Studies on protoporphyrin biosynthetic pathway in Saccharomyces cerevisiae ; characterization of the tetrapyrrole intermediates

Recent Advances in Photodynamic Therapy against Fungal Keratitis

Fungal keratitis is a serious clinical infection on the cornea caused by fungi and is one of the leading causes of blindness in Asian countries. The treatment options are currently limited to a few antifungal agents. With the increasing incidence of drug-resistant infections, many patients fail to respond to antibiotics. Riboflavin-mediated corneal crosslinking (similar to photodynamic therapy (PDT)) for corneal ectasia was approved in the US in the early 2000s. Current evidence suggests that PDT could have the potential to inhibit fungal biofilm formation and overcome drug resistance by using riboflavin and rose bengal as photosensitizers. However, only a few clinical trials have been initiated in anti-fungal keratitis PDT treatment. Moreover, the removal of the corneal epithelium and repeated application of riboflavin and rose bengal are required to improve drug penetration before and during PDT. Thus, an improvement in trans-corneal drug delivery is mandatory for a successful and efficient treatment. In this article, we review the studies published to date using PDT against fungal keratitis and aim to enhance the understanding and awareness of this research area. The potential of modifying photosensitizers using nanotechnology to improve the efficacy of PDT on fungal keratitis is also briefly reviewed.

Plant Photodynamic Stress: What's New?

In the 1970's, an unconventional stressful photodynamic treatment applied to plants was investigated in two directions. Exogenous photosensitizer treatment underlies direct photodynamic stress while treatment mediating endogenous photosensitizer over-accumulation pinpoints indirect photodynamic stress. For indirect photodynamic treatment, tetrapyrrole biosynthesis pathway was deregulated by 5-aminolevulenic acid or diphenyl ether. Overall, photodynamic stress involves the generation of high amount of reactive oxygen species leading to plant cell death. All these investigations were mainly performed to gain insight into new herbicide development but they were rapidly given up or limited due to the harmfulness of diphenyl ether and the high cost of 5-aminolevulinic acid treatment. Twenty years ago, plant photodynamic stress came back by way of crop transgenesis where for example protoporphyrin oxidases from human or bacteria were overexpressed. Such plants grew without dramatic effects of photodamage suggesting that plants tolerated induced photodynamic stress. In this review, we shed light on the occurrence of plant photodynamic stress and discuss challenging issues in the context of agriculture focusing on direct photodynamic modality. Indeed, we highlighted applications of exogenous PS especially porphyrins on plants, to further develop an emerged antimicrobial photodynamic treatment that could be a new strategy to kill plant pathogens without disturbing plant growth.

Photodynamic antifungal chemotherapy

The growing resistance against antifungal drugs has renewed the search for alternative treatment modalities, and antimicrobial photodynamic therapy (PDT) seems to be a potential candidate. Preliminary findings have demonstrated that dermatophytes and yeasts can be effectively sensitized in vitro and in vivo by administering photosensitizers (PSs) belonging to four chemical groups: phenothiazine dyes, porphyrins and phthalocyanines, as well as aminolevulinic acid, which, while not a PS in itself, is effectively metabolized into protoporphyrin IX. Besides efficacy, PDT has shown other benefits. First, the sensitizers used are highly selective, i.e., fungi can be killed at combinations of drug and light doses much lower than that needed for a similar effect on keratinocytes. Second, all investigated PSs lack genotoxic and mutagenic activity. Finally, the hazard of selection of drug resistant fungal strains has been rarely reported. We review the studies published to date on antifungal applications of PDT, with special focus on yeast, and aim to raise awareness of this area of research, which has the potential to make a significant impact in future treatment of fungal infections.

A comprehensive overview of photodynamic therapy in the treatment of superficial fungal infections of the skin

Photodynamic therapy (PDT) is a two-step procedure, involving the topical or systemic administration of a photosensitizer followed by selective illumination of the target lesion with visible light, which triggers the oxidative photodamage and subsequent cell death within the target area. In dermatology, PDT has proven to be a useful treatment for a variety of malignant tumors and selected inflammatory diseases. In addition, PDT of several infective viral or bacterial skin diseases has been investigated. These investigations grew out of the positive findings of studies of another important use of PDT: that of disinfection of blood products. Up to now, little has been published concerning the application of PDT to fungi, probably due to the fact that research funding has been mainly directed towards blood disinfection, and these pathogens show a low risk of transfusion transmission. However, preliminary findings have demonstrated that dermatophytes and yeasts can be effectively sensitized in vitro by administering photosensitizers belonging to four chemical groups: phenothiazine dyes, porphyrins and phthalocyanines, as well as aminolevulinic acid, which, while not a photosensitizer in itself, is effectively metabolized into protoporphyrin IX. Besides efficacy, PDT has shown other benefits. First, the sensitizers used are highly selective, i.e., fungi were killed at combinations of drug and light doses much lower than that needed for a similar effect on keratinocytes. Second, all investigated photosensitizers lack genotoxic and mutagenic activity. Finally, the hazard of selection of drug resistant fungal strains was never reported. This paper intends to provide a comprehensive overview of investigative studies about the effects of PDT on yeasts and dermatophytes, and bring attention to this application of PDT which we believe very important in that skin mycosis is so common and PDT is not only cost-effective, but also has the advantages of being highly selective and avoiding the occurrence of drug resistant strains.

