A mechanism-based fluorogenic probe for the cytochrome P-450 cholesterol side chain cleavage enzyme

Resorufin-based responsive probes for fluorescence and colorimetric analysis

The fluorescence imaging technique has attracted increasing attention in the detection of various biological molecules in situ and in real-time owing to its inherent advantages including high selectivity and sensitivity, outstanding spatiotemporal resolution and fast feedback. In the past few decades, a number of fluorescent probes have been developed for bioassays and imaging by exploiting different fluorophores. Among various fluorophores, resorufin exhibits a high fluorescence quantum yield, long excitation/emission wavelength and pronounced ability in both fluorescence and colorimetric analysis. This fluorophore has been widely utilized in the design of responsive probes specific for various bioactive species. In this review, we summarize the advances in the development of resorufin-based fluorescent probes for detecting various analytes, such as cations, anions, reactive (redox-active) sulfur species, small molecules and biological macromolecules. The chemical structures of probes, response mechanisms, detection limits and practical applications are investigated, which is followed by the discussion of recent challenges and future research perspectives. This review article is expected to promote the further development of resorufin-based responsive fluorescent probes and their biological applications.

Synthesis of dinorcholane and 5β-cholane derivatives

The convenient synthesis of seven dinorcholane and 5β-cholane derivatives, 3β-acetoxy-23,24-dinorchol-5-en-22-oyl methoxy anhydride (2), 3β-acetoxy-23,24-dinorchol-5-en-22-oyl chloride (3), 3β-acetoxy-23,24-dinorchol-5-en-22-ol (4), 3α-acetoxy-5β-cholan-24-oic acid (6), 3α-acetoxy-5β-cholan-24-oic acid ethyl ester (7), 3-oxo-5β-cholan-24-oic acid (8) and 3-oxo-5β-cholan-24-oic acid ethyl ester (9) have been described. Full spectroscopic data for these compounds are presented.

22-NBD-cholesterol as a novel fluorescent substrate for cholesterol-converting oxidoreductases

Docking simulations and experimental data indicate that 22-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)-23,24-bisnor-5-cholen-3β-ol (22-NBD-cholesterol), a common fluorescent sterol analog, binds into active sites of bovine cytochrome P450scc and microbial cholesterol dehydrogenases (CHDHs) and then undergoes regiospecific oxidations by these enzymes. The P450scc-dependent system was established to realize N-dealkylation activity toward 22-NBD-cholesterol, resulting in 7-nitrobenz[c][1,2,5]oxadiazole-4-amine (NBD-NH(2)) formation as a dominant fluorescent product. Basing on LC-MS data of the probes derivatized with hydroxylamine or cholesterol oxidase, both pregnenolone and 20-formyl-pregn-5-en-3β-ol were deduced to be steroidal co-products of NBD-NH(2), indicating intricate character of the reaction. Products of CHDH-mediated conversions of 22-NBD-cholesterol were defined as 3-oxo-4-en and 3-oxo-5-en derivatives of the steroid. Moreover, the 3-oxo-4-en derivative was also found to be formed after 22-NBD-cholesterol incubation with pathogenic bacterium Pseudomonas aeruginosa, indicating a possible application of the reaction for a selective and sensitive detection of some microbes. The 3-keto-4-en derivative of 22-NBD-cholesterol may be also suitable as a new fluorescent probe for steroid hormone-binding enzymes or receptors.

Assessment of acyl groups and reaction conditions in the competition between perhydrolysis and hydrolysis of acyl resorufins for developing an indicator reaction for fluorometric analysis of hydrogen peroxide

