Yuan P M, Talent J M, Gracy R W
Mech Ageing Dev. 1981 Oct;17(2):151-62. doi: 10.1016/0047-6374(81)90081-6.
Triosephosphate isomerase exhibits acidic electrophoretic subforms in many tissues and these isozymes appear to increase during aging of erythrocytes and the eye lens. Incubation of the pure enzyme under mild alkaline conditions results in the generation of acidic forms which are identical to those found in vivo. Structural analysis of these isozymes from both in vivo and in vitro studies showed that they are the result of deamidation of two specific asparagines (Asn-15 and Asn-71). These labile asparagines are located in the subunit-subunit contact sites, and the deamidations introduce a total of four new negative charges in the contact site. The positions of the new aspartic acid residues are juxtaposed, thus creating charge-charge interactions which cause the dimeric enzyme to dissociate more readily. These studies (1) explain the molecular basis for the acidic isozymes observed in many tissues, (2) show that the deamination process is spontaneous and requires no intrinsic cell factors, (3) show that the deamination occurs in a sequential fashion with the deamidation of Asn-71 preceding the deamidation of Asn-15, and (4) suggest that proteolytic degradation processes may become altered during aging resulting in the accumulation of the deamidated intermediates of the normal catabolic process.
磷酸丙糖异构酶在许多组织中呈现酸性电泳亚型，并且这些同工酶在红细胞和晶状体老化过程中似乎会增加。在温和碱性条件下孵育纯酶会导致产生与体内发现的酸性形式相同的形式。对这些来自体内和体外研究的同工酶的结构分析表明，它们是两个特定天冬酰胺（Asn-15和Asn-71）脱酰胺作用的结果。这些不稳定的天冬酰胺位于亚基-亚基接触位点，脱酰胺作用在接触位点总共引入四个新的负电荷。新天冬氨酸残基的位置并列，从而产生电荷-电荷相互作用，导致二聚体酶更容易解离。这些研究（1）解释了在许多组织中观察到的酸性同工酶的分子基础，（2）表明脱氨过程是自发的，不需要内在细胞因子，（3）表明脱氨以顺序方式发生，Asn-71的脱酰胺作用先于Asn-15的脱酰胺作用，并且（4）表明蛋白水解降解过程在老化期间可能会改变，导致正常分解代谢过程的脱酰胺中间体积累。
