Mechanism of protection by sterols against polyene antibiotics

Antifungal Activity of Select Essential Oils against Candida auris and Their Interactions with Antifungal Drugs

Candida auris is an emerging fungal pathogen that commonly causes nosocomial blood infections in the immunocompromised. Several factors make this pathogen a global threat, including its misidentification as closely related species, its ability to survive for weeks on fomites, and its resistance to commonly prescribed antifungal drugs, sometimes to all three classes of systemic antifungal drugs. These factors demonstrate a need for the development of novel therapeutic approaches to combat this pathogen. In the present study, the antifungal activities of 21 essential oils were tested against C. auris. Several essential oils were observed to inhibit the growth and kill C. auris, Candida lusitaniae, and Saccharomyces cerevisiae when in direct contact and at concentrations considered safe for topical use. The most effective essential oils were those extracted from lemongrass, clove bud, and cinnamon bark. These essential oils also elicited antifungal activity in gaseous form. The efficacies of formulations comprised of these three essential oils in combination with fluconazole, amphotericin B, flucytosine, and micafungin were explored. While synergism was neither observed with cinnamon bark oil nor any of the antifungal drugs, lemongrass oil displayed synergistic, additive, and indifferent interactions with select drugs. Formulations of clove bud oil with amphotericin B resulted in antagonistic interactions but displayed synergistic interactions with fluconazole and flucytosine. These essential oils and their combinations with antifungal drugs may provide useful options for surface disinfection, skin sanitization, and possibly even the treatment of Candida infections.

Membrane activity of the pentaene macrolide didehydroroflamycoin in model lipid bilayers

Didehydroroflamycoin (DDHR), a recently isolated member of the polyene macrolide family, was shown to have antibacterial and antifungal activity. However, its mechanism of action has not been investigated. Antibiotics from this family are amphiphilic; thus, they have membrane activity, their biological action is localized in the membrane, and the membrane composition and physical properties facilitate the recognition of a particular compound by the target organism. In this work, we use model lipid membranes comprised of giant unilamellar vesicles (GUVs) for a systematic study of the action of DDHR. In parallel, experiments are conducted using filipin III and amphotericin B, other members of the family, and the behavior observed for DDHR is described in the context of that of these two heavily studied compounds. The study shows that DDHR disrupts membranes via two different mechanisms and that the involvement of these mechanisms depends on the presence of cholesterol. The leakage assays performed in GUVs and the conductance measurements using black lipid membranes (BLM) reveal that the pores that develop in the absence of cholesterol are transient and their size is dependent on the DDHR concentration. In contrast, cholesterol promotes the formation of more defined structures that are temporally stable.

Anti-Candida albicans effectiveness of citral and investigation of mode of action

CONTEXT

Candidiasis is a mycosis caused by Candida species, which is of clinical importance due to the increase in resistant yeasts. Candida infection has been a serious health problem due to the inappropriate use of antibiotics. Therefore, it is necessary to study molecules with an antifungal action. Citral is a monoterpene with known pharmacological properties, including antimicrobial action.

OBJECTIVE

The aim of this work was to determine the minimum inhibitory concentration (MIC) and minimum fungicidal concentration (MFC) of citral and the probable mode of action.

MATERIALS AND METHODS

The MIC of citral was determined by the broth microdilution method using Sabouraud dextrose medium. Additionally, the interference of citral in cell wall (sorbitol assay) and the binding of citral to ergosterol and cholesterol were studied, carried out by broth microdilution method.

RESULTS

The MIC and MFC of citral were 512 and 1024 µg/mL, respectively. The MIC of amphotericin B was 1 µg/mL. The mechanism of action did not involve either the cell wall or ergosterol. However, the presence of cholesterol increased the MIC of citral to 1024 µg/mL, indicating there is some interaction between citral and cholesterol. Amphotericin B was used as the positive control, and it showed a high MIC in the presence of ergosterol (32 µg/mL), while in the presence of cholesterol MIC increased to 4 µg/mL.

CONCLUSION

Citral inhibits the growth of C. albicans. The probable mechanism of action did not involve the cell wall or ergosterol. Citral is able to interact with cholesterol. More studies are necessary to describe their effects completely.

