Adrenal microsomal hydroxylating system: purification and substrate binding properties of cytochrome P-450C-21

Differential behavior of the sub-sites of cytochrome 450 active site in binding of substrates, and products (implications for coupling/uncoupling)

The cytochrome P450 catalyzes hydroxylation of many substrates in the presence of O(2) and specific electron transport system. The ternary complex S-Fe(+)O(2) with substrate and O(2) bound to their respective sites on the reduced enzyme is an important intermediate in the formation of the hydroxylating species. Then the active site may be considered as having two sub-sites geared for entirely different types of functionally relevant interactions. The two sites are the substrate binding site, the specific protein residues (Site I), and the L(6) position of the iron (Site II) to which O(2) binds upon reduction. In the ferric enzyme, when substrate binds to Site I, the low spin six-coordinated P450 is converted to the readily reducible high spin five coordinated state. Certain amines and OH compounds, such as products of P450-catalyzed reactions, can bind to Site II resulting in six coordinated inhibited complexes. Then the substrate and product interactions with the two sub-sites can regulate the functional state of the enzyme during catalysis. Product interactions have received very little attention. CYP101 is the only P450 in which X-ray and spectroscopic data on all three structures, the substrate-free, camphor-bound and the 5-exo-OHcamphor-bound are available. The substrate-free CYP101 is low spin and six-coordinated with a water molecule ligated at the L(6) position of the iron. The substrate camphor binds to Site I, and releases the L(6) water despite its inability to bind to this site, indicating that Site I binding can inhibit Site II ligation. The product 5-exo-OHcamphor in addition to binding to Site I, binds to Site II through its -OH group forming Fe-O bond, resulting in the low spin six-coordinated complex. New temperature-jump relaxation kinetic data indicating that Site II ligation inhibits Site I binding are presented. It appears that the Site I and Site II function as interacting sub-sites. The inhibitory allosteric interactions between the two sub-sites are also reflected in the data on binding of the substrate camphor (S) in the presence of the product 5-exo-OH camphor (P) to CYP101 (E). The data are in accordance with the two-site model involving the ternary complex ESP. The affinity of the substrate to the product-bound enzyme as well as the affinity of the product to the substrate-bound enzyme decreased with increase in product concentration, which is consistent with mixed inhibition indicative of inhibitory allosteric interactions between the two sub-sites. Implications of these observations for coupling/uncoupling mechanisms are discussed in the light of the published findings consistent with the two-site behavior of the P450 active site. In addition, kinetic data indicating that the transient high spin intermediate may have to be taken into account for understanding how some P450s have been able to express appreciable hydroxylation activities in the absence of substrate-induced low to high spin transition, observable by the traditional static spectroscopy, are presented.

20beta-hydroxy-C21-steroid 20beta-oxidase activity of cytochrome P450c21 purified from bovine adrenocortical microsomes

We showed previously that cytochrome P450c21 (CYP21A1) from bovine adrenocortical microsomes has a putative 20beta-oxidase activity for 20beta-hydroxyprogesterone leading to a conversion to progesterone [M. Tsubaki, K. Morimoto, S. Tomita, S. Miura, Y. Ichikawa, A. Miyatake, F. Masuya, H. Hori, Biochim. Biophys. Acta 1259 (1995) 89-98]. We have extended the investigation on the 20beta-oxidase activity of cytochrome P450c21. Combination of 17alpha, 20beta-dihydroxyprogesterone with purified cytochrome P450c21 in oxidized form caused an induction of a typical type I difference spectrum (a peak at 390 nm and a trough at 420 nm) with a spectral dissociation constant of 2.3 microM. EPR spectrum at low temperature (15 K) exhibited an appearance of a new low-spin signal at gz=2.42, gy=2.21, and gx=1.966 and an increase in intensity of the high-spin signal (g=8.06, 3.54) upon formation of the enzyme-steroid complex, as previously found for 17alpha-hydroxyprogesterone and 20beta-hydroxyprogesterone. The enzymatic activity for 17alpha, 20beta-dihydroxyprogesterone was confirmed in a reconstituted system consisting of cytochrome P450c21 and NADPH-cytochrome P450 reductase. 17alpha,20beta-Dihydroxyprogesterone was converted to 17alpha-hydroxyprogesterone via the 20beta-oxidase reaction. The high turnover numbers of the 20beta-oxidase activity for 20beta-hydroxy-c21-steroids suggests that this activity is likely to have some physiological roles. Cytochrome P450c21 and 20beta-hydroxysteroid dehydrogenase may regulate the androgen biosynthesis catalyzed by cytochrome P450c17alpha with controlling the concentration of 20beta-hydroxy-C21-steroids.

Electron paramagnetic resonance investigation of cytochrome P-450c21 from bovine adrenocortical microsomes: a new enzymatic activity

