Control of substrate flux and adrenal cytochrome P-450

Magnetic resonance study of the structure and functions of cytochrome P450

Cytochrome P450 is a membrane-bound enzyme providing oxidation of numerous organic compounds in organisms. The objective of this review is to show the wide possibilities that are provided by Electron Spin Resonance (ESR) and Nuclear Magnetic Resonance (NMR) techniques to the study of the structure and functions of this unique enzyme. High sensitivity of ESR spectra of cytochrome P450 to its functional state and interaction with substrates and inhibitors is illustrated. NMR and proton relaxation make it possible to obtain unique information about the structure of the active center of cytochrome P450 under physiological conditions. ESR and NMR methods allow one to obtain structural data on location of substrates, inhibitors, and their spin-labeled analogs with respect to Fe3+ ions in the enzyme-active center. Of special interest seems to be coupling of ESR with the affinity modification method. For this purpose, the spin-labeled analogs of cytochrome P450 substrates containing alkylating groups were used. As a result, an important datum has been obtained on the structure of active centers of cytochrome P450 in microsomes and in a highly purified state. In conclusion, the problems of the structure and functions of cytochrome P450, which can be most efficiently resolved with the use of magnetic resonance methods, are discussed.

The effect of ether stress and cycloheximide treatment on cholesterol binding and enzyme turnover of adrenal cortical cytochrome P450scc

Relative rate constants for the formation of pregnenolone from cytochrome P450scc bound cholesterol in adrenal cortical mitochondria of stressed, stressed plus cycloheximide treated and dexamethasone treated rats were calculated from the ratios of initial rates of pregnenolone formation and the pregnenolone induced difference spectrum. In mitochondria from adrenals removed under aerobic conditions in vivo, the rate constant for the enzyme in stressed rats is twice a high as the rate constant for the enzyme from the stressed plus cycloheximide group, and four times as high as that for the enzyme from dexamethasone treated rats. Anoxia for 5 min in the intact gland increases the rate constant in all groups. Pregnenolone difference spectra are higher in mitochondria from stressed plus cycloheximide treated rats than in mitochondria from stressed rats, when adrenals are removed aerobically. It is concluded that ACTH increases cholesterol binding to cytochrome P450scc, by increasing either the enzymes affinity for its substrate or the availability of cholesterol and in addition promotes turnover of the enzyme. Both of these effects of ACTH are inhibited by cycloheximide.

Regulation of steroidogenesis in the ovine corpus luteum

To examine the factor affecting LH-induced progesterone production in vitro in ovine luteal slices, an experimental procedure was employed wherein each slice served as its own control. The role of microfilaments in steroidogenesis was studied in luteal slices treated with cytochalasin B (an inhibitor of microfilament function). Cytochalasin B treatment resulted in significant reduction of progesterone production by luteal slices in response to LH and the addition of serum to the medium did not alter this effect. The ability of luteal slices to respond to LH with increased progesterone secretion was restored when cytochalasin B was removed from the medium. Further studies indicated that inhibition of LH-induced progesterone production by treatment with cytochalasin B was not a result of a change in: 1) cyclic adenosine 3',5'-monophosphate production in response to LH; 2) mitochondrial membrane permeability to cholesterol; or 3) activity of 3 beta-hydroxysteroid dehydrogenase, delta 5, delta 4-isomerase enzyme complex. The possibility existed that microfilaments were necessary for cholesterol transport to mitochondria in response to LH stimulation. However, mitochondrial cholesterol content was unchanged in response to LH in the presence or absence of aminoglutethimide (an inhibitor of cholesterol side-chain cleavage enzyme activity) as determined by uptake of 3H-cholesterol or total content determined by gas-liquid chromatography. Further, treatment with cytochalasin B had no effect on mitochondrial cholesterol content. These results suggest a role for microfilaments in LH-induced progesterone production at a point prior to the conversion of cholesterol to pregnenolone.

Interaction of prostaglandins with adrenal microsomal cytochrome P-450 in the guinea pig

Studies were carried out to investigate the effects of prostaglandins (PG) in vitro on adrenal microsomal steroid and drug metabolism in the guinea pig. The addition of PGE1, PGE2, PGA1, PGF1 alpha or PGF2 alpha to isolated adrenal microsomes produced typical type I difference spectra. The sizes of the spectra (delta A385-420) produced by prostaglandins were smaller than those produced by various steroids including progesterone, 17-hydroxyprogesterone and 11 beta-hydroxyprogesterone. However, the affinities of prostaglandins and steroids for adrenal microsomal cytochrome P-450, as estimated by the spectral dissociation constants, were similar. Prior addition of prostaglandins to isolated adrenal microsomes did not affect steroid binding to cytochrome P-450 or the rate of steroid 21-hydroxylation. In contrast, prostaglandins inhibited adrenal metabolism of ethylmorphine and diminished the magnitude of the ethylmorphine-induced spectral change in adrenal microsomes. The results indicate that prostaglandins inhibit adrenal drug metabolism by interfering with substrate binding to cytochrome P-450. Since 21-hydroxylation was unaffected by PG, different cytochrome P-450 moieties are probably involved in adrenal drug and steroid metabolism.