Perhydrolysis of acetyl resorufin (AR) was reported previously to work as a fluorometric indicator reaction for glucose determination using only glucose oxidase. However, hydrolysis of AR in blank solution rendered the working concentration range of this method less than two orders of magnitude. To exclude or at least significantly reduce this interference, acyl groups and reaction conditions in the competition between perhydrolysis and hydrolysis of various acyl resorufins were assessed. Fluorometric evaluation of reactions in the presence or absence of H202 in phosphate buffer (pH 7.5, 100 mm)-CH3CN at 25 degrees C demonstrated that in tert-butylacetyl, isobutyryl, cyclohexanecarbonyl and pivaloyl resorufins (TBAR, IBR, CHR and PVR, respectively) among 10 acyl resorufins examined here, the competitive situation was shifted in a much more favorable way to perhydrolysis than in AR, although fluorometric responses due to their H2O2-dependent deacylation were suppressed in comparison with AR. Examination of the effects of pH, components and concentrations of buffers as well as reaction temperature established reaction conditions that not only allowed perhydrolysis of each of these four compounds to prevail over hydrolysis more effectively, but also improved the H2O2-based fluorometric responses. Thus, perhydrolysis of TBAR, IBR, CHR and PVR in phosphate buffer (pH 8.0, 20 mM)-CH3CN at 25 degrees C worked effectively as fluorometric indicator reactions for H2O2 analysis, affording a calibration curve over a concentration range of three orders of magnitude. Taking sensitivity, reproducibility and the response for blank solution into consideration, PVR seemed to be the best choice as a fluorochromogen for H2O2 determination under these conditions. For H2O2 analysis at lower pH, perhydrolysis of IBR in phosphate buffer (pH 7.5, 20 mm)-CH3CN was shown to effectively function as an indicator reaction. Applicability of the fluorometric methods with PVR and IBR to blood glucose determination was also discussed, comparing with Trinder's method with phenol, 4-aminoantipyrine and peroxidase (POD).

Hydrogen peroxide-induced deacetylation of acetyl resorufin as a novel indicator reaction for fluorometric detection of glucose using only glucose oxidase

Hydrogen peroxide (H2O2)-induced deacetylation of non-fluorescent acetyl resorufin (1) to fluorescent resorufin (2) as a novel indicator reaction for fluorometric detection of glucose using only glucose oxidase (GOD) is described. When a 1:1:1 mixture of 1 (in CH3CN), glucose, and GOD (each in pH 7.4 phosphate buffer) was incubated at 25 degrees C under aerobic conditions, the resulting solution turned yellow to fluorescent pink due to 2. The formation of 2 was markedly retarded on incubation under anaerobic conditions. When a mixture of 1 and H2O2 was incubated under aerobic conditions, the formation of 2 was noted as in the case of the enzymatic reaction of 1. These results demonstrated that the observed color change is brought about through deacetylation of 1 to 2 induced by H2O2 generated in GOD-catalyzed oxidation of glucose. With regard to the fluorometric traces of the enzymatic reaction with 1 (0.2 mM), GOD (0.5 mg/ml), and glucose at 25 degrees C, fluorescence intensity exhibited a linear relationship against glucose concentration between 0.2 and 2.0 mm, with a correlation coefficient of 0.997. Neither ascorbic acid, uric acid, nor bilirubin significantly interfered with the transformation of 1 to 2 through GOD-catalyzed oxidation of glucose.

Real-time fluorescence analysis on molecular mechanisms for regulation of cytochrome P450scc activity upon steroidogenic stimulation in adrenocortical cells

Real-time fluorescence analysis revealed that the activity of cytochrome P450scc was related to Ca2+ signals arising from extracellular NADPH, ACTH and ATP stimulation in adrenocortical fasciculata cells. The side-chain cleavage reaction by cytochrome P450scc was measured with 3beta-hydroxy-22,23-bisnor-5-cholenyl ether (cholesterol-resorufin) by observing the distinct increase in fluorescence upon conversion of cholesterol-resorufin to resorufin and pregnenolone. Adrenocorticotropic hormone (ACTH) induced a relatively small stimulation of the P450scc activity. A significant production of resorufin was revealed after stimulation of cell cultures with 100 pM, 1 nM of ACTH for 3 h. On the other hand, extracellular NADPH was found to rapidly and greatly stimulate the resorufin production in intact cells immediately after the addition of 50-500 microM NADPH. The extracellular NADPH stimulation was prevented by the addition of thapsigargin and EGTA which abolished Ca2+ oscillations induced by NADPH. Suramin, a specific antagonist of the P2y type ATP receptor, also completely abolished the NADPH-induced cholesterol-resorufin conversion. These results imply that extracellular NADPH (membrane impermeable) produced Ca2+ oscillations through its binding to ATP receptor thereby stimulating the activity of P450scc. The application of 45-500 microM extracellular ATP to cells did not, however, significantly increase the resorufin production. These three stimulators produced very different types of Ca2+ signals. ACTH induced mainly a series of Ca2+ spikes superimposed on a long-lasting basal Ca2+ elevation. The Ca2+ signals induced by NADPH showed predominantly a series of Ca2+ spikes without elevation of the basal Ca2+ concentration. Only long-lasting Ca2+ elevation was induced by extracellular ATP. The stimulation of cytochrome P450scc may thus be correlated with the different patterns of Ca2+ signals.

