Preparation and characterization of submitochondrial fractions from adrenal cells

The mechanism of specific binding of free cholesterol by the steroidogenic acute regulatory protein: evidence for a role of the C-terminal alpha-helix in the gating of the binding site

Steroidogenesis depends on the delivery of free cholesterol to the inner mitochondrial membrane by StAR (steroidogenic acute regulatory protein). Mutations in the StAR gene leads to proteins with limited cholesterol-binding capacity. This gives rise to the accumulation of cytoplasmic cholesterol, a deficit in steroid hormone production and to the medical condition of lipoid congenital adrenal hyperplasia. A detailed understanding of the mechanism of the specific binding of free cholesterol by StAR would be a critical asset in understanding the molecular origin of this disease. Previous studies have led to the proposal that the C-terminal alpha-helix 4 of StAR was undergoing a folding/unfolding transition. This transition is thought to gate the cholesterol-binding site. Moreover, a conserved salt bridge (Glu169-Arg188) in the cholesterol-binding site is also proposed to be critical to the binding process. Interestingly, some of the documented clinical mutations occur at this salt bridge (E169G, E169K and R188C) and in the C-terminal alpha-helix 4 (L275P). In the present study, using rationalized mutagenesis, activity assays, CD, thermodynamic studies and molecular modelling, we characterized the alpha-helix 4 mutations L271N and L275P, as well as the salt bridge double mutant E169M/R188M. The results provide experimental validation for the gating mechanism of the cholesterol-binding site by the C-terminal alpha-helix and the importance of the salt bridge in the binding mechanism. Altogether, our results offer a molecular framework for understanding the impact of clinical mutations on the reduction of the binding affinity of StAR for free cholesterol.

Cholesterol binding is a prerequisite for the activity of the steroidogenic acute regulatory protein (StAR)

Steroidogenesis depends on the delivery of cholesterol from the outer to the inner mitochondrial membrane by StAR (steroidogenic acute regulatory protein). However, the mechanism by which StAR binds to cholesterol and its importance in cholesterol transport are under debate. According to our proposed molecular model, StAR possesses a hydrophobic cavity, which can accommodate one cholesterol molecule. In the bound form, cholesterol interacts with hydrophobic side-chains located in the C-terminal alpha-helix 4, thereby favouring the folding of this helix. To verify this model experimentally, we have characterized the in vitro activity, overall structure, thermodynamic stability and cholesterol-binding affinity of StAR lacking the N-terminal 62 amino acid residues (termed N-62 StAR). This mature form is biologically active and has a well-defined tertiary structure. Addition of cholesterol to N-62 StAR led to an increase in the alpha-helical content and T degrees (melting temperature), indicating the formation of a stable complex. However, the mutation F267Q, which is located in the C-terminal helix interface lining the cholesterol-binding site, reduced the biological activity of StAR. Furthermore, the cholesterol-induced thermodynamic stability and the binding capacity of StAR were significantly diminished in the F267Q mutant. Titration of StAR with cholesterol yielded a 1:1 complex with an apparent K(D) of 3 x 10(-8). These results support our model and indicate that StAR can readily bind to cholesterol with an apparent affinity that commensurates with monomeric cholesterol solubility in water. The proper function of the C-terminal alpha-helix is essential for the binding process.

HEPATIC, EXTRAHEPATIC, MICROSOMAL, AND MITOCHONDRIAL ACTIVATION OF THEN-HYDROXYLATED PRODRUGS BENZAMIDOXIME, GUANOXABENZ, AND RO 48-3656 ([[1-[(2S)-2-[[4-[(HYDROXYAMINO)IMINOMETHYL]BENZOYL]AMINO]-1-OXOPROPYL]-4-PIPERIDINYL]OXY]-ACETIC ACID)

In previous studies, it was shown that liver microsomes from rabbit, rat, pig, and human are involved in the reduction of N-hydroxylated amidines, guanidines, and amidinohydrazones of various drugs and model compounds (Drug Metab Rev 34: 565-579). One responsible enzyme system, the microsomal benzamidoxime reductase, consisting of cytochrome b5, its reductase, and a cytochrome P450 isoenzyme, was isolated from pig liver microsomes (J Biol Chem 272:19615-19620). Further investigations followed to establish whether such enzyme systems are also present in microsomes of other organs such as brain, lung, and intestine. In addition, the mitochondrial reduction in human and porcine liver and kidney preparations was studied. The reductase activities were measured by following the reduction of benzamidoxime to benzamidine, guanoxabenz to guanabenz, and Ro 48-3656 ([[1-[(2S)-2-[[4-[(hydroxyamino)iminomethyl]benzoyl]amino]-1-oxopropyl]-4-piperidinyl]oxy]-acetic acid) to Ro 44-3888 ([[1-[(2S)-2-[[4-(aminoiminomethyl)benzoyl]amino]-1-oxopropyl]-4-piperidinyl]oxy]-acetic acid). Interestingly, preparations of all tested organs were capable of reducing the three compounds. The highest specific rates were found in kidney followed by liver, brain, lung, and intestine, and usually the mitochondrial reduction rates were superior. From the determined characteristics, similarities between the enzyme systems in the different organs and organelles were detected. Furthermore, properties of the benzamidoxime reductase located in the outer membrane of pig liver mitochondria were studied. In summary, these results demonstrate that in addition to the microsomal reduction, mitochondria are involved to a great extent in the activation of amidoxime prodrugs. The importance of extrahepatic metabolism in the reduction of N-hydroxylated prodrugs is demonstrated.

