到ECF中的水分子数量尚不清楚。在本研究中，使用质子核磁共振方法并在两个磁场强度下对体外水和NAA模型进行扩散测量，测定了NAA对水扩散系数的影响，并确定了每个NAA分子的结合水分子比率（K）。对于在100mM和3特斯拉下测量的NAA，K = 24，在7特斯拉下K = 14。基于这些结果，表观K（NAA）与场强成反比，并且计算得出场强因子为2.55mmol水/单位特斯拉，在没有任何外加磁场强度的情况下，K（NAA）将为32。
