Pregnenolone separation from cholesterol using Sephadex LH-20 mini-columns

Neuroendocrine Regulation of Anxiety: Beyond the Hypothalamic-Pituitary-Adrenal Axis

The central nervous system regulates and responds to endocrine signals, and this reciprocal relationship determines emotional processing and behavioural anxiety. Although the hypothalamic-pituitary-adrenal (HPA) axis remains the best-characterised system for this relationship, other steroid and peptide hormones are increasingly recognised for their effects on anxiety-like behaviour and reward. The present review examines recent developments related to the role of a number of different hormones in anxiety, including pregnane neurosteroids, gut peptides, neuropeptides and hormonal signals derived from fatty acids. Findings from both basic and clinical studies suggest that these alternative systems may complement or occlude stress-induced changes in anxiety and anxiety-like behaviour. By broadening the scope of mechanisms for depression and anxiety, it may be possible to develop novel strategies to attenuate stress-related psychiatric conditions. The targets for these potential therapies, as discussed in this review, encompass multiple circuits and systems, including those outside of the HPA axis.

Catalytic properties of cytochrome P-450scc purified from the human placenta: comparison to bovine cytochrome P-450scc

Cytochrome P-450scc was purified from the human placenta by extraction of mitochondria with cholate and Emulgen 911, chromatography on phenyl-Sepharose and DEAE-Sephacel, and ammonium sulphate fractionation. The catalytic properties of the purified human cytochrome P-450scc were analysed in Tween-20 micelles and compared to those of bovine adrenal cytochrome P-450scc analysed in the same system. Both enzymes had the same Km for cholesterol and were stimulated by cardiolipin when the cholesterol concentration was subsaturating. Examination of the rates of pregnenolone synthesis from 20 alpha-hydroxycholesterol, 22R-hydroxycholesterol and 20 alpha, 22R-dihydroxycholesterol by human and bovine cytochromes P-450scc revealed that the first hydroxylation (22R position) was rate-limiting for both in Tween-20 micelles. The rate of the 22R-hydroxylation was further decreased when a 20 alpha-hydroxyl group was already present on the cholesterol side-chain. The second hydroxylation occurred at about the same rate as the third hydroxylation for both enzymes. The rate of side-chain cleavage of 25-hydroxycholesterol by human cytochrome P-450scc in Tween-20 micelles was low, the highest rate being about 1% of the Vmax for cholesterol. Substrate inhibition was seen with high concentrations of 25-hydroxycholesterol. Conversion of 25-hydroxycholesterol to pregnenolone was accompanied by a build-up of products with intact side-chains, which were probably intermediates of the reaction. Side-chain cleavage of 25-hydroxycholesterol by bovine cytochrome P-450scc showed similar characteristics to the human enzyme, except that the highest velocity observed was approx. 25% of the Vmax for cholesterol. Rates of cleavage of 25-hydroxycholesterol by both enzymes were higher in dioleoylphosphatidylcholine vesicles than in Tween-20, but were still well below the Vmax for cholesterol and showed substrate inhibition. This study shows that there is close similarity in catalytic properties between human and bovine cytochromes P-450scc which suggests that the active site of the cytochrome is highly conserved.

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.

ACTH control of cholesterol side-chain cleavage at adrenal mitochondrial cytochrome P-450scc. Regulation of intramitochondrial cholesterol transfer

Rat adrenal mitochondria exhibit a linear 2-fold accumulation of cholesterol for 20 min following either in vivo ether stress or ACTH administration, providing cholesterol metabolism is inhibited by aminoglutethimide (AMG). Additional cycloheximide (CX) pretreatment only slightly decreases this increase, but the location of accumulation shifts from the inner membrane to the outer membrane, implying a decreased cholesterol transfer from outer to inner membrane. Although the capacity of outer mitochondrial membranes was saturated after a 10-min treatment with CX, a 20-min treatment resulted in further retention of cholesterol in intact mitochondria that was not recovered in the isolated membranes. An additional pool of loosely bound cholesterol is proposed for CX mitochondria. These studies provide evidence that the CX-sensitive step of adrenal steroidogenesis attributed to loss of a labile ACTH regulatory protein (Pedersen, R.C. and Brownie, A.C. (1983) Proc. Natl. Acad. Sci. U.S.A. 80, 1882-1886) involves cholesterol transfer from the outer to the inner mitochondrial membrane. ACTH also enhances the PI and PE content of the outer membranes by a CX-sensitive mechanism that may contribute to intramitochondrial cholesterol transport. CX treatment does not affect cholesterol uptake by the inner membrane from phospholipid vesicles. The initial rate of endogenous metabolism in isolated inner membranes is insensitive to pretreatment (2 nmol/nmol P-450/min). The duration of this linear rate was increased 4-fold by AMG treatment while this increase was prevented by CX treatment. The kinetics indicate differences in inner membrane reactive cholesterol levels. Inner membranes also contained a fraction of unreactive cholesterol that is insensitive to pretreatment. Cholesterol-P-450scc complex formation for all pretreatments fits a single hyperbolic function of the reactive cholesterol content of the inner mitochondrial membrane (Kd = 0.025 mol cholesterol/mol phospholipid), and is activated over 5-fold upon mitochondrial disruption. All changes in inner membranes caused by CX can, therefore, be attributed solely to the restricted cholesterol access in vivo.