Probable Mechanism(s) of Antifungal Activity of SJA-95, a Heptaene Polyene Antibiotic

A new strain, streptomyces sp. S. 24 was isolated from a soil sample collected from Japan. The strain produced heptaene polyene antibiotic, SJA-95, in submerged culture and found to elicit promising antifungal activity against yeasts, filamentous fungi and clinical isolates, both in vitro and in vivo. Experimental studies were carried out using biological methods to understand the probable mechanism(s) of antifungal activity of SJA-95. Our experimental findings suggest that SJA-95 binds more avidly to ergosterol, the sterol in fungal cell membranes, than to cholesterol found in mammalian cell membranes. Such preferential binding of SJA-95 to ergosterol might help to establish its usefulness as a chemotherapeutic agent with lesser adverse reactions.

Polyene-resistant mutants of Aspergillus fennelliae: sterol content and genetics

Mutants of Aspergillus fennelliae (Neosartorya fennelliae) resistant to relatively high levels of amphotericin B and low levels of nystatin were obtained by successive transfers of wild type in the presence of increasing concentrations of the polyenes. The resistance of the mutants to the polyenes was accompanied by both qualitative and quantitative changes in the sterol composition of the cells. Those resistant to amphotericin B (AF5-AB1 and p-AB1) lacked ergosterol, the major sterol of the wild type, but contained a new sterol clearly distinguished by the pattern of ultraviolet spectrophotometry and thin-layer chromatography. The mutants resistant to nystatin, however, contained both ergosterol and a new sterol, but the former was produced in a much reduced amount, as compared with the wild type. Genetic analysis indicated that the lack of ergosterol is closely associated with a reduced growth rate, poor asexual reproduction, and the loss of sexual reproduction. Growth studies revealed that the addition of ergosterol to the media did not affect the growth pattern of the mutants. Mutants resistant to amphotericin B showed an increased minimal inhibitory concentration for nystatin, pimaricin, and filipin. Mutants resistant to nystatin, however, conferred increased minimal inhibitory concentration for pimaricin and filipin but not for amphotericin B.

Effect of amphotericin B and clotrimazole on lymphocyte stimulation

Human lymphocytes were stimulated in vitro by phytohemagglutinin, concanavalin A, pokeweed mitogen, purified protein derivative-tuberculin, and allogenic cells. The deoxyribonucleic acid synthesis of the lymphocytes was inhibited in increasing degree by 4 to 8 mug of amphotericin B per ml of culture irrespective of lymphocyte stimulant used. The effect of clotrimazole on the deoxyribonucleic acid synthesis varied between experiments with different and also between experiments with the same stimulant. Ten micrograms of clotrimazole per ml was generally inhibiting, whereas in one experiment 2 mug or more per ml inhibited the purified protein derivative-induced deoxyribonucleic acid synthesis. The effects of amphotericin B and clotrimazole were neutralized by serum.

Antifungal agents: mode of action, mechanisms of resistance, and correlation of these mechanisms with bacterial resistance

The increased use of antibacterial and antifungal agents in recent years has resulted in the development of resistance to these drugs. The significant clinical implication of resistance has led to heightened interest in the study of antimicrobial resistance from different angles. Areas addressed include mechanisms underlying this resistance, improved methods to detect resistance when it occurs, alternate options for the treatment of infections caused by resistant organisms, and strategies to prevent and control the emergence and spread of resistance. In this review, the mode of action of antifungals and their mechanisms of resistance are discussed. Additionally, an attempt is made to discuss the correlation between fungal and bacterial resistance. Antifungals can be grouped into three classes based on their site of action: azoles, which inhibit the synthesis of ergosterol (the main fungal sterol); polyenes, which interact with fungal membrane sterols physicochemically; and 5-fluorocytosine, which inhibits macromolecular synthesis. Many different types of mechanisms contribute to the development of resistance to antifungals. These mechanisms include alteration in drug target, alteration in sterol biosynthesis, reduction in the intercellular concentration of target enzyme, and overexpression of the antifungal drug target. Although the comparison between the mechanisms of resistance to antifungals and antibacterials is necessarily limited by several factors defined in the review, a correlation between the two exists. For example, modification of enzymes which serve as targets for antimicrobial action and the involvement of membrane pumps in the extrusion of drugs are well characterized in both the eukaryotic and prokaryotic cells.

