Biomedical subjects

N S Ryder

Publications and source records attributed to N S Ryder.

44 records · Page 3Linked to original sources

PubMed1985

Inhibition of squalene epoxidase by allylamine antimycotic compounds. A comparative study of the fungal and mammalian enzymes.

The inhibition of squalene epoxidase by the allylamine antimycotic agents naftifine and compound SF 86-327 was investigated, with particulate enzyme preparations from the pathogenic yeasts Candida albicans and Candida parapsilosis and from rat liver. Both naftifine and compound SF 86-327 were potent inhibitors of the Candida epoxidases and showed apparently non-competitive kinetics with respect to the substrate squalene. The Ki values for naftifine and compound SF 86-327 in the C. albicans system were 1.1 microM and 0.03 microM respectively. The C. parapsilosis enzyme was slightly more sensitive to inhibition. Varying the concentrations of cofactors or the soluble cytoplasmic fraction (S200) had no effect on the inhibition. The epoxidase from rat liver was much less sensitive (Ki for compound SF 86-327 was 77 microM). The inhibition was also qualitatively different from that in Candida, being competitive with respect to squalene and also with respect to the S200 fraction. S200 fraction derived from C. albicans also antagonized the inhibition of the epoxidase from liver, but the liver S200 fraction did not affect inhibition of the Candida enzyme by compound SF 86-327. There was no evidence for an irreversible or mechanism-based inhibition of either the fungal or the mammalian epoxidase. The selective inhibition of squalene epoxidase was sufficient to account for the known antimycotic action of the compounds.

Allylamine↗

PubMed1985

Synthesis of new polyoxin derivatives and their activity against chitin synthase from Candida albicans.

Two analogues of L-alanylpolyoxin C with a modified peptide bond were synthesized and tested for inhibition of chitin synthase in Candida albicans. N-Methylation of the peptide bond (compound 13) or the replacement of it by NH2CH2 (compound 9) led to loss of activity in the enzyme assay. A novel analogue (compound 5) of nikkomycin was synthesized from uracil polyoxin C and (2S,3R)-3-hydroxyhomotyrosine, a component of echinocandin C. Despite high activity in the chitin synthase assay, 5 had no inhibitory effect on cells of C. albicans.

Antifungal Agents↗

PubMed1985

Effect of allylamine antimycotic agents on fungal sterol biosynthesis measured by sterol side-chain methylation.

Sterol side-chain (C-24) methylation was assayed by incorporation of radioactivity from [Me-14C]methionine into the ergosterol fraction in cells of the pathogenic fungi Candida albicans, Candida parapsilosis and Trichophyton mentagrophytes. Methylation at C-24 occurred after nuclear demethylation in all cases. The method was used to measure ergosterol biosynthesis inhibition by the allylamine antimycotics naftifine and SF 86-327, which are known to block squalene epoxidation. In C. albicans cells treated with SF 86-327 (1 mg l-1) to fully inhibit squalene epoxidation, C-24 methylation continued for several hours at about 40% of the control rate. This residual biosynthesis was probably due to methylation of endogenous sterol precursors. The degree of residual biosynthesis in the three fungi correlated well with their susceptibility to SF 86-327. The highly susceptible dermatophyte T. mentagrophytes had negligible residual sterol biosynthesis. These differences were not due to inhibition of methionine uptake. For naftifine (100 mg l-1) there was evidence of a second inhibitory action in C. albicans. A cell-free assay indicated that this was due to direct inhibition of the C-24 methyltransferase.

Adenosine↗

PubMed1985

Specific inhibition of fungal sterol biosynthesis by SF 86-327, a new allylamine antimycotic agent.

