Erecińska M, Zaleska M M, Chiu L, Nelson D
Department of Pharmacology, University of Pennsylvania School of Medicine, Philadelphia 19104-6084.
J Neurochem. 1991 Aug;57(2):491-8. doi: 10.1111/j.1471-4159.1991.tb03778.x.
Isolated rat brain synaptosomes accumulated L-asparagine with a Km value of 348 microM and a Vmax value of 3.7 nmol/mg of protein/min at 28 degrees C. Uptake of L-asparagine was inhibited by the presence of L-glutamine, whereas transport of L-glutamine was blocked by L-asparagine. Alanine, serine, cysteine, threonine, and, in particular, leucine were also inhibitory whereas alpha-(methylamino)isobutyrate, ornithine, lysine, arginine, and glutamate were much less effective blockers. Transport of L-asparagine had a substantial sodium-dependent component, whereas that of the D-stereoisomer was almost unaffected by the presence or absence of the cation. L-Asparagine was accumulated to a maximal gradient, [L-Asn]i/[L-Asn]o, of 20-30, and this value was reduced to 5-6 by withdrawal of sodium or addition of high [KCI]. A plot of log [Na+]o/[Na+]i against the log [L-Asn]i/[L-Asn]o had a slope close to I, which indicates that a single sodium ion is transported inward with each asparagine molecule. It is postulated that uptake of L-asparagine occurs, to a large extent, in cotransport with Na+ and that it utilizes the sodium chemical gradient and the membrane electrical potential as the source of energy. The similarity between the L-asparagine and L-glutamine transport systems and the reciprocal inhibition of influx of the two amino acids suggest that the same mechanism is responsible for glutamine accumulation. This could explain the high [Gln]i maintained by the brain in vivo.
在28℃时，分离出的大鼠脑突触体积累L-天冬酰胺，其米氏常数（Km）值为348微摩尔，最大反应速度（Vmax）值为3.7纳摩尔/毫克蛋白质/分钟。L-谷氨酰胺的存在会抑制L-天冬酰胺的摄取，而L-天冬酰胺会阻断L-谷氨酰胺的转运。丙氨酸、丝氨酸、半胱氨酸、苏氨酸，尤其是亮氨酸也具有抑制作用，而α-（甲基氨基）异丁酸、鸟氨酸、赖氨酸、精氨酸和谷氨酸的阻断作用则小得多。L-天冬酰胺的转运有很大一部分依赖于钠离子，而D-立体异构体的转运几乎不受阳离子存在与否的影响。L-天冬酰胺积累到最大梯度，即细胞内浓度与细胞外浓度之比[L-Asn]i/[L-Asn]o为20 - 30，通过去除钠离子或添加高浓度[KCl]，该值会降至5 - 6。以log [Na+]o/[Na+]i对log [L-Asn]i/[L-Asn]o作图，斜率接近1，这表明每个天冬酰胺分子向内转运时伴随一个钠离子。据推测，L-天冬酰胺的摄取在很大程度上是与钠离子协同转运的，并且它利用钠离子的化学梯度和膜电位作为能量来源。L-天冬酰胺和L-谷氨酰胺转运系统之间的相似性以及两种氨基酸流入的相互抑制表明，相同的机制负责谷氨酰胺的积累。这可以解释大脑在体内维持的高[Gln]i水平。
