Sanghvi A, Grassi E, Diven W
Proc Natl Acad Sci U S A. 1983 Apr;80(8):2175-8. doi: 10.1073/pnas.80.8.2175.
A loss in cholesterol 7 alpha-hydroxylase activity [cholesterol 7 alpha-monooxygenase; cholesterol,NADPH:oxygen oxidoreductase (7 alpha-hydroxylating), EC 1.14.13.17] was seen when rat liver microsomes were incubated in the presence of Ca2+, Mg2+, or Mn2+. The loss in enzyme activity was complete within only 5 min of incubation with Ca2+ and Mn2+, whereas Mg2+ required 10 to 15 min of incubation with microsomes to produce a similar inhibition. This effect of metal ions could be blocked if the incubations were carried out in phosphate buffer. Similarly, preincubation of microsomes in the presence of NaF completely prevented the loss in enzyme activity due to Ca2+ and Mg2+ ions, but only partially the loss due to Mn2+. These results suggest metal ion activation of an endogenous microsomal phosphatase, which in turn may inactivate cholesterol 7 alpha-hydroxylase through its dephosphorylation. Further, a dialyzable microsomal factor appears to be essential for stabilizing the enzyme, because dialysis of a microsomal suspension results in a considerable loss of enzyme activity.
当大鼠肝脏微粒体在Ca2+、Mg2+或Mn2+存在的情况下进行孵育时，胆固醇7α-羟化酶活性[胆固醇7α-单加氧酶；胆固醇，NADPH：氧氧化还原酶(7α-羟化)，EC 1.14.13.17]出现损失。与Ca2+和Mn2+孵育仅5分钟内酶活性就完全丧失，而Mg2+则需要与微粒体孵育10至15分钟才能产生类似的抑制作用。如果在磷酸盐缓冲液中进行孵育，金属离子的这种作用可以被阻断。同样，在NaF存在下对微粒体进行预孵育可完全防止由于Ca2+和Mg2+离子导致的酶活性丧失，但只能部分防止由于Mn2+导致的酶活性丧失。这些结果表明金属离子激活了内源性微粒体磷酸酶，而该磷酸酶反过来可能通过使胆固醇7α-羟化酶去磷酸化而使其失活。此外，一种可透析的微粒体因子似乎对稳定该酶至关重要，因为对微粒体悬浮液进行透析会导致酶活性大量丧失。