The yeast mitochondrial intermembrane space: purification and analysis of two distinct fractions

We have developed a protocol for the sequential release of the intermembrane space (IMS) content of Saccharomyces cerevisiae mitochondria. Two distinct fractions were obtained: a soluble IMS with cytochrome b2 as key marker and a salt-extractable IMS with cytochrome c as key marker. The identity of several proteins was determined by amino-terminal amino acid sequencing. The IMS fractions were devoid of contaminations from cytosol and mitochondrial outer and inner membranes. By subtraction analysis, the protein profiles of soluble and salt-extractable IMS fractions were depleted of contaminating bands derived from matrix proteins. The fractionation method will provide the basis for the further analysis of IMS proteins and characterization of their functions in bioenergetics, mitochondrial biogenesis, and regulatory processes.

GTP-binding proteins in adrenocortical mitochondria

We have identified two GTP-binding proteins in mitochondria from bovine adrenal cortex (fasciculata). Sub-mitochondrial particles were fractionated into inner membrane, contact point and outer membrane vesicles on sucrose density gradients. These sub-mitochondrial fractions were identified by the presence of enzyme markers and electron microscopy. Photoaffinity labelling with [gamma-32P]GTP identified a 45 kDa GTP-binding protein in outer mitochondrial membranes and a 19 kDa protein in the contact points. The molecular weight of 45 kDa and requirement for Mg2+ ions raise the possibility that this protein is an alpha subunit of a heterotrimeric GTP-binding protein or a novel GTP-binding protein. The specificity of nucleotide binding, the requirement for low concentrations of Mg2+ (0.1 mM) and molecular weight of 19 kDa suggest that this protein is a typical member of the so-called small GTP-binding protein family. The location of 45 kDa in the outer membrane and that of 19 kDa in the contact points suggest roles for these proteins in the interaction with the extramitochondrial environment and in the regulation of mitochondrial membranes, respectively.

Regulated expression of cytochrome P-450scc (cholesterol-side-chain cleavage enzyme) in cultured cell lines detected by antibody against bacterially expressed human protein

The first step in the synthesis of steroids is catalysed by cytochrome P-450ssc (cholesterol-side-chain cleavage enzyme). We have investigated the synthesis of this enzyme in three cultured cell lines at the protein and hormone secretion levels. Hormone levels were measured by an enzyme immunoassay using a monoclonal antibody against progesterone. The protein level was detected using polyclonal antibodies directed against a P-450scc fusion protein overproduced in Escherichia coli. Utilizing a bacteriophage T7 promoter expression system, a large amount of human P-450scc fusion protein was produced and easily purified. P-450scc was synthesized in the mouse adrenal tumour cell line Y1 and human choriocarcinoma cell line JEG-3, but not in monkey kidney cell line COS-1. The production of P-450scc in Y1 and JEG-3 cells was stimulated by 8-bromo cyclic AMP, the effect of which was not observed until 6 h after induction and was more pronounced at 24 h. Y1 and JEG-3 cells exhibited a difference in progesterone secretion after induction.

The binding of cortisol to adrenal mitochondria

Binding of tritiated cortisol to adrenal zona glomerulosa mitochondria was studied and compared with that of corticosterone. Cortisol was shown to bind specifically to the inner membrane of zona glomerulosa mitochondria. Corticosterone and cortisol had similar apparent association constants (Ka) and concentrations of binding sites. The methodology was validated by obtaining similar Ka from both binding plots and kinetic data. Cortisol binding was inhibited by pretreatment with sodium dithionite, and displaced by deoxycorticosterone, corticosterone, 18-hydroxy-corticosterone, 11 beta-hydroxy-18-ethynyl-progesterone and metyrapone, but not by cholesterol. These results suggest that cortisol and corticosterone bind to the same cytochrome P-450.

Effects of ACTH on the last step of aldosterone biosynthesis

The production of tritiated aldosterone and tritiated SM (a saponifiable 18-hydroxycorticosterone derivative) by rat adrenals were studied at various incubation times in absence or presence of two concentrations of ACTH. Tritiated 18-hydroxycorticosterone or 18-deoxyaldosterone served as precursors. The lower ACTH concentration (150 pM) increased the production of tritiated aldosterone. Whereas, the higher ACTH concentration (1.5 microM) stimulated tritiated aldosterone production at shorter incubation time (30 min), while after 60 min it inhibited. This time dependency would reflect variations in the levels of endogenous steroids. On the other hand, the effects of ACTH on tritiated SM production were opposite to those on tritiated aldosterone. In effect, while 150 pM ACTH inhibited SM production, 1.5 microM ACTH stimulated it. These results suggest that ACTH promotes opposite effects on the productions of aldosterone and SM and therefore both productions would be coordinated under the regulation of ACTH.