Effects of mepartricin, a polyene macrolide agent, on fecal excretion and serum concentration of estrogen and number of prostatic estrogen receptors in immature rats

BACKGROUND

Mepartricin, an antifungal agent, was investigated for effects on fecal excretion and serum concentration of sex steroids and the number of sex steroid prostatic receptors in immature rats.

METHODS

Mepartricin was orally administered at 2.5, 5, and 10 mg/kg once daily for 2 weeks. Fecal estrogen and testosterone excretions, serum estrogen, testosterone and luteinizing hormone concentrations, and numbers of prostatic estrogen and androgen receptors were assayed. Prostate weight was also monitored.

RESULTS

Fecal estrogen excretion showed a dose-dependent increase, which was significant for the two higher dosages. Conversely, the serum estrogen concentration and prostatic estrogen receptors were significantly decreased. No significant changes in fecal testosterone excretion, serum testosterone and luteinizing hormone concentrations, and prostatic androgen receptors were observed. Prostate weight was significantly reduced at 5 mg/kg, but we did not observe dose-dependency.

CONCLUSIONS

Mepartricin increases fecal excretion of estrogen by binding with it in the intestinal tract, which results in reducing the serum estrogen concentration and number of prostatic estrogen receptors.

Location and role of sterol at nystatin-binding sites

Lampen, J. O. (Rutgers, The State University, New Brunswick, N.J.), Peter M. Arnow, Zofia Borowska, and Allen I. Laskin. Location and role of sterol at nystatin-binding sites. J. Bacteriol. 84:1152-1160. 1962.-The polyene antifungal antibiotics nystatin and N-acetylcandidin were bound rapidly at 0 C by isolated cell walls and derived polysaccharides or by protoplast membranes from Saccharomyces cerevisiae strain LK2G12. These binding sites were relatively inaccessible or unreactive in the intact cell, since polyene uptake by protoplasts, log-phase cells, or stationary-phase cells was slow, especially at 0 C. Binding by the membrane appears to be the critical event in cell damage; thus, uptake of nystatin by the cell wall may actually be protective. Binding by all cell forms showed little reversibility. Bound radioactive nystatin or N-acetyl (1-C(14)) candidin was not displaced during incubation of log-phase yeast cells with a large excess of unlabeled polyene. Saturation of the polyenic moiety of nystatin to form the perhydro compound eliminated almost completely the affinity of the molecule for the yeast cell. Essentially all of the polyene bound by protoplasts was present on the membrane. It was removed by treatment of the protoplasts with the sterol-complexing agent digitonin. A variety of evidence is offered that the binding site on the membrane contains a sterol. (The membrane sterol was mostly unesterified ergosterol.) This hypothesis is consistent with the ability of certain exogenous sterols to complex with polyenes and prevent their binding and growth-inhibiting action for fungi. The bound sterol of the wall structure may also participate in polyene binding, although a possible function of the wall polysaccharides cannot be excluded. The specific binding structure(s) appears to be absent from bacteria, since nystatin was not taken up even by heated or benzene-treated bacteria or by bacterial protoplasts. It should be noted that bacteria, which are insensitive to the polyene antibiotics, generally contain only traces of sterol, if any. Considerable quantities of sterol are present in fungi, algae, certain protozoa, and animal cells, all of which are sensitive in some degree.

The effect of aggregation state of amphotericin-B on its interactions with cholesterol- or ergosterol-containing phosphatidylcholine monolayers

