Protein carboxymethyltransferase and its physiological significance

蛋白质羧甲基转移酶是一种细胞质酶，以s -腺苷基l -蛋氨酸为甲基供体，甲基化蛋白质底物中的侧链羧基。然而，该酶的天然底物及其生理功能尚不清楚。因此，在本研究中，我们打算研究其在大脑中的内源性底物，并发现以下观察结果。当脑切片与l -蛋氨酸孵育或当细胞质部分与s -腺苷型l -蛋氨酸孵育时，一些多肽被甲基化。细胞质中的两种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。高分子量微管相关蛋白比微管蛋白或卵清蛋白具有更高的甲基化活性，作为标准底物：每mol高分子量微管相关蛋白可甲基化约20 mmol，每mol微管蛋白可甲基化约5、2 mmol的微管蛋白和10 mmol的卵清蛋白。在脑细胞中发现了其他几种接受甲基的蛋白质，尽管这些蛋白质尚未被鉴定。特别是，细胞质部分除了含有微管蛋白外，还含有一个30 KD蛋白作为主要的甲基接受蛋白。该蛋白可能在细胞活动中具有重要作用，因为在老年大鼠脑中未检测到与s -腺苷l -蛋氨酸体外甲基化的30 KD蛋白。众所周知，微管在细胞有丝分裂中起着重要的作用。我们还知道，在癌细胞中，细胞分裂频繁而活跃，而脑细胞则不分裂。因此，我们研究了腹水-肝癌细胞微管的甲基接受活性，发现微管也是该酶的良好底物。细胞分裂和羧甲基化之间似乎没有关系。需要进一步的研究来确定这种反应的生理意义。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