Loss of cholesterol 7 alpha-hydroxylase activity in vitro in the presence of bivalent metal ions and by dialysis of rat liver microsomes

Reversible inactivation of cytochrome P450 by alkaline earth metal ions: auxiliary metal ion induced conformation change and formation of inactive P420 species in CYP101

The effects of the divalent alkaline-earth metal ions (Ca2+ and Mg2+) on the substrate binding affinity, spin-state transition at the heme active site, conformational properties as well as the stability of the active form of cytochrome P450cam (CYP 101) have been investigated using various spectroscopic and kinetic methods. The divalent cations were found to have two types of effects on the enzyme. At the initial stage the alkaline-earth metal ion facilitated enhanced binding of the substrate and formation of the high-spin form of the heme active center of the enzyme compared to that in absence of any metal ion. However, analogous to many other mono-valent metal ions, the alkaline-earth metal ions were also less efficient than K+ in promoting the substrate binding and spin-transition properties of the enzyme. The auxiliary metal ions were shown to cause small but distinct change in the circular dichroism spectra of the substrate-free enzyme in the visible region, indicating that the tertiary structure around the heme was perturbed on binding of the auxiliary metal ion to the enzyme. The effect of the auxiliary metal ion was found to be more prominent in the WT enzyme compared to the Y96F mutant of P450cam suggesting that the Tyr 96 residue plays an important role in mediating the effects of the auxiliary metal ions to the active site of the enzyme. At the second stage of interaction, the alkaline-earth metal ions were found to slowly convert the enzyme into an inactive P420 form, which could be reversibly re-activated by addition of KCl. The results have been discussed in the light of understanding the mechanism of inactivation of certain mammalian P450 enzymes by these alkaline-earth metal ions.

Regulation of three key enzymes in cholesterol metabolism by phosphorylation/dephosphorylation

Our laboratories have investigated the role of phosphorylation/dephosphorylation in the regulation of three key enzymes in cholesterol metabolism. 3-Hydroxy-3-methylglutarylcoenzyme A (HMG-CoA) reductase (EC 1.1.1.34), the major regulatory enzyme in cholesterol biosynthesis, is inhibited by phosphorylation. Acyl-CoA:cholesterol O-acyltransferase (ACATase; EC 2.3.1.26) and cholesterol 7 alpha-hydroxylase (EC 1.14.13.7), key regulatory enzymes in the utilization of cholesterol, are activated by phosphorylation. In view of these results, we propose that short-term regulation of the concentration of intracellular unesterified cholesterol is achieved by a coordinate phosphorylation/dephosphorylation of these three enzymes. For example, if cholesterol enters the liver cell, HMG-CoA reductase would be inhibited by phosphorylation and biosynthesis of cholesterol would be reduced; however, reactions utilizing cholesterol would be activated, due to the phosphorylation of ACATase and cholesterol 7 alpha-hydroxylase. Thus, the phosphorylation/dephosphorylation of these three enzymes provides an elegant short-term mechanism for the homeostasis of intracellular unesterified cholesterol.

Evidence for phosphorylation/dephosphorylation of rat liver acyl-CoA:cholesterol acyltransferase

Acyl-coenzyme A:cholesterol O-acyltransferase (ACATase; EC 2.3.1.26) is a membrane-bound microsomal enzyme that catalyzes the formation of long-chain fatty-acyl cholesterol esters in rat liver and other tissues. This enzyme is important in regulating the concentration of unesterified cholesterol in the cell. Having recently demonstrated that rat liver 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA reductase; EC 1.1.1.34), the major regulatory enzyme in cholesterol biosynthesis, undergoes in vivo phosphorylation and inactivation after a single cholesterol meal, we decided to test the hypothesis that the enzyme ACATase, important in cholesterol utilization and storage, is also subject to regulation by phosphorylation/dephosphorylation. The results show that rat liver ACATase can be reversibly inactivated/activated, in vitro, by incubation conditions that favor dephosphorylation/phosphorylation. Activation was also achieved by using a partially purified protein kinase extracted from microsomes. It is significant that HMG-CoA reductase is inactivated by phosphorylation whereas ACATase is activated by phosphorylation. ACATase is, therefore, regulated by phosphorylation in a manner exactly opposite to that of HMG-CoA reductase. We propose that the coordinate regulation of ACATase and HMG-CoA reductase by phosphorylation/dephosphorylation provides a mechanism for short-term intracellular cholesterol homeostasis.