Amphotericin B (AmB) is the most effective antibiotic used in the treatment of systemic fungal infections. It is generally thought that the activity of this drug results from its interaction with ergosterol, the main sterol of fungi membranes. However, AmB also interacts with cholesterol, the major sterol of mammal membranes, thus limiting the usefulness of this drug due to its relatively high toxicity. The aim of the present work is to study the molecular basis of the interactions of AmB with these sterols contained in a DOPC film by using the monolayer technique. Two different concentrations of the sterols in the film (13 and 30%) at an initial surface pressure of 30 mN/m were studied, which correspond to conditions found in various biological membranes. Four concentrations of AmB in the subphase, ranging from a molecularly dispersed to a highly aggregated state of the drug were studied. Our results show that the monomeric form of AmB interacts with the ergosterol containing film solely. On the other hand, when AmB is dispersed as a pre-micellar or as a highly aggregated state in the subphase, a very significant selectivity of its interactions between the two sterols is observed which is shown in our experimental results by a difference of 8 mN/m in the surface pressure when AmB interacts with ergosterol as compared to cholesterol. We show that the activity of AmB is most likely related to the micellar form of the antibiotic. In addition, we observe that upon increasing the amount of ergosterol in the film, the insertion of AmB is largely promoted, results that are discussed in terms of the molecular organization of the sterols within the monolayer film. We show that these results provide a better understanding of the action of AmB (activity/toxicity) at the membrane level.

CYTOLOGY OF CANDIDA ALBICANS AS INFLUENCED BY DRUGS ACTING ON THE CYTOPLASMIC MEMBRANE

Gale, Glen R. (Veterans Administration Hospital, Durham, N.C.) Cytology of Candida albicans as influenced by drugs acting on the cytoplasmic membrane. J. Bacteriol. 86:151-157. 1963.-An electron microscopic comparison was made of the cytological responses of Candida albicans to benzalkonium chloride, amphotericin B, and filipin, all of which are thought to exert their primary pharmacological actions on the cytoplasmic membrane of susceptible microorganisms. After 5 to 15 min of exposure to benzalkonium chloride, most of the cytoplasm became less dense, and intact nuclei, mitochondria, or intracytoplasmic membranes were not observed. The remaining dense cytoplasm contained numerous small holes. The polyene antifungal agents, amphotericin B and filipin, caused a reduction in cytoplasmic density, but had no observable effect on nuclei or mitochondria. The intervals before onset of changes induced by the polyene agents correlated with the known time-dependent binding of these drugs by cells, and the decreased electron scattering induced by all three agents was compatible with their known abilities to cause a loss of integrity of the cytoplasmic membrane with resultant leakage of cytoplasmic components. No consistent morphological effect of any of these drugs on the cytoplasmic membrane could be shown.

Filipin and its interaction with cholesterol in aqueous media studied using static and dynamic light scattering

Aggregation of filipin in aqueous medium and filipin-induced changes in cholesterol micelles have been studied using intensity and dynamic light scattering. The dependencies of filipin aggregate dimensions on concentration, solvent, and temperature were studied, and revealed that the aggregates do not have a well-defined geometry, i.e., a critical micelle concentration cannot be detected and stable structures are not formed. The aggregates are of size Rg approximately 110 nm and Rh approximately 63 nm, referring to the radius of gyration and hydrodynamic radius, respectively. In the concentration range studied (1 microM < C < 30 microM), a low molecular weight species (monomer/dimer) is always present together with the aggregates. In ethanol/water mixtures, large (Rg approximately 500 nm), narrow distribution aggregates are formed in the water volume fraction range 0.45 < phi H2O < 0.65. Aggregation also occurs on changing the temperature; In the range 7-37 degrees C, smaller aggregates (10-30 nm form and the process is only partially reversible. No pronounced effect of filipin on the structure of the cholesterol micelles was observed (a small increase in Rg and Rh is noted). These results rule out any "specificity" for the filipin interactions with cholesterol, which has been considered a key event in the filipin biochemical mode of action. A reevaluation of this question is suggested and some alternatives are advanced.

Antifungal susceptibility testing

Unlike antibacterial susceptibility testing, reliable antifungal susceptibility testing is still largely in its infancy. Many methods have been described, but they produce widely discrepant results unless such factors as pH, inoculum size, medium formulation, incubation time, and incubation temperature are carefully controlled. Even when laboratories agree upon a common method, interlaboratory agreement may be poor. As a result of numerous collaborative projects carried out both independently and under the aegis of the Subcommittee on Antifungal Susceptibility Testing of the National Committee for Clinical Laboratory Standards, the effects of varying these factors have been extensively studied and a standard method which minimizes interlaboratory variability during the testing of Candida spp. and Cryptococcus neoformans has been proposed. This review summarizes this work, reviews the strengths and weaknesses of the proposed susceptibility testing standard, and identifies directions for future work.