Purification and immunological characterization of human placental mitochondrial cytochrome P-450

Human placental mitochondrial cytochrome P-450 was purified to electrophoretic homogeneity by hydrophobic, anion exchange and cation exchange column chromatography. The specific content of the purified protein was 15.7 nmol/mg protein and it showed a single band mol. wt 48,000 D in SDS-gel electrophoresis. When reconstituted with bovine adrenal adrenodoxin reductase and adrenodoxin it converted cholesterol to pregnenolone (cholesterol side-chain cleavage activity, CSCC) at the rate of 1 pmol/min/pmol P-450. Antibodies against the purified protein were raised in rabbits. Inhibition studies demonstrated 85% inhibition of placental CSCC activity at an antibody/protein ratio of 10:1. Placental microsomal aromatase activity was inhibited by 47% at the same antibody/protein ratio. The antibody inhibited bovine mitochondrial CSCC activity by 87% at the same antibody/protein ratio. Placental microsomal 7-ethoxycoumarin O-deethylase, aryl hydrocarbon hydroxylase and 7-ethoxyresorufin O-deethylase activities were not significantly inhibited by the antibody. The results indicate that the purified protein catalyzes cholesterol side-chain cleavage reaction, human placental microsomal aromatase and bovine adrenal mitochondrial P-450scc may share common antigenic determinants with placental P-450scc, but the placental microsomal xenobiotic-metabolizing cytochrome(s) is (are) distinctly different.

Adrenal cortical 11 beta-hydroxylase and side-chain cleavage enzymes. Requirement for the A- or B-pyridyl ring in metyrapone for inhibition

The adrenal cortical enzyme systems, 11 beta-hydroxylase, P-450 11 beta, and the side-chain cleavage complex, P-450 scc, differ only in their cytochrome P-450s. Structural modifications of metyrapone, an inhibitor of cytochrome P-450 enzyme systems, have been made to determine the requirement for the A- or B-pyridyl ring for inhibition of P-45011 beta and P-450 scc activities. Three new analogs of metyrapone (A-phenylmetyrapone, B-phenylmetyrapone and diphenylmetyrapone) were synthesized and evaluated as inhibitors using a crude, defatted bovine adrenal cortical mitochondrial preparation. Characterization of the mitochondrial preparation demonstrated: enhancement of both activities by the addition of 15.0 microM adrenodoxin, the addition of 1% ethanol decreased both activities less than 10%, and the apparent Km of deoxycorticosterone for P-45011 beta was 6.8 microM and the apparent Km of cholesterol for P-450 scc was 21.6 microM. Inhibition of P-45011 beta and P-450 scc activities with these compounds demonstrated: the B-pyridyl ring of metyrapone is required for inhibition of both activities whereas requirement for the A-ring is less stringent, and the four metyrapone analogs were more selective inhibitors of P-45011 beta activity. These studies suggest that the A-phenyl metyrapone analog is a good candidate for further development of a selective adrenocortical radiopharmaceutical.

Cholesterol side-chain cleavage activity in human placenta and bovine adrenals: an one-step method for separation of pregnenolone formed in vitro

Cholesterol side-chain cleavage (CSCC) activity towards exogenous cholesterol was quantified by an one-step reversed-phase minicolumn method for the separation of pregnenolone formed in the reaction. The assay is rapid and reproducible. The method is linear for up to 2 mg of placental mitochondrial protein and up to 1 mg of bovine adrenal mitochondrial protein in the incubata over 30 min and 5 min reaction times, respectively. Average Km and Vmax values were 14.1 microM and 3.4 pmol/min/mg for the placental preparation and 1.5 microM and 20.7 pmol/min/mg for the bovine adrenal mitochondrial preparation. In human placenta, the mitochondrial fraction contained most of the CSCC activity. Inhibition studies showed that aminoglutethimide (500 microM) inhibited both placental and bovine adrenal activities at the same level (about 80-90% inhibition) but androstenedione (500 microM), metyrapone (500 microM), benzo(a)pyrene (800 microM) and Emulgen 911 (0.05%) were more effective in human placental preparations. Neither of the activities were inhibited to any great extent by alpha-naphthoflavone (500 microM), SKF 525A (500 microM) or 7-ethoxycoumarin (1 mM).