Mechanisms of corticotropin action in rat adrenal cells. I. The effects of inhibitors of protein synthesis and of microfilament formation on corticosterone synthesis

The contributions of protein synthesis and formation of microtubules and microfilaments to corticotropin-stimulated steroidogenesis in rat adrenal cell suspensions has been assessed by use of a series of inhibitors to each function. Five inhibitors of protein synthesis (cycloheximide, puromycin, blastocidin S, anisomycin, and trichodermin) each exhibited time-dependent inhibition of corticotropin-stimulated steroidogenesis. For the first 30 min, steroidogenesis was more extensively inhibited than protein synthesis, after which the effectiveness of the inhibitors diminished on steroidogenesis but not on protein synthesis. The reversal effect was not observed at high levels of inhibitors. One inhibitor of microfilament formation (cytochalasin B) and four inhibitors of microtubule formation (colchicine, podophyllotoxin, vinblastine sulfate and griseofulvin) inhibited steroidogenesis without inhibiting protein synthesis and without any reversal effect with prolonged incubation. The actions of all ten inhibitors were shown to be fully reversible. Cell superfusion of adrenal cells showed that the decay of steroidogenesis upon addition of all the protein synthesis inhibitors was similar to decay upon removal of corticotropin from the medium (t1/2 = 4--6 min). Recoveries from inhibition upon removal of the inhibitors were similar to each other and comparable to initial corticotropin stimulation of the cells (lag of 3--5 min, t1/2=7--9 min). Similar kinetics of inhibition and recovery were observed for vinblastine sulfate while a direct inhibition of cytochrome P-450scc by aminoglutethimide was complete within 1 min and was rapidly reversed. Injection of each inhibitor (all classes) into hypophysectomized rats inhibited the elevation of plasma corticosterone by corticotropin. The extent of cholesterol combination with cytochrome P-450scc in adrenal mitochondria isolated from these rats was also decreased by all of the inhibitors. Decreases in plasma corticosterone correlated directly with decreases in cholesterol combination with cytochrome P-450scc (r=0.94). It is concluded that protein synthesis and steroidogenesis must be intimately coupled probably due to the requirement of a labile protein for cholesterol transport to cytochrome P-450scc. An involvement of microtubules and microfilaments in this process is clearly indicated.

On the requirement for protein synthesis during corticotropin-induced stimulation of cholesterol side chain cleavage in rat adrenal mitochondrial and solubilized desmolase preparations

Following simple homogenization, significant amounts of mitochondrial-derived, cholesterol side chain cleaving enzyme (desmolase) activity are recovered in rat adrenal 105 000 X g-supernatant fraction. Corticotropin administration enhances soluble desmolase activity, and cycloheximide potentiates this effect. The lipid droplet fraction which has no desmolase activity markedly enhances pregnenolone synthesis in the soluble desmolase preparations, presumably by supplying free cholesterol substrate. Corticotropin particularly with cycloheximide pretreatment, enhances lipid fraction activity. Thus increased cholesterol availability may largely explain the corticotropin effect on the soluble desmolase system. Since protein synthesis is required for corticotropin activity in intact mitochondria, but not in calcium-swollen mitochondria or the soluble enzyme system, the labile protein apparently required during corticotropin action may function to overcome a "barrier" which exists only in the intact mitochondria and restrains cholesterol side chain cleavage.

Effect of Ca2+, ruthenium red and ageing on pregnenolone production by mitochondrial fractions from normal and luteinizing hormone treated rat testes

The effect of Ca2+ in vitro on pregnenolone production rates under various incubation conditions by mitochondrial fractions isolated from testes of normal rats and of rats after in vivo treatment with luteinizing hormone has been investigated. Concentrations of Ca2+ in the range of 0.1-0.5 mM stimulated succinate supported pregnenolone production in mitochondrial fractions from both control and luteinizing hormone treated testes. When mitochondrial fractions were isolated in 0.25 M sucrose without additions, Ca2+ in vitro increased succinate supported pregnenolone production rates in mitochondrial fractions isolated from control testes to a greater extent than in mitochondrial fractions, from luteinizing hormone treated testes. Production rates in control mitochondrial fraction, incubated in the presence of initial Ca2+ concentrations of 0.7 mM and higher were almost similar to production rates in relevant luteinizing hormone treated mitochondria.