[Photochemical Processes of Cell DNA Damage by UV Radiation of Various Wavelengths: Biological Consequences].

Photochemical reactions in cell DNA are induced in various organisms by solar UV radiation and may lead to a series of biological responses to DNA damage, including apoptosis, mutagenesis, and carcinogenesis. The chemical nature and the amount of DNA lesions depend on the wavelength of UV radiation. UV type B (UVB, 290-320 nm) causes two main lesions, cyclobutane pyrimidine dimers (CPDs) and, with a lower yield, pyrimidine (6-4) pyrimidone photoproducts (6-4PPs). Their formation is a result of direct UVB photon absorption by DNA bases. UV type A (UVA, 320-400 nm) induces only cyclobutane dimers, which most likely arise via triplet-triplet energy transfer (TTET) from cell chromophores to DNA thymine bases. UVA is much more effective than UVB in inducing sensitized oxidative DNA lesions, such as single-strand breaks and oxidized bases. Of the latter, 8-oxo-dihydroguanine (8-oxodG) is the most frequent, being produced in several oxidation processes. Many recent studies reported novel, more detailed information about the molecular mechanisms of the photochemical reactions that underlie the formation of various DNA lesions. The information is mostly summarized and analyzed in the review. Special attention is paid to the oxidation reactions that are initiated by reactive oxygen species (ROS) and radicals generated by potential endogenous photosensitizers, such as pterins, riboflavin, protoporphyrin IX, NADH, and melanin. The review discusses the role that specific DNA photoproducts play in genotoxic processes induced in living systems by UV radiation of various wavelengths, including human skin carcinogenesis.

[New photo-induced effects of reactivation and protection of yeast cells under lethal UVB radiation].

Brief exposure of yeasts to low-intensity monochromatic light (400-730 nm) has revealed the effects of photoreactivation and photoprotection of the cells inactivated by medium wave UVB radiation (290-320 nm). The red spectral region with a maximum at 680 nm has been found to be the most active in the initiation of photoreactivation and photoprotection. It has been noted that, according to the regularities investigated, these processes differ fundamentally from the known processes of enzymatic photoreactivation and photoprotection, which have a spectral response limited by, respectively, blue (< 450 nm) and near (< 380 nm) UV light. The data obtained make possible to consider the observed effects of photoreactivation and photoprotection as the manifestation of functioning of some light-dependent defense system capable of increasing the resistance of cells to UVB radiation.

[Protection of Saccharomyces cerevisiae against oxidative and radiation-caused damage by alkyl hydroxybenzenes]. / Zashchita Saccharomyces cerevisiae alkiloksibenzolami ot okislitel'nogo i radiatsionnogo porazheniia.

The effects of C7-alkylhydroxybenzene (C7-AHB) and p-hydroxyethylphenol (tyrosol), chemical analogs of microbial anabiosis autoregulators, on the viability of yeast cells under oxidative stress were investigated. The stress was caused by reactive oxygen species (ROS) produced under gamma irradiation of cell suspensions using doses of 10-150 krad at an intensity of 194 rad/s or by singlet oxygen generated in cells photosensibilized with chlorin e6 (10 micrograms/l). C7-AHB was found to exert a protective effect. The addition of 0.05-0.16 vol% of C7-AHB to cell suspensions 30 min before irradiation protected yeast cells from gamma radiation (50 krad). The protective effect of C7-AHB manifested itself both in the preservation of cell viability during irradiation and in the recovery of their capacity to proliferate after irradiation. In our studies on photodynamic cell inactivation, the fact that the phenolic antioxidant C7-AHB protects cells from intracellular singlet oxygen was revealed for the first time. The analysis of difference absorption spectra of oxidized derivatives of C7-AHB demonstrated that the protective mechanism of C7-AHB involves the scavenging of ROS resulting from oxidative stress. The fact that tyrosol failed to perform a photoprotective function suggests that the antioxidant properties of microbial C7-AHB are not related to their chaperon functions. The results obtained make an important addition to the spectrum of known antioxidant and antistress effects of phenolic compounds.

[The photodynamic inactivation of Candida guilliermondii in the presence of photodithazine]. / Fotodinamicheskaia inaktivatsiia drozhzhei Candida guilliermondii v prisutstvii fotoditazina.

Photodithazine, a glucosamine salt of chlorin e6, enhanced the inactivation of Candida guilliermondii cells by visible light. The sensitizing effect of photodithazine was found to be related to free or cell surface-bound molecules of this dye. Sodium azide (a singlet oxygen quencher) and propyl gallate (an inhibitor of lipid peroxidation) protected yeast cells from the photodithazine-enhanced photoinactivation.

Near-UV activation of yeast growth.

Near-UV radiation (337 nm) at fluences of 2-10 kJ m-2 activates, during the lag-phase, subsequent growth of the yeast Candida guilliermondii. Photostimulation occurs only after a temperature-dependent time interval (1-2 h) between short-period irradiation and the onset of cell growth on a nutrient medium. A photoactivated enzymatic synthesis of an intermediate metabolite regulating cell growth is proposed as the cause of the biochemical changes responsible for the photostimulatory effect. On the basis of the observed disappearance of photostimulation after adding para-chlorphenylalanine (a specific inhibitor of serotonin synthesis), serotonin is suggested to act as the active metabolite. The regulatory function of serotonin is confirmed by the stimulatory effect of exogenous serotonin and its dependence on concentration, which resembles the dependence on near-UV fluence.

[A photosynergistic lethal effect in the irradiation of yeasts with long-wave ultraviolet and visible light]. / Fotosinergicheskii letal'nyi éffekt pri obluchenii drozhzhei dlinnovolnovym ul'trafioletovym i vidimym svetom.

A lethal synergistic effect which is expressed as the nonadditive summing of the damaging effect of each irradiation separately has been found during the investigation of combined action of longwave ultraviolet (UV) rays (337 nm or 365 nm) and visible light (400-600 nm) on the yeast cells. Based on the data on different mechanisms of lethal effect of longwave UV and visible light, it has been suggested that the basis of the photosynergistic effect is the mutual intensification of the photo-destructive processes occurring in different intracellular structures and processes induced by different endogenous sensitizers.