Cytochrome P-450c21 (CYP21A1) purified from bovine adrenocortical microsomes was investigated by electron paramagnetic resonance (EPR) spectroscopy to clarify the interactions among heme active center, protein surroundings, water molecules and bound substrates or analogues. The low-spin EPR signals of the oxidized enzyme (as purified) consisted of two species; one at gz = 2.39, gy = 2.23, and gx = 1.925 (component A) and the other at gz = 2.42, gy = 2.23, and gx = 1.916 (component B). The component A is probably representing a product-bound form, whereas the component B indicates either occupation of the substrate-binding site with a substrate analogue or absence of steroid at the site. Upon addition of progesterone, the component A signal completely disappeared and the intensity of high-spin signal (g = 8.06, 3.54) increased. Addition of 17 alpha-hydroxyprogesterone caused a development of a new low-spin signal at gz = 2.42, gy = 2.21, and gx = 1.966 (component C) and a further increase in intensity of the high-spin signal (g = 8.06, 3.54). Addition of 20 beta-hydroxyprogesterone caused an increase in intensity of the component C signal (and the g = 8 high-spin signal) even stronger than did 17 alpha-hydroxyprogesterone. These observations suggest that 20 beta-hydroxyprogesterone binds to the cytochrome P-450c21 active center in a very similar manner as 17 alpha-hydroxyprogesterone does and, therefore, may be a metabolizable substrate. A new enzymatic pathway catalyzed by cytochrome P-450c21 was confirmed with a reconstituted enzymatic system consisting of cytochrome P-450c21, NADPH-cytochrome P-450 reductase and an NADPH-generating system. 20 beta-Hydroxyprogesterone was converted to progesterone via a putative 20 beta-oxidase reaction in a comparable turnover number to that of the 21-hydroxylation of 17 alpha-hydroxyprogesterone.

On the model controversy for substrate-induced spin-state transition in cytochrome P450: (a new perspective)

Three models have been proposed for substrate-induced spin-state transition in cytochrome P450. These are referred to as two-, three- and four-state models. In this communication the three models are reviewed with respect to their experimental basis and their ability to accommodate the results reported on the effects of substrates on spin-state and reduction of various P450's. In addition, a new perspective is presented.

Binding of substrates to cytochrome P450 enzymes: mathematical artifacts in the application of difference spectrophotometry

Investigation of the difference absorption spectrum has been a common method for studying many aspects of hemoproteins such as cytochrome P450. The difference spectrophotometric technique, as applied in studies on binding of substrates to cytochrome P450, has resulted in certain mathematical artifacts which have been mistaken for real changes in binding equilibria. The purpose of this paper is to point out these artifacts.

ACTH-induced increases in rabbit adrenal immunoreactive P-450(17 alpha) and P-450(21)

Rabbit adrenal 17 alpha-hydroxylase activity has previously been shown to increase dramatically following ACTH stimulation. The present study was designed to determine whether the increase in enzyme activity could be correlated with an increase in P-450(17 alpha) protein measured by immunoblotting using an anti-porcine P-450(17 alpha) antibody. It was found that the total and specific contents of rabbit adrenal immunoreactive P-450(17 alpha) were increased 6- to 8-fold and 4-fold, respectively, after ACTH stimulation. The results were similar whether the detection system was 125I-labeled protein A or an alkaline phosphatase-conjugated second antibody. Corresponding increases in 17 alpha-hydroxylase activity were also observed but were slightly less than the increases in immunoreactive P-450(17 alpha), suggesting that not all of the protein was enzymatically active. Comparatively, immunoreactive P-450(21) was increased only 1.3-fold. Antibodies to porcine P-450(17 alpha) and bovine P-450(21) reacted monospecifically with the homologous rabbit and guinea pig proteins as judged by the detection of single bands on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Inhibition studies showed that in an assay using 125 micrograms per ml of microsomal protein ACTH-stimulated rabbit adrenal 17 alpha-hydroxylase activity was inhibited 72% at a 100 mg per ml concentration of the anti-porcine P-450(17 alpha); however, 47% inhibition was observed at the same concentration of anti-bovine P-450(21). Pre-immune IgG had no effect. Molecular weight, Mr, determinations by SDS-PAGE showed both rabbit and guinea pig P-450(17 alpha) to be 52 kDa; rabbit P-450(21), 54 kDa; and guinea pig P-450(21), 49 kDa.(ABSTRACT TRUNCATED AT 250 WORDS)

18-Substituted progesterone derivatives as inhibitors of aldosterone biosynthesis

The synthesis of new progesterone derivatives substituted at the 18 methyl group is described. These compounds are designed as 18-monooxygenase, cytochrome P-450-dependent potential kcat inhibitors. Preliminary results on the in vitro biological investigation of these modified progesterones are presented.

Heterogeneity of the bovine adrenal steroid 21-hydroxylase

The results presented indicate that purified cytochrome P-45021 which migrated upon SDS gel electrophoresis essentially as a single band, is further separable into different species by ion-exchange chromatography. The P-450 eluted from the CM-Sephadex column at different points along the buffer concentration gradient, exhibited significant differences in (1) the 21-hydroxylation of 17 alpha-OH-progesterone compared to progesterone and (2) the Type I spectral change produced by 17 alpha-OH-progesterone compared to that due to delta 4-androstenedione. These results indicate that the purified P-450 which appeared homogeneous contains different species differing in net charge and steroid preferences for 21-hydroxylation and binding. The ratio, 21-hydroxylation of 17 alpha-OH-progesterone/progesterone ranged between 2.6 and 0.86 suggesting that the purified preparation is a mixture of 17 alpha-OH-progesterone preferring and progesterone preferring species. Possible molecular bases for the heterogeneity of the 21-hydroxylase are discussed.

Synthesis and activity of new inhibitors of aldosterone biosynthesis

A new family of aldosterone biosynthesis inhibitors, designed as 18-mono-oxygenase, cytochrome-P450-dependent, potential Kcat inhibitors, is described. These compounds are progesterone derivatives substituted at the 18-methyl group. Preliminary results on the in vitro biological evaluation of these modified progesterones are presented. Aldosterone biosynthesis is completely inhibited by 18-vinyl progesterone 5 at a concentration of 0.8 microM and by 18-ethynyl progesterone 6 at 8 microM. It appears that products designed as alkylating agents for the prosthetic heme group are the most potent inhibitors in that series.