Choline sulphokinase, free of endogenous choline, APS kinase and PAPS-degrading enzymes, but containing some ATP sulphurylase, was prepared from Aspergillus nidulans.The pH optimum of the enzyme was 7.8 and maximum stability was in the region pH 8.5–10.5.Neither Mg2+ nor other metal ions, were essential for activity. Inhibition by sulphydryl reagents suggested the necessity for free SH groups in the enzyme.Only 3′-phosphoadenosine-5′-phosphosulphate (PAPS) could act as the sulphate donor and of 49 compounds tested, including many choline analogues, only three, choline, dimethylethylaminoethanol and dimethylaminoethanol could act as sulphate acceptors; the Km values being 12, 20 and 25 mM respectively. The binding sites on the enzyme for PAPS and choline appear to be independent of one another.A number of compounds were found to be inhibitors of the reaction the most potent of which was hexadecyltrimethylammonium bromide. The inhibition by p-nitrophenol, carnitine and dimethylaminopropanel-ol was competitive.The transfer of sulphate from PAPS to choline by choline sulphokinase appears to be irreversible, at least in vitro.
从曲霉菌中提取的
胆碱硫激酶不含内源性
胆碱、
APS激酶和P
APS降解酶,但含有一些
ATP硫酰化酶。该酶的最佳pH值为7.8,在pH 8.5-10.5范围内具有最大稳定性。
镁离子或其他
金属离子对酶的活性均不重要。
硫醇试剂的抑制作用表明酶中必须存在游离的SH基团。只有3′-
磷酸腺苷-5′-
磷酸硫酯(P
APS)可以作为
硫酸盐供体,在测试的49种化合物中,包括许多
胆碱类似物,只有三种化合物(
胆碱、二甲基乙基
氨基
乙醇和二甲基
氨基
乙醇)可以作为
硫酸盐受体;其Km值分别为12、20和25 mM。酶上P
APS和
胆碱的结合位点似乎相互独立。发现许多化合物可以抑制反应,其中最有效的化合物是
十六烷基三甲基溴化铵。
对硝基苯酚、肉碱和二甲基
氨基
丙醇的抑制作用是竞争性的。
胆碱硫激酶将
硫酸盐从P
APS转移到
胆碱的过程似乎是不可逆的,至少在体外是这样。