Photodynamic therapy of interdigital mycoses of the feet with topical application of 5-aminolevulinic acid

BACKGROUND

Findings of in vitro studies have demonstrated that dermatophytes and yeasts can be effectively photosensitized after topical delivery of 5-aminolevulinic acid (ALA). This procedure, called photodynamic therapy (PDT), seems to lack mutagenic activity and hazard of selection of drug-resistant strains.

METHODS

Twenty percent ALA preparation in Eucerin cream was applied under an occlusive dressing to skin lesions of nine patients with clinical and microbiological evidence of interdigital mycosis of the feet. After 4 h, lesions were irradiated with 75 J/cm(2) of broad-band red light. Interdigital lesions of the other foot served as control (treated with only light or only ALA). After 7 days from the first treatment, no further treatment was delivered if lesions were not clinically evident and direct microscopic examination was negative. Otherwise, three additional weekly treatments were delivered. Four weeks after the last treatment, patients had a final follow-up clinical and laboratory examination.

RESULTS

Clinical and microbiological recovery was seen in six out of nine patients after one (four cases) or four (two cases) treatments. However, after 4 weeks, recurrences were seen in four patients. Overall tolerability was always good.

CONCLUSION

Under the conditions employed in the present study, ALA-PDT had good therapeutic effects on interdigital mycosis of the feet. However, recurrences were quick. In vivo environmental conditions, i.e. temperature, humidity and pH of the interdigital skin, could induce a poor cell uptake of ALA and a deficient biosynthesis of photosensitizing protoporphyrin IX. In addition, the irregular tridimensional shape of this peculiar anatomical area could lead to a non-uniform delivery of light and/or ALA cream. However, the present results can stimulate further studies on the PDT of superficial skin mycoses.

Genetic and biochemical characterization of mutants of Saccharomyces cerevisiae blocked in six different steps of heme biosynthesis

Heme-deficient mutants of Saccharomyces cerevisiae have been isolated from two isogenic strains with the use of an enrichment method based on photodynamic properties of Zn-protoporphyrin. They defined seven non-overlapping complementation groups. A mutant representative of each group was further analysed. Genetic analysis showed that each mutant carried a single nuclear recessive mutations. Biochemical studies showed that the observed accumulation and/or excretion of the different heme synthesis precursors by the mutant cells correlated well with the enzymatic deficiencies measured in acellular extracts. Six of the seven mutants were blocked in a different enzyme activity: 5-aminolevulinate synthase, porphobilinogen synthase, uroporphyrinogen I synthase, uroporphyrinogen decarboxylase, coproporphyrinogen III oxidase and ferrochelatase. The other mutant had the same phenotype as the mutant deficient in ferrochelatase activity. However, it possessed a normal ferrochelatase activity when measured in vitro, so this mutant was assumed to be deficient in protoporphyrinogen oxidase activity or in the transport and/or reduction of iron. The absence of PBG synthesis led to a total lack of uroporphyrinogen I synthase activity. The absence of heme, the end product, led to an important increase of coproporphyrinogen III oxidase activity, while the activity of 5-aminolevulinate synthase, the first enzyme of the pathway, was not changed. These results are discussed in terms of possible modes of regulation of heme synthesis pathway in yeast.

Analysis of heme biosynthesis in catalase and cytochrome deficient yeast mutants

Mutants of Saccharomyces cerevisiae, described as catalase and cytochromes deficient (Pachecka et al., 1974), have been analyzed for heme biosynthesis ability. Some enzymatic activities involved in protoheme synthesis were measured in acellular extracts, whereas whole cells were analyzed for cytochrome spectra and for possible accumulation of porphyrin synthesis intermediates. A good correlation was found between these in vitro and in vivo studies. Results show that two mutants were impaired in 5-aminolevulinate synthesis, two mutants were devoid of uroporphyrinogen I synthetase activity and one mutant presented defects in coproporphyrinogen III oxidase activity.

[Study of the mechanism of the effect of extracellular pH on the synthesis of the oxidative complex (cytochrome a+a3) of Bacillus coagulans: relationship to the "glucose effect" and role of excreted coproporphyrin III (author's transl)]

During the "respiratory adaptation" of Bacillus coagulans, it was possible to dissociate the kinetics of cytochrome a and a3 synthesis with carbon monoxide. The synthesis of cytochrome a3 is preferentially repressed when the pH of the incubation medium is pH 6.5 instead of pH 5.5. However, though the total synthesis of tetrapyrrole compounds is the same at both pH values, the excretion of coproporphyrin III is much increased at pH 6.5. Bacillus coagulans, sensitive to the "glucose effect", shows the "pH effect" only in the presence of high glucose concentrations. The repression of the oxidase complex synthesis by a slight increase of the extracellular pH appears directly related to the increase of the extracellular coproporphyrin III.