Smolin L A, Benevenga N J
Am J Clin Nutr. 1984 May;39(5):730-7. doi: 10.1093/ajcn/39.5.730.
Competition between homocyst(e)ine and cyst(e)ine for binding sites on plasma proteins was examined both in vivo and in vitro. Plasma-free cyst(e)ine concentrations were elevated in rats fed diets adequate or deficient in vitamin B6 containing 2.4% L-cystine; however, plasma protein-bound cysteine was not increased. Feeding high cystine diets did not slow the accumulation or decrease the concentration of plasma protein-bound or free homocyst(e)ine in vitamin B6-deficient rats. An in vitro experiment demonstrated that plasma protein-bound cysteine increased when plasma was incubated with increasing concentrations of cysteine, and decreased with increasing homocysteine concentrations. Plasma protein-bound homocysteine concentration was increased by increasing the concentration of homocysteine in the incubation medium; however, increasing the cysteine concentration failed to decrease bound homocysteine. The affinity of homocysteine for binding sites on plasma proteins appeared to be high because cysteine did not displace the bound homocysteine. Therefore, feeding a high cystine diet is unlikely to cause a decrease in bound homocysteine in homocystinuric patients due to competition for binding sites, but may still be beneficial because plasma cystine concentrations are below normal in individuals with homocystinuria.
在体内和体外研究了同型半胱氨酸和胱氨酸对血浆蛋白结合位点的竞争。给喂食含2.4% L-胱氨酸的维生素B6充足或缺乏日粮的大鼠，其血浆游离胱氨酸浓度升高；然而，血浆蛋白结合的半胱氨酸并未增加。给维生素B6缺乏的大鼠喂食高胱氨酸日粮，并未减缓血浆蛋白结合或游离同型半胱氨酸的积累或降低其浓度。一项体外实验表明，当血浆与浓度不断增加的胱氨酸孵育时，血浆蛋白结合的半胱氨酸增加，而与同型半胱氨酸浓度增加时则降低。通过增加孵育介质中同型半胱氨酸的浓度，血浆蛋白结合的同型半胱氨酸浓度增加；然而，增加胱氨酸浓度未能降低结合的同型半胱氨酸。由于胱氨酸不能取代结合的同型半胱氨酸，同型半胱氨酸对血浆蛋白结合位点的亲和力似乎很高。因此，给同型胱氨酸尿症患者喂食高胱氨酸日粮不太可能因竞争结合位点而导致结合的同型半胱氨酸减少，但可能仍然有益，因为同型胱氨酸尿症患者的血浆胱氨酸浓度低于正常水平。