Inhibition of steroidogenesis by luteal cells of early pregnancy in the rat in response to in vitro administration of a gonadotropin-releasing hormone agonist

Previous studies from this laboratory have demonstrated that the administration of a gonadotropin-releasing hormone agonist (GnRH-Ag) in vivo in early or mid-pregnancy to rats induces antifertility effects by suppressing the luteal production of progesterone (P4) within 24h with a concomitant increase in luteal lipid droplets and decreases in the luteal cytochrome P450 side chain cleavage (P450scc) enzyme and its mRNA content. These observations suggest a direct inhibitory effect of GnRH-Ag on the corpus luteum. Here we demonstrate a suppressive effect of GnRH-Ag in vitro on the basal P4, pregnenolone (P5) and 20 alpha-dihydroprogesterone (20 alpha-DHP) production by luteal cells obtained during early pregnancy in rats. We further studied its effect on two key enzymes, namely P450scc and 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD), which participate in the conversion of cholesterol to P5 and conversion of P5 to P4, respectively. We observed that two doses of GnRH-Ag, 10(-4) and 10-7 M, suppress the basal P4 production in vitro after 12 h of incubation by luteal cells; P4 remained suppressed after 48 h of incubation. Basal P5 production was also suppressed after luteal cells were incubated for 12 h with 10(-4) M and 10(-7) M GnRH-Ag, but incubation for 48 h with GnRH-Ag failed to alter P5 production by these cells. 20 alpha-DHP production was suppressed after incubating the luteal cells with both doses of GnRH-Ag for 12 h. GnRH-Ag inhibited P450scc activity after 12 h of incubation and 3 beta-HSD protein content at all time periods measured. These results suggest that GnRH exerts a direct inhibitory effect on luteal steroidogenesis. This inhibition is due to its suppressive effect on P450scc and/or 3 beta-HSD and not due to an increase in P4 metabolites.

Hormone stimulated steroid biosynthesis in granulosa cells studied with a fluorogenic probe for cytochrome P-450SCC

The regulation of steroidogenesis by luteinizing hormone (LH) was studied in granulosa cells during follicular development using a fluorescent reporter assay based on the metabolism of a fluorescent probe specific for cytochrome P-450SCC (cholesterol side-chain cleavage enzyme). Intact granulosa cells or mitochondria were obtained from the first (F1) second (F2) and third (F3) largest preovulatory follicles of the hen ovary and incubated with the fluorogenic substrate. Metabolism of this substrate by cytochrome P-450SCC generates the highly fluorescent resorufin anion (the fluorescent reporter). In both mitochondria and intact granulosa cells, incubated with the fluorescent substrate, an increase in resorufin fluorescence was observed and the increase was greater in samples derived from F1 than in samples from F2 or F3. In cells, LH added simultaneously with the P-450SCC substrate significantly increased resorufin fluorescence above control values in a time- and dose-dependent manner up to 2-3 h after the incubation was initiated. Forskolin and 8-bromo-cAMP also stimulated metabolism of the P-450SCC substrate significantly by 15 min. When granulosa cells were preincubated with LH before exposure to the P-450SCC substrate resorufin fluorescence was significantly attenuated compared to controls (not exposed to LH in the preincubation period). The decrease in resorufin fluorescence observed when cells were pretreated with LH, may be due to the release of cholesterol from endogenous pools and its competition with the exogenous fluorogenic for the substrate P-450SCC enzyme. In granulosa cells that were preloaded with the P-450SCC substrate, the stimulatory effect of LH treatment remained constant from 30 min to 2 h after hormone addition. The results show that this fluorescent probe can be used in a rapid assay for the continuous measurement of the acute effects of hormone agonists on cholesterol conversion to pregnenolone in steroidogenic cells.