Brunauer L S, Clarke S
J Biol Chem. 1986 Sep 25;261(27):12538-43.
We have measured the rate of accumulation of amino acid residues in human erythrocyte membrane and cytosolic proteins which give D-aspartic acid upon acid hydrolysis. These residues would include D-aspartic acid, D-asparagine, as well as the beta-transpeptidation product, D-isoaspartic acid. Measurements made using age (density) fractionated cells indicate that racemization at these residues occurs on membrane proteins with a t1% (the time required to convert 1% to the D configuration) of about 38.6 days. Fractionation of membrane components revealed a faster rate of racemization for intrinsic proteins than for extrinsic proteins. On the other hand, significant age-dependent racemization was not detected for cytosolic proteins, and the calculated t1% value for these proteins is at least 4 times larger. These results suggest that in the 120-day life span of an erythrocyte, significant racemization of membrane (but not cytosolic) proteins can occur. We have also determined that the rates of accumulation of these residues for erythrocyte membrane and cytosolic proteins incubated in vitro are similar to those observed in vivo. These observations are discussed in terms of the possible cellular metabolism of racemized proteins.
我们已测量了人红细胞膜和胞质蛋白中氨基酸残基的积累速率，这些蛋白在酸水解时会产生D-天冬氨酸。这些残基将包括D-天冬氨酸、D-天冬酰胺以及β-转肽产物D-异天冬氨酸。使用按年龄（密度）分级分离的细胞进行的测量表明，这些残基的消旋化发生在膜蛋白上，其t1%（将1%转化为D构型所需的时间）约为38.6天。膜成分的分级分离显示，内在蛋白的消旋化速率比外在蛋白快。另一方面，未检测到胞质蛋白有明显的年龄依赖性消旋化，这些蛋白的计算t1%值至少大4倍。这些结果表明，在红细胞120天的寿命中，膜蛋白（而非胞质蛋白）可能会发生明显的消旋化。我们还确定，体外培养的红细胞膜和胞质蛋白中这些残基的积累速率与体内观察到的速率相似。本文根据消旋化蛋白可能的细胞代谢对这些观察结果进行了讨论。