SF 86-327 is a new antimycotic agent of the allylamine type. Its primary action appears to be the inhibition of ergosterol biosynthesis at the point of squalene epoxidation, as was previously found with the related compound naftifine. Biosynthesis was measured by incorporation of [14C]acetate into sterols in cells of Candida albicans, Candida parapsilosis, Torulopsis glabrata, and the dermatophyte Trichophyton mentagrophytes. There was a positive correlation between the SF 86-327 concentrations needed for inhibition of growth and of sterol synthesis in these four fungi. The greater antifungal efficacy of SF 86-327 in comparison with naftifine was also reflected in the relative activities of the two compounds as sterol synthesis inhibitors. Inhibition was maximal at neutral pH. A similar degree of inhibition was found in cell-free extracts when [14C]mevalonate was used as substrate. In all cases, inhibition of sterol synthesis was accompanied by a parallel accumulation of labeled squalene. SF 86-327 and naftifine had no significant effect on initial enzymes of the ergosterol pathway, measured by incorporation of [14C]acetyl coenzyme A, or on steps distal to squalene epoxidation, measured by conversion of labeled squalene 2,3-epoxide or lanosterol. Both allylamines were highly selective for fungal, as opposed to mammalian, sterol biosynthesis. SF 86-327 caused slight inhibition of squalene epoxidation in a rat liver cell-free system, but at concentrations three to four orders of magnitude greater than those required for inhibition of the fungal pathway.

Allylamine↗

PubMed1984

Properties of a particulate squalene epoxidase from Candida albicans.

The properties and requirements of squalene epoxidase and effects of some inhibitors were investigated in the pathogenic yeast Candida albicans. A washed 'microsomal' fraction converted radiolabelled squalene to 2,3-oxidosqualene and lanosterol. Minimum requirements for activity were molecular oxygen, NADH or NADPH, and FAD. Epoxidase activity was stimulated by up to 100% by addition of the soluble cytoplasmic fraction, which itself contained negligible epoxidase activity. This stimulation was most powerful at low concentrations of enzyme, or high concentrations of squalene. Divalent cations did not stimulate activity and EDTA was not inhibitory. An apparent Km for squalene of 50 microM was determined in the presence of soluble cytoplasm. Epoxidase activity was destroyed by Triton X-100, deoxycholate or Cu2+, and partially inhibited by thiol reagents, rotenone and antimycin A. The enzyme was not inhibited by cyanide or by several inhibitors of cytochrome P-450.

Candida albicans↗

PubMed1984

Allylamine derivatives: new class of synthetic antifungal agents inhibiting fungal squalene epoxidase.

A new class of synthetic antifungal agents, the allylamines , has been developed by modification of naftifine , a topical antimycotic. SF 86-327, the most effective of these compounds so far, is highly active in vitro against a wide range of fungi and exceeds clinical standards in the oral and topical treatment of guinea pig dermatophytoses. SF 86-327 is a powerful specific inhibitor of fungal squalene epoxidase, a key enzyme in sterol biosynthesis.

Allylamine↗

PubMed1984

Effect of the antimycotic drug naftifine on growth of and sterol biosynthesis in Candida albicans.

Naftifine, a new antimycotic drug of the allylamine class, is a potent inhibitor of ergosterol biosynthesis in Candida albicans. Treated cells showed a dose-dependent drop in ergosterol content; the level was reduced by 60% at concentrations of greater than 50 mg/liter, causing total inhibition of growth. This inhibition coincided with a heavy accumulation of the sterol precursor squalene. Radiolabeling experiments showed that the inhibition of sterol synthesis was complete within 10 min of exposure of cells to the compound. Control cells incorporated [14C]acetate into nonsaponifiable lipids composed primarily of ergosterol, whereas naftifine-treated cells accumulated only labeled squalene. When the drug was removed by washing cells thoroughly in 1% Tween 80, the accumulated squalene was further metabolized to ergosterol. A similar pattern of inhibition was observed in sterol biosynthesis from [14C]mevalonate in a cell-free system. At 50 mg/liter, naftifine gave greater than 99% inhibition of sterol biosynthesis both in whole cells and in cell extracts of C. albicans. The primary action of naftifine appears to be the blocking of fungal squalene epoxidation.

Acetates↗

PubMed1980

The effect of the 3-hydroxy-3-methylglutaryl CoA reductase inhibitor ML-236b on phytosterol synthesis in Acer pseudoplatanus tissue culture.

Sycamore cell cultures were incubated with various labelled sterol precursors. ML-236B, a fungal metabolite, caused virtually total inhibition of acetate or leucine incorporation into sterols, while mevalonate incorporation was unaffected. Sterol synthesis from endogenous precursors, measured by incorporation of [Me-14C] methionine into the side chain, continued at a reduced rate for at least 6 h after addition of the inhibitor.

Carbon Radioisotopes↗