Loss of Cholesterol 7 α -hydroxylase Activity in vitro in the Presence of Bivalent Metal Ions and by Dialysis of Rat Liver Microsomes

A loss in cholesterol 7α -hydroxylase activity [cholesterol 7α -monooxygenase; cholesterol, NADPH: oxygen oxidoreductase (7α -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 7α -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.