Liposomal cholesterol binding to steroid-free cytochrome P450scc: effects of fatty acyl and head groups in phospholipids

Inhibition and stimulation of activity of purified recombinant CYP11A1 by therapeutic agents

In vertebrates, the biosynthesis of steroid hormones is initiated by cytochrome P450 CYP11A1 which converts cholesterol to pregnenolone. We investigated whether some of the experimental and FDA-approved therapeutic agents alter the activity of CYP11A1 in the reconstituted system in vitro. We found that under the experimental conditions used and when phospholipids are included, ketoconazole, posaconazole, carbenoxolone, and selegiline inhibit CYP11A1-mediated production of pregnenolone by at least 67%. Conversely, pemirolast, clobenpropit, desogestrel, dexmedetomidine, and tizanidine stimulate the enzyme activity by up to 70%. We then evaluated the identified inhibitors and activators for spectral binding to CYP11A1 and their effect on enzyme activity in the absence of phospholipids. The data obtained provide insight into how different drugs interact with CYP11A1 and demonstrate that P450 association with the lipid bilayer determines, in many cases, a drug's effect on enzyme activity.

Features of the retinal environment which affect the activities and product profile of cholesterol-metabolizing cytochromes P450 CYP27A1 and CYP11A1

The retina is the sensory organ in the back of the eye which absorbs and converts light to electrochemical impulses transferred to the brain. Herein, we studied how retinal environment affects enzyme-mediated cholesterol removal. We focused on two mitochondrial cytochrome P450 enzymes, CYPs 27A1 and 11A1, which catalyze the first steps in metabolism of cholesterol in the retina and other tissues. Phospholipids (PL) from mitochondria of bovine neural retina, retinal pigment epithelium, liver and adrenal cortex were isolated and compared for the effect on kinetic properties of purified recombinant CYPs in the reconstituted system in vitro. The four studied tissues were also evaluated for the mitochondrial PL and cholesterol content and levels of CYPs 27A1, 11A1 and their redox partners. The data obtained were used for modeling the retinal environment in the in vitro enzyme assays in which we detected the P450 metabolites, 22R-hydroxycholesterol and 5-cholestenoic acid, unexpectedly found by us in the retina in our previous studies. The effect of the by-product of the visual cycle pyridinium bis-retinoid A2E on kinetics of CYP27A1-mediated cholesterol metabolism was also investigated. The results provide insight into the retina's regulation of the enzyme-mediated cholesterol removal.

Phospholipids modify substrate binding and enzyme activity of human cytochrome P450 27A1

Cytochrome P450 27A1 (P450 27A1) is an important metabolic enzyme involved in bile acid biosynthesis and the activation of vitamin D3 in mammals. Recombinant P450 27A1 heterologously expressed in Escherichia coli was found to be copurified with phospholipids (PLs). The PL content varied in different preparations and was dependent on the purification protocol. A link between the increased amounts of PLs and deterioration of the enzyme substrate binding properties was also observed. Tandem negative ionization mass spectrometry identified phosphatidylglycerol (PG) as the major PL copurified with P450 27A1. Subsequent reconstitution of P450 into exogenous PG vesicles assessed the effect of this contamination on substrate binding and enzyme activity. Two other PLs, phosphatidylethanolamine (PE) and phosphatidylserine (PS), were also tested. PG and PE increased the Kd for 5beta-cholestane-3alpha,7alpha,12alpha-triol and cholesterol binding, whereas PS had no effect on either substrate binding. PG and PE did not significantly alter 5beta-cholestane-3alpha,7alpha,12alpha-triol hydroxylase activity and even stimulated cholesterol hydroxylase activity. PS inhibited 5beta-cholestane-3alpha,7alpha,12alpha-triol hydrolyase activity and had no effect on cholesterol hydroxylase activity. Our study shows the potential for PLs to regulate the activity of P450 27A1 in vivo and alter the amount of cholesterol degraded through the "classical" and "alternative" bile acid biosynthetic pathways.

Lipid regulation of bovine cytochrome P450(11) beta activity

Purified bovine adrenocortical cytochrome P450(11) beta has been reconstituted into phospholipid vesicles using a detergent dialysis procedure. Using this reconstituted system, we have examined the effect of changes in the fatty acyl substituents of the lipids on the catalytic activity of the enzyme. The studies reported here show that cytochrome P450(11) beta exhibits a completely different response to changes in the fatty acyl groups from that shown by cytochrome P450scc. Cytochrome P450(11) beta displays maximal activity in lipid vesicles composed of saturated lipids, such as dipalmitoyl and dimyristoyl phosphatidylcholines, with turnover numbers ranging from 35 to 60 min-1. Incremental increases of phospholipids such as diphytanoyl and dioleoyl phosphatidylcholines result in a progressive inhibition of 11 beta hydroxylase activity; most of this kinetic effect is attributable to a significant decrease in Vmax accompanied by modest changes in Km for the steroid substrate deoxycorticosterone. Diphosphatidyl glycerol (cardiolipin), which has been previously shown to activate cytochrome P450scc, is a potent inhibitor of the 11 beta hydroxylase activity of cytochrome P450(11) beta, with half maximum inhibition observed in vesicles containing 4-5 mol% diphosphatidyl glycerol. Kinetic analysis demonstrates that this inhibition by diphosphatidyl glycerol is reflected in both a decrease in Vmax and relatively large increases (up to sevenfold) in Km for the steroid substrate. These effects on the 11 beta hydroxylase activity may have important implications for the in vivo regulation of not only the 11 beta hydroxylase activity, but also the other catalytic activities of this enzyme, particularly 18- and 19-hydroxylase and oxidase activities.

