Jeffery D R, Roth J A
J Neurochem. 1984 Mar;42(3):826-32. doi: 10.1111/j.1471-4159.1984.tb02755.x.
Catechol-O-methyltransferase (COMT; E.C. 2.1.1.6) from human frontal cortex occurs in both a soluble and membrane-bound form. Attempts to solubilize the membrane-bound transferase by repeated washing or by extraction into solutions of high ionic strength were unsuccessful. The finding that Triton X-100 was capable of solubilizing membrane-bound COMT suggested that the membrane-bound transferase is an integral membrane protein. The membrane-bound and soluble enzymes did not differ in their requirements for magnesium ions or in their pH-activity profiles; both enzymes showed an optimum near pH 8.0 when assayed in phosphate buffer. In addition, the two enzymes did not differ in the degree of inhibition caused by CaCl2, both enzymes displaying 65% inhibition at 2.5 mM CaCl2. The competitive inhibitors tropolone and nordihydroguaiaretic acid displayed Ki values for the membrane-bound transferase five- to 10-fold lower than those observed for the soluble transferase. Solubilization of membrane-bound COMT in Triton X-100 resulted in an increase in the apparent Km value of the membrane-bound transferase for dopamine. The increase in Km appeared to be due to apparent competitive inhibition by Triton X-100 and reached a limiting value of approximately 80 microM. These results confirm that membrane-bound COMT is an integral membrane protein that may be structurally distinct from soluble COMT.
人额叶皮质中的儿茶酚-O-甲基转移酶（COMT；E.C. 2.1.1.6）以可溶性和膜结合形式存在。通过反复洗涤或提取到高离子强度溶液中来溶解膜结合转移酶的尝试均未成功。发现Triton X-100能够溶解膜结合的COMT，这表明膜结合转移酶是一种整合膜蛋白。膜结合酶和可溶性酶对镁离子的需求以及它们的pH-活性曲线并无差异；在磷酸盐缓冲液中测定时，两种酶在pH 8.0左右均表现出最佳活性。此外，两种酶对CaCl2引起的抑制程度也没有差异，在2.5 mM CaCl2时，两种酶均表现出65%的抑制率。竞争性抑制剂托酚酮和去甲二氢愈创木酸对膜结合转移酶的Ki值比对可溶性转移酶观察到的值低五到十倍。在Triton X-100中溶解膜结合的COMT会导致膜结合转移酶对多巴胺的表观Km值增加。Km值的增加似乎是由于Triton X-100的表观竞争性抑制作用，并且达到了约80 microM的极限值。这些结果证实膜结合的COMT是一种整合膜蛋白，其结构可能与可溶性COMT不同。