Conformation of polyene antibiotic, filipin III: CD and 1H NMR studies

Detailed studies on the solution conformation of polyene antibiotic, filipin III using circular dichroism (CD) and proton nuclear magnetic resonance techniques have been carried out. In dimethyl sulfoxide (DMSO), filipin III exhibits concentration dependent aggregation-monomeric at lower and oligomeric at higher concentrations of the antibiotic. At concentrations used for 1H NMR studies (6 x 10(-3)M) the molecule coexists as monomeric and oligomeric species. However, titration experiments indicated that, in a mixed solvent system of DMSO:methanol (2:3 v/v) the antibiotic exists only as a monomer. Complete 1H NMR assignments and the conformation of the monomer filipin III have been determined by the combined use of DQFCOSY and ROESY experiments in DMSO:methanol solvent system.

Fluorescence study of the macrolide pentaene antibiotic filipin in aqueous solution and in a model system of membranes

The polyene antibiotic filipin (a pentaene) has been studied using photophysical techniques. The polyene self-aggregates in water with a critical micellar concentration of 2 microM. Two approaches were used to evaluate the aggregate dimensions: (a) a lower limit of 10 nm for the aggregate radius was obtained from energy transfer experiments; (b) a formula for rationalizing the turbidity spectrum was derived, and from its application a spherical shape of radius about 50 nm was deduced. The low value for the fluorescence anisotropy of the aggregate (r = 0.02) is compatible with a very loose structure, i.e. the chromophore has very efficient depolarization dynamics that is not controlled by the aggregate size. The Stern-Volmer plot of aggregated filipin fluorescence quenching by iodide is non-linear, presenting a downward curvature. A model was used for the interpretation of these data, along with a study of the quenching in transient state; it was concluded that all the components of the decay are affected by the quencher, i.e. the aggregate has a very open structure with respect to the iodide ion. The partition constants of the polyene, Kp, between a model system of membranes (small unilamellar vesicles of dipalmitoylglycerophosphocholine) and the aqueous phase were determined from anisotropy measurements; the values obtained were Kp (gel phase) = (3.4 +/- 0.8) x 10(3) and Kp (liquid crystal phase) = (7.7 +/- 2.2) x 10(2). The observation that the polyene incorporation is efficient is at variance with the belief that the presence of sterols are essential for the interaction of polyene antibiotics with membranes [for review see Bolard, J. (1986) Biochim. Biophys. Acta 864, 257-304].

Collaborative investigation of variables in susceptibility testing of yeasts

A multicenter study was performed to evaluate the effect of medium, incubation time (24 and 48 h), and temperature (30 and 35 degrees C) on intra- and interlaboratory variations in MICs of flucytosine, amphotericin B, and ketoconazole for yeasts. Testing was performed on coded isolates of Candida species (11 strains) and Cryptococcus neoformans (2 strains) by using a standard macrodilution protocol 11 laboratories. Four chemically defined media buffered to pH 7.0 with morpholinepropanesulfonic acid were evaluated, including buffered yeast nitrogen base, synthetic amino acid medium-fungal, RPMI 1640 medium, and high-resolution antifungal assay medium. Intralaboratory variability was less than or equal to fourfold for 97% of the replicate sets of data. The highest level of interlaboratory agreement, irrespective of antifungal agent or incubation conditions, was observed with RPMI 1640 medium. Intralaboratory variability was less than or equal to fourfold for 93% of the determinations with ketoconazole and 100% with flucytosine tested in RPMI 1640 medium at 35 degrees C for 24 h. Variability in amphotericin B results was less than or equal to fourfold for 81% of the determinations in RPMI 1640 medium at 35 degrees C for 48 h. The rank order of MICs within each antifungal test group was similar among the various laboratories and was generally in agreement with the reference rank order regardless of the test medium that we used.

