Taylor P M, Kaur S, Mackenzie B, Peter G J
Department of Anatomy and Physiology, University of Dundee, UK.
J Exp Biol. 1996 Apr;199(Pt 4):923-31. doi: 10.1242/jeb.199.4.923.
We have measured rates of uptake of arginine, glutamine, glutamate, serine, phenylalanine and glycine in Xenopus laevis oocytes cultured for periods of up to 24h in saline in the presence or absence of a mixture of 20 amino acids at concentrations approximating those in Xenopus plasma. Amino acid supplementation increased the total intracellular amino acid concentration from 8.2 to 18.4 nmol per oocyte. Specific Na(+)-dependent amino acid transporters (systems B0,+, Xag-) exhibit 'adaptive regulation' (up-regulation during amino acid deprivation and down-regulation during amino acid supplementation). Na(+)-independent transporters of glutamate, glutamine and glycine (including system asc) display an opposite modulation in activity, which may help to combat amino-acid-induced oxidative stress by increasing the supply of glutathione precursors. Single amino acids at physiological plasma concentrations (0.47 mmol l-1 L-alanine, 0.08 mmol l-1 L-glutamate) mimicked at least some effects of the amino acid mixture. The mechanisms of transport modulation do not appear to include trans-amino acid or membrane potential effects and, in the case of Na(+)-independent transport, are independent of protein or mRNA synthesis. Furthermore, activation of protein kinase C by phorbol 12-myristate 13-acetate did not significantly affect endogenous glutamine and glutamate transport. The Xenopus oocyte appears to possess endogenous signalling mechanisms for selectively modulating the activity of amino acid transport proteins expressed in its surface membranes, a factor for consideration when using oocytes as an expression system for structure-function studies of cloned amino acid transporters.
我们测定了非洲爪蟾卵母细胞中精氨酸、谷氨酰胺、谷氨酸、丝氨酸、苯丙氨酸和甘氨酸的摄取速率。这些卵母细胞在含有或不含20种氨基酸混合物的盐溶液中培养长达24小时，氨基酸混合物的浓度接近非洲爪蟾血浆中的浓度。补充氨基酸后，每个卵母细胞的细胞内总氨基酸浓度从8.2纳摩尔增加到18.4纳摩尔。特定的Na⁺依赖性氨基酸转运体（系统B0,⁺、Xag⁻）表现出“适应性调节”（氨基酸缺乏时上调，氨基酸补充时下调）。谷氨酸、谷氨酰胺和甘氨酸的Na⁺非依赖性转运体（包括系统asc）在活性上表现出相反的调节，这可能通过增加谷胱甘肽前体的供应来对抗氨基酸诱导的氧化应激。生理血浆浓度的单一氨基酸（0.47毫摩尔/升L-丙氨酸、0.08毫摩尔/升L-谷氨酸）模拟了氨基酸混合物的至少一些作用。转运调节机制似乎不包括转氨基酸或膜电位效应，就Na⁺非依赖性转运而言，与蛋白质或mRNA合成无关。此外，佛波酯12-肉豆蔻酸酯13-乙酸酯激活蛋白激酶C对内源性谷氨酰胺和谷氨酸转运没有显著影响。非洲爪蟾卵母细胞似乎具有内源性信号传导机制，可选择性调节其表面膜中表达的氨基酸转运蛋白的活性，这是在将卵母细胞用作克隆氨基酸转运体结构-功能研究的表达系统时需要考虑的一个因素。
