G. Y. Fraikin

NEAR‐UV ACTIVATION OF ENZYMATIC CONVERSION OF 5‐HYDROXYTRYPTOPHAN TO SEROTONIN

Abstract— Near‐UV (337 nm) photoactivation of the 5‐hydroxytryptophan decarboxylation reaction producing serotonin has been observed. The photoactivation effect was investigated as a function of fluence rate and fluence, and pH. Photoactivation of decarboxylase activity was found to occur at nearly neutral pH values (low activity of the enzyme in the dark). The findings indicate that the effect of light is similar to a pH shift toward the acid region, which causes the enzyme conversion from the inactive to active form. Pyridoxal phosphate, the decarboxylase cofactor, in the form of an adduct absorbing at 330–340 nm, is suggested as a candidate for the role of the photoactive chromophore of decarboxylase.

Phytochrome system of the yeast Candida guilliermondii and recovery from ultraviolet injury.

The effect of red (660 nm) and far-red (730 nm) light on the stability of the yeast Candida guilliermondii to lethal u.v. radiation has been studied. Reactivation and protection were exhibited for 30 min after treatment with red light and were abolished by far-red exposure applied within this time period. The temperature dependence of the reactivation effect was also studied. The data obtained showed that the properties of recovery and protection against u.v. exposure are associated with the phytochrome system of the yeast.

INVOLVEMENT OF NEAR‐UV‐INDUCED SYNTHESIS OF SEROTONIN IN PHOTOPROTECTION AND IN POTENTIATION OF FAR‐UV LETHALITY IN THE YEAST CANDIDA GUILLIERMONDII

Abstract— This paper considers mechanisms of near‐UV (334nm) induced photoprotection as well as potentiation of far‐UV (254 nm) lethality in Candida guilliermondii. Using exogenous precursors of serotonin, it appears that the above two mechanisms involve photoactivated synthesis of serotonin. It has been postulated that the serotonin effect could take place by binding to DNA.

REPAIR OF 313-NM INDUCED LESIONS AND PHOTOPROTECTION IN YEAST CANDIDA GUILLIERMONDII

Abstract. The present communication is concerned with the effects of near‐UV radiation (300–380 nm) on yeast Candida guilliermondii. It was found that certain doses of 313 nm irradiation caused inactivation of the yeast which was exhibited in a way different from the lethal action of far‐UV radiation. It was also found that the cells inactivated by 313 nm are capable of recovering vitality, if incubated for some time in a non‐nutrient medium. The yeast inactivated by far‐UV radiation also proved to be capable of recovering, though to a lesser degree. Both 334 nm radiation and non‐lethal doses at 313 nm induced the photoprotective effect against far‐UV damage. The effect was exhibited if there was a certain time interval (2–4 h) between the exposures to photoprotective light and subsequent far‐UV radiation. Within this time interval the extent of photoprotection was dependent on temperature.

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.

TWO‐PHOTON INACTIVATION, PHOTOREACTIVATION AND PHOTOPROTECTION IN YEAST CELLS IRRADIATED BY 266 NM‐LASER RADIATION

Abstract— The quadratic dependence of inactivation of yeast cells on laser fluence rate at 266 nm suggests the occurrence of two‐photon photochemical reactions in DNA, producing photoproducts similar to those found with ionizing radiation. Observations of a significant diminution of photoreacti‐vation and the disappearance of photoprotection at high laser fluence rates provide additional evidence for this.

ENHANCEMENT OF THE FAR‐UV LETHALITY IN YEAST CANDIDA GUILLIERMONDII BY NEAR‐UV POST‐IRRADIATION

Abstract— In experiments with the non‐photoreactivable yeast Candida guilliermondii, radiations at 313, 334 and 365 nm, having no effect on untreated cell populations, produced an ‘enhancing’ effect on the lethality of 254 nm‐pretreated cells. Wavelengths in the visible region of the spectrum did not exhibit a similar effect.