Prevention of amphotericin-B-induced nephrotoxicity by loading with sodium chloride: a report of 1291 days of treatment with amphotericin B without renal failure

An open, prospective study was performed to evaluate the clinical usefulness of sodium chloride loading for prevention of amphotericin-B-induced nephrotoxicity in 37 patients requiring 44 courses of amphotericin B treatment. The median duration of the treatment course was 22 days (range, 9-136 days), and mean cumulative dose per patient was 1117 mg (range, 231-7831 mg). During amphotericin B treatment, all patients received 50 to 100 ml of 10% sodium chloride (85 to 171 mmol NaCl) via an intravenous line for prevention of amphotericin-B-induced nephrotoxicity evaluated by serum creatinine levels. Using this regimen, none of the patients developed significant nephrotoxicity (increase in serum creatinine of more than twice baseline level, or serum creatinine level greater than or equal to 2.0 mg/dl, respectively) despite the co-administration of other potentially nephrotoxic drugs. It was not necessary to discontinue treatment with amphotericin B in any of the patients. There were no side effects due to sodium chloride loading. Our results demonstrate that sodium chloride loading is useful for the prevention of amphotericin-B-induced nephrotoxicity.

Mechanism of action of amphotericin B on Leishmania donovani promastigotes

The growth of Leishmania donovani promastigotes in a liquid medium was completely inhibited by amphotericin B at a concentration of 0.3 microgram ml-1 (0.3 microM). Continuous release of small molecules that absorb at 260 nm and 280 nm was observed after contact with the drug. Uptake of [U-14C]glucose was inhibited in cells treated with the drug. An immediate release of isotopic glucose and its metabolites from preloaded cells could be demonstrated after incubation with amphotericin B (0.4 microM). Inhibition of respiration by the drug was a comparatively slower process. All the above effects could be effectively prevented in the presence of either cholesterol or ergosterol. The primary site of action of amphotericin B on L. donovani promastigote cells appears to be membrane sterols that result in a loss of the permeability barrier to small metabolites. An interesting biochemical similarity, thus, emerges between flagellated protozoa and fungi.

Nystatin-resistance of candida albicans isolates from two cases of oral candidiasis

Two cases of oral candidiasis are described which failed to respond to nystatin therapy when used in combination with triamcinolone acetonide. The isolates of C. albicans obtained from the patients after treatment showed high in vitro resistance to nystatin when tested in combination with triamcinolone acetonide. Triamcinolone acetonide was detected in the saliva of both patients after treatment. Addition of this saliva to the isolates of C. albicans obtained after treatment was found to confer nystatin resistance. Both patients were treated with miconazole nitrate and a mycological and clinical cure was obtained in one of the cases.

Action of peptidolipidic antibiotics of the iturin group on erythrocytes. Effect of some lipids on hemolysis

Iturin A, bacillomycin L and bacillomycin L dimethyl ester have a strong lytic activity upon human erythrocytes while iturin C is totally inactive. The hemolytic action of the antibiotics is inhibited by free cholesterol as well as by cholesterol included in mixed liposomes of phosphatidylcholine-cholesterol and to a lesser extent by phosphatidylcholine liposomes. This inhibition is the result of an interaction between the antibiotic and added lipids which diminishes the concentration of free antibiotic available to lyse erythrocytes. The inhibitory effect of liposomes on hemolysis demonstrated the affinity of the antibiotic for artificial membrane, especially those containing cholesterol.

Effects of amphotericin B, nystatin, and other polyene antibiotics on chitin synthase

The effects of amphotericin B (AmB), nystatin, filipin, and pimaricin were tested chitin synthase (EC 2.4.1.16) (chitosomes from yeast cells of Mucor rouxii). AmB and nystatin inhibited the enzyme at concentrations greater than or equal to 10 micrograms/ml, filipin was weakly inhibitory, and pimaricin had no effect. The inhibition of chitin synthase by AmB appears to be noncompetitive, with a Ki value of about 0.13 mM. the effect of nystatin was more complex and included a sharp stimulation of chitin synthase activity at approximately 50 micrograms/ml. Our findings suggest the existence of binding sites (sterols?) on the chitosome that are selective for certain polyenes and that play a role in the operation of chitin synthase. Because the minimal growth inhibitory concentrations of AmB or nystatin are lower than the concentrations that inhibit chitin synthase in vitro, the possibility of chitosomal chitin synthase being a primary target for the antifungal action of these polyenes seems unlikely.

