Craig W S, Kyte J
J Biol Chem. 1980 Jul 10;255(13):6262-9.
Sodium and potassium ion-activated adenosine triphosphatase is known to be composed of at least two different polypeptides, alpha and beta. When a detergent-treated supernatant preparation of the enzyme is reacted with the cross-linking reagent, cupric phenanthroline, a single, covalent heterodimer is formed. This product is formed from one of each of the two polypeptides. The remaining, unreacted alpha and beta chains maintain a constant ratio to each other throughout the reaction. The same heterodimer is formed in membrane-bound enzyme when reacted with several other cross-linking reagents. The protein mass ratio between the chains in the native enzyme, determined by two methods, is 2.15 +/- 0.16. Using this value and a value of 121,000 +/- 6,000 for the molecular weight of the larger polypeptide, a molecular weight of 56,000 +/- 7,000 can be calculated for the protein portion of the smaller polypeptide. Upon removal of a substantial portion of the carbohydrate from the smaller polypeptide, a change in its electrophoretic mobility is observed, while that of the larger polypeptide remains unaffected. The apparent length of this unglycosylated small chain is 450 residues, corresponding to a molecular weight of 51,000. Taken together, these results demonstrated that the two polypeptides of the (Na+ + K+)-ATPase exist in an equimolar, noncovalent association in the native enzyme, and that the protein molecular weight of the minimum asymmetric unit is 177,000 +/- 13,000, Previous results which address the question of the quaternary structure of the ATPase are re-examined in light of these determinations.
钠钾离子激活的三磷酸腺苷酶已知至少由两种不同的多肽α和β组成。当用去污剂处理过的该酶上清液制剂与交联剂菲咯啉铜反应时，会形成一个单一的共价异二聚体。该产物由两种多肽各一个组成。在整个反应过程中，剩余未反应的α链和β链彼此保持恒定比例。当膜结合酶与其他几种交联剂反应时，也会形成相同的异二聚体。通过两种方法测定，天然酶中两条链之间的蛋白质质量比为2.15±0.16。利用这个值以及较大多肽分子量为121,000±6,000，可计算出较小多肽蛋白质部分的分子量为56,000±7,000。从小多肽中去除大部分碳水化合物后，观察到其电泳迁移率发生变化，而较大多肽的电泳迁移率保持不变。这条未糖基化小链的表观长度为450个残基，对应分子量为51,000。综上所述，这些结果表明，（Na⁺+K⁺）-ATP酶的两种多肽在天然酶中以等摩尔非共价结合形式存在，并且最小不对称单元的蛋白质分子量为177,000±13,000。根据这些测定结果，重新审视了以前关于ATP酶四级结构问题的研究结果。