Cholesterol metabolism by purified cytochrome P-450scc is highly stimulated by octyl glucoside and stearic acid exclusively in large unilamellar phospholipid vesicles

Cholesterol side-chain cleavage (CSCC) catalyzed by purified bovine adrenal mitochondrial cytochrome P-450scc is highly dependent on the vesicles that supply cholesterol. Six-fold higher rates are achieved with large unilamellar dioleoylphosphatidylcholine vesicles (diameter 150 nm) prepared by octyl glucoside (OG) dialysis (DOPC-LUV) than with small sonicated vesicles (diameter 30 nm) (DOPC-SUV) (Vmax = 25 and 4 min-1, respectively. Extensive dialysis that may remove OG decreased Vmax rates for DOPC-LUV almost to rates seen with DOPC-SUV. These dialyzed DOPC-LUV were, however, very sensitive to addition of OG (EC50 = 2.5 microM, 4.3-fold stimulation) while DOPC-SUV were only weakly affected (EC50 = 100 microM, 1.6-fold stimulation). This enhancement of CSCC in LUV by OG only occurred when the cholesterol:DOPC exceeded 0.1 and was associated with a 15-fold increase in the Km for cholesterol. Structural changes in both SUV and LUV at high cholesterol:DOPC ratios (0.1-1) were indicated by decreases in internal volume that were insensitive to OG and did not affect the external diameters. Stearic acid produced a similar stimulation of CSCC in LUV (EC50 = 50 microM) and had no effect on SUV. The Vmax for CSCC, produced by OG activation of DOPC-LUV, is comparable to the highest attained for cytochrome P-450scc (Tween 20/cholesterol). In LUV, a minor proportion of OG (1-5% of cholesterol) is thus sufficient to generate a domain of reactive cholesterol that maintains a near-optimum turnover. This increased CSCC was paralleled by increased binding of cholesterol to P-450scc, suggesting that this cholesterol is more readily donated by the membrane to the cytochrome.

Quantitation of interaction between cytochrome P-450scc and adrenodoxin--analysis in the median UV-region by second derivative spectroscopy

Interaction between the essential protein components of the bovine adrenal mitochondrial enzyme system (cytochrome P-450scc, adrenodoxin and adrenodoxin reductase) were studied in the median UV-region utilizing second derivative difference spectroscopy. Complex formation of cytochrome P-450scc with adrenodoxin induces a signal in the second derivative difference spectrum which can be attributed to tyrosine due to its minimum at 283 nm. Based on this signal cytochrome P-450scc was titrated with adrenodoxin in dependence on different effectors (reductase, phospholipid, cholesterol). The dissociation constants (Kd) of the P-450scc/adrenodoxin complexes derived therefrom revealed an increasing affinity between both components starting from titrations in buffer solution without additional components up to the completely reconstituted system. A high affinity between P-450scc and adrenodoxin corresponds to a high turnover rate of cholesterol. Dissociation constants of the P-450scc/adrenodoxin complex were also derived from spectral changes in the Soret region. But these data do not correlate with the substrate turnover.

Transduction of ACTH signal from plasma membrane to mitochondria in adrenocortical steroidogenesis. Effects of peptide, phospholipid, and calcium

The conversion of cholesterol to pregnenolone by adrenocortical mitochondria is the rate-limiting step in steroidogenesis. This process is stimulated dramatically by the action of ACTH through the sequential reactions, in which adenyl cyclase, cAMP-dependent protein kinase, cholesterol esterase and ribosomal protein synthesis are all involved. The de novo synthesized protein, the so-called labile protein with a half-life of approx 10 min, is believed to stimulate the cholesterol side chain cleavage reaction by an unknown mechanism. Available evidence indicates that the electron on transfer reaction from NADPH to P-450scc is mediated rapidly by adrenodoxin reductase and p-450 scc. In addition, these redox components are inactivated slowly with a half-life of 3.5 days after hypophysectomy. It is known that the corticoid output from adrenocortical cells starts within 5 min and reaches the maximum after 10-15 min of ACTH administration to animals. One can assume that under normal physiological conditions, both O2 and NADPH are not limiting. Additionally, mitochondrial inner membranes are poor in cholesterol. In this context, the availability of substrate cholesterol to P450scc is the most likely candidate for the regulatory mechanism.

Importance of the unsaturated fatty acyl group of phospholipids in their stimulatory role on rat adrenal mitochondrial steroidogenesis

We have investigated the relationship between chemical properties of various phospholipids and their steroidogenic activity for adrenal mitochondria prepared from dexamethasone/cycloheximide-treated quiescent rats. Phospholipids studied include those purified from bovine and rat adrenal mitochondria, obtained from commercial sources, and reduced by catalytic hydrogenation. All phospholipids were subjected to analysis of their fatty acyl groups and examined for their steroidogenic activities. From these experiments, we came to the following conclusions: The degree of unsaturation in the fatty acyl moiety correlates with their steroidogenic activities regardless of head groups. Namely, polyunsaturation appears to be more important than monounsaturation with a relative insensitivity toward their head groups. Saturated phospholipids exhibit an inhibition for steroidogenic activity. Cardiolipins, which are steroidogenic, appear exceptional. Their head groups may partially participate in the activity in addition to their high content of unsaturated fatty acids. The importance of the adrenoyl (C22:4) group in phospholipids is suggested.