Effect of fatty acids on action of polyene antibiotics

Fatty acids cause a decrease in the absorption spectra of the antifungal polyene macrolide antibiotics nystatin, filipin, candicidin, and amphotericin B. For nystatin, filipin, and candicidin, this decrease in absorption could be correlated with the activity of the fatty acids in protecting the yeast Saccharomyces cerevisiae against the action of these antibiotics. With amphotericin B a correlation was observed between the decrease in absorption caused by certain derivatives of fatty acids and the protective action of these derivatives against the activity of amphotericin B on yeast. It is concluded that, like the sterols, fatty acids also interact with the polyene antibiotics and thereby reduce their effective concentrations.

Filipin prevents and reverses insulin stimulation of rat adipocyte phosphodiesterase

A membrane fraction prepared from isolated rat adipocytes contained an insulin-sensitive cyclic nucleotide phosphodiesterase (EC 3.1.4.17) which catalyzed the hydrolysis of both adenosine 3',5'-monophosphate (cAMP) and guanosine 3',5'-monophosphate (cGMP). The rate of hydrolysis of cGMP was about one-third that of cAMP. The hydrolysis of the two nucleotides appeared to be assoicated with one catalytic site: one nucleotide interfered with the hydrolysis of the other, in a manner predictable from the kinetic constants in that the Km of one nucleotide as a substrate was comparable to its Ki as an inhibitor of the hydrolysis of the other nucleotide. Incubation of the adipocytes with insulin increased the Vmax of phosphodiesterase without affecting the Km values for either substrate. After adipocytes had been treated with filipin, a membrane perturbant, at a concentration that did not cause cell lysis, the response of phosphodiesterase to insulin was obliterated. Further, the insulin-stimulated phosphodiesterase activity was reversed when hormone-treated cells were subsequently incubated with this agent. These results suggest that the response of membrane phosphodiesterase to insulin is impaired once adipocytes have been exposed to filipin, either preceding or following the incubation with insulin.

Effects of filipin and steroids on phytochrome pelletability

Red light given to dark-grown etiolated leaves of Hordeum vulgare L. in vivo or to crude homogenates increases the phytochrome content of the 20,000 g pellet on centrifugation. The steroids cholesterol and stigmasterol inhibit this red light-induced phytochrome pelletability. Filipin (a polyene antibiotic, which is known to combine with steroids) inhibits red light-induced phytochrome pelletability. Filipin and steroids at the appropriate concentration applied together prevent the inhibition caused by either when applied alone. These results suggest that phytochrome may bind to a steroid component of membranes. The phospholipid phosphatidyl choline dipalmitoyl has no effect on red light-induced phytochrome pelletability. Preliminary evidence demonstrates a direct association of soluble phytochrome in its active form and steroids. The physiological significance of red light-induced pelletability and the primary mechanism of phytochrome action are discussed in terms of a hypothetical steroid-binding site.

Sterol-polyene antibiotic complexation: probe of membrane structure

Polyene antibiotics are useful tools for studying the role of sterols in biological membranes. The interaction of polyene antibiotics with membrane-bound sterols in artificial membrane systems, prokaryotic and eukaryotic cells, and lipid-containing viruses is reviewed. The pentaene macrolide, filipin, is shown to serve as a probe of phosphatidylcholine-sterol interaction and of the localization of cholesterol in the membrane of mycoplasmas.

Protection by unsaturated lecithin against the imidazole antimycotics, clotrimazole and miconazole

The activity of egg lecithin in preventing the antifungal action of the two imidazole antimycotics, clotrimazole and miconazole, was confirmed. However, addition of this phospholipid could not relieve an existing imidazole inhibition. Compared with egg lecithin, reduced egg lecithin showed no such protective effect. The addition of egg lecithin to an aqueous suspension of the imidazole drugs changed the absorption profile of the imidazole, suggesting a low solubility and, consequently, a lower effective concentration; however, the addition of reduced egg lecithin did not produce any change in the adsorption. These results indicate that the preventive effect of egg lecithin on imidazole inhibition may be a consequence of preferential in vitro interaction of the drug with unsaturated phospholipid to form a hydrophobic complex.