Protein carboxymethyltransferase and its physiological significance

蛋白质羧基甲基转移酶是一种细胞溶质酶，其以S-腺苷L-甲硫氨酸作为甲基供体，甲基化蛋白质底物中的侧链羧基。然而，这种酶的天然底物以及生理功能一直是未知的。因此，在本研究中，我们试图研究其在脑内的内源性底物，并发现以下观察结果：当脑切片与L-蛋氨酸孵育时，或当胞浆部分与S-腺苷L-蛋氨酸孵育时，几种多肽被甲基化。细胞质中的两个甲基接受蛋白被鉴定为微管蛋白和高分子量微管相关蛋白。转移到这些蛋白质上的甲基部分在碱性pH下是不稳定的。高分子量微管相关蛋白显示出比微管蛋白或卵清蛋白更高的甲基接受活性，所述微管蛋白或卵清蛋白用作标准底物：约20 mmol的高分子量微管相关蛋白 ...更多信息 s，2 mmol微管蛋白和10 mmol卵清蛋白的甲基化每摩尔的每种蛋白质。特别是，除了微管蛋白外，胞质组分还含有30 KD蛋白作为主要的甲基接受蛋白。在老年大鼠脑中未发现30 KD的S-腺苷甲硫氨酸甲基化蛋白，因此该蛋白可能在细胞活动中起重要作用。众所周知，癌细胞的细胞分裂频繁而活跃，而脑细胞则不分裂。因此，我们研究了肝癌腹水细胞微管的甲基接受活性，发现微管也是该酶的良好底物。细胞分裂与羧甲基化之间似乎没有联系，需要进一步的研究来确定这一反应的生理意义。少

Protein carboxylmethyltransferase is a cytosolic enzyme that methylates side-chain carboxyl groups in proteinaceous substrates, with S-adenosyl L-methionine as the methyl donor. However, natural substrates as well as the physiological function of this enzyme have been unknown. Therefore, in this study we intended to examine its endogenous substrates in brain and found the following observations.Several polypeptides were methylated when brain slices were incubated with L-methionine or when cytosolic fraction were incubated with S-adenosyl L-methionine. Two methyl-accepting proteins in the cytoplasm were identified as tubulin and high molecular weight microtubule-associated proteins. The muthyl-moiety transferred to these proteins was labile at alkaline pH. The high molecular weight microtubule-associated proteins showed higher methyl-accepting activity than tubulin or ovalbumin, which was used as a standard substrate: about 20 mmol of high molecular weight microtubule-associated protein … More s, 2 mmol of tubulin and 10 mmol of ovalbumin were methylated per mol of each protein.Several other methyl-accepting proteins were found in brain cells, although these proteins have not yet been identified. In particular, the cytosolic fraction contains a 30 KD protein as a major methyl-accepting protein in addition to microtubule proteins. This protein may have some significant role in cellular activities, because no 30 KD protein methylated in vitro with S-adenosyl L-methionine was detected in the brains of aged rats.It is well-known that microtubules plays a great role in cell-mitosis. It is also known that in cancer cells cell-division occurrs frequently and actively, whereas brain cells do not divide. Therefore we studied the methyl-accepting activity of microtubules of ascites-hepatoma cells and found that the microtubules was also a good substrate for this enzyme. There seems no relation between cell division and carboxylmethylation.Further studies are required to determine the physiological significance of this reaction. Less