Van Winkle L J, Mann D F, Wasserlauf H G, Patel M
Department of Biochemistry, Chicago College of Osteopathic Medicine, Downers Grove, IL 60515.
Biochim Biophys Acta. 1992 Jun 30;1107(2):299-304. doi: 10.1016/0005-2736(92)90416-j.
L-Glutamate and L-cystine appeared to compete for transport via a mediated Na(+)-independent transport process in 1- and 2-cell conceptuses. Not only did these substances competitively inhibit each others' uptake by conceptuses, but their Ki values for inhibition were about equal to their Km values for transport in 1-cell conceptuses. Moreover, the transport process interacted strongly with L-amino acids that had 3-6 atoms in a chain between their negatively charged groups, whereas it interacted weakly or not at all with amino acids that did not have these characteristics or that were N-methylated. Transport of anionic amino acids was not altered greatly by pH in the range 4.5-8.0, but transport of L-cystine was much faster at higher pH values. The slower cystine transport at lower pH values was due primarily to protonation of its second amino group. A small increase in the degree of deprotonation of cystine's carboxyl groups also probably contributed slightly to its faster transport at higher pH values. By all of these criteria, the transport process in conceptuses appears to be a form of amino acid transport system xc-. System xc- activity was several-fold higher in 1- than in 2-cell conceptuses. Similarly, L-glutamate uptake by unfertilized eggs was relatively rapid, whereas it was much slower in immature, fully-grown oocytes. System xc- activity in 1-cell conceptuses also appeared to increase in response to the oxidative stress of culture, whereas no such increase was observed for 2-cell conceptuses. We suggest that this transient increase in the activity of system xc- activity during development of 2-cell conceptuses from immature, fully-grown oocytes could help protect unfertilized and fertilized eggs from oxidative stresses in situ.
在1细胞和2细胞期的孕体中，L-谷氨酸和L-胱氨酸似乎通过一种介导的、不依赖钠离子的转运过程进行转运竞争。这些物质不仅能竞争性抑制孕体对彼此的摄取，而且它们的抑制常数（Ki）值大约等于其在1细胞期孕体中的转运米氏常数（Km）值。此外，该转运过程与带负电荷基团之间链上有3至6个原子的L-氨基酸强烈相互作用，而与不具有这些特征或被N-甲基化的氨基酸相互作用微弱或根本不相互作用。在4.5至8.0的pH范围内，阴离子氨基酸的转运没有太大变化，但L-胱氨酸在较高pH值下的转运速度要快得多。较低pH值下胱氨酸转运较慢主要是由于其第二个氨基的质子化。胱氨酸羧基去质子化程度的小幅增加可能也对其在较高pH值下更快的转运略有贡献。根据所有这些标准，孕体中的转运过程似乎是氨基酸转运系统xc-的一种形式。1细胞期孕体中的系统xc-活性比2细胞期孕体中的高几倍。同样，未受精卵对L-谷氨酸的摄取相对较快，而在未成熟的、完全成熟的卵母细胞中则慢得多。1细胞期孕体中的系统xc-活性似乎也会因培养的氧化应激而增加，而2细胞期孕体中未观察到这种增加。我们认为，从未成熟的、完全成熟的卵母细胞发育到2细胞期孕体过程中，系统xc-活性的这种短暂增加有助于原位保护未受精卵和受精卵免受氧化应激。
