Ultraviolet Light-induced Changes Research Articles

Sub-Bandgap Laser Light-Induced Excess Carrier Transport Between Surface States and Two-Dimensional Electron Gas Channel in AlGaN/GaN Structure

Variations of the channel resistance of a AlGaN/GaN high electron mobilities transistor caused by subsequently incident photons with sub-bandgap energies after ultraviolet light-induced changes became stable were studied. Temperature-dependent measurements yielded a 0.342 eV thermal activation energy and wavelength-varying measurements yielded a 0.8 eV cut-off photon energy. The ratio between the two values is very close to the theoretical value of the ratio between the valence and conduction band discontinuities. A qualitative description about the transports of excess carriers through the band discontinuities, is also proposed and consistent with the experimental results.

Ultraviolet light-induced changes in the glucose-6-phosphate dehydrogenase activity of porcine corneas.

Corneas have the high activity of the pentose phosphate pathway. To clarify the relevance of the pentose phosphate pathway to their antioxidant defense, we examined the activity of glucose-6-phosphate dehydrogenase (G6PDH), the rate-determining enzyme of the pathway, in corneas exposed to ultraviolet light (UV). Fresh porcine eye globes were exposed to ultraviolet light A (UVA) or ultraviolet light C (UVC), and the tear film-removed eye globes to UVA. After UV exposure, the corneas were dissected from the eye globes and extracted with a saline solution, and the G6PDH activity in the extract was assayed. The G6PDH activity of unilateral opaque corneas, their paired, transparent corneas, and normal corneas, all of which were obtained from fresh, UV-unexposed eye globes, and the L-lactate dehydrogenase (LDH) activity of these opaque and normal corneas also were assayed. The G6PDH activity of corneas increased with UVA exposure, and decreased with long-term UVC exposure, although it increased with short-term UVC exposure. A UV-blocking contact lens screened corneas from the UVA-induced increase. Removal of the tear film enhanced the UVC-induced decrease. The G6PDH activity of unilateral opaque corneas was lower than that of paired, transparent corneas or normal corneas, all of which were obtained from fresh, UV-unexposed eye globes. The LDH activity of the opaque corneas was much higher than that of the normal corneas. Exposure of corneas to UVA or a small dose of UVC enhances the G6PDH activity, i.e., the pentose phosphate pathway. This activity enhancement may play an important role in corneal antioxidant defense against UV-induced oxidative stress. However, exposure of corneas to large doses of UVC appears to damage the pathway.

Ultra-violet light-induced changes in membrane properties in secretory cells

Illumination with ultra-violet is used widely in physiological experiments for the photolysis of caged compounds. In the peptidergic cells of the pituitary gland, as well as cultured PC12 cells, ultra-violet light was found to produce changes in a number of