Hsu B Y, Corcoran S M, Segal S
Biochim Biophys Acta. 1986 Jul 10;859(1):79-87. doi: 10.1016/0005-2736(86)90320-2.
In papain-treated rat renal brush-border membrane vesicles, cystine uptake was enhanced under sodium gradient conditions. This effect was not observed when sodium was equilibrated across the vesicle membrane or when sodium was completely absent from the incubation medium. The increased rate of cystine uptake occurred within the first two minutes of incubation and coincided with the period of increased flux of sodium known to occur after papain treatment. Under sodium gradient conditions, the Vmax of cystine uptake by treated vesicles was 65% greater while the Km was 25% lower than the value observed in untreated membranes. The increased cystine uptake after papain treatment occurred when medium cystine was in the electroneutral form. In the absence of a sodium gradient, cystine uptake by control membranes was insensitive to changes in membrane potential and this was unaltered after papain treatment. Exposure of the membranes to papain also resulted in a profound decrease in cystine binding which occurs in native membranes incubated with cystine. The fact that cystine uptake is unchanged under sodium equilibration and even enhanced under sodium gradient conditions suggests that the component of cystine binding is not essential for cystine transport and may represent non-specific binding to membrane proteins.
在木瓜蛋白酶处理的大鼠肾刷状缘膜囊泡中，在钠梯度条件下胱氨酸摄取增强。当钠在囊泡膜两侧达到平衡时或当孵育培养基中完全没有钠时，未观察到这种效应。胱氨酸摄取速率的增加发生在孵育的前两分钟内，并且与木瓜蛋白酶处理后已知发生的钠通量增加的时期一致。在钠梯度条件下，处理过的囊泡摄取胱氨酸的Vmax比未处理膜中观察到的值大65%，而Km低25%。当培养基中的胱氨酸呈电中性形式时，木瓜蛋白酶处理后胱氨酸摄取增加。在没有钠梯度的情况下，对照膜对胱氨酸的摄取对膜电位的变化不敏感，并且在木瓜蛋白酶处理后这一点未改变。将膜暴露于木瓜蛋白酶还导致与胱氨酸孵育的天然膜中发生的胱氨酸结合显著减少。胱氨酸摄取在钠平衡下不变甚至在钠梯度条件下增强这一事实表明，胱氨酸结合成分对于胱氨酸转运不是必需的，并且可能代表与膜蛋白的非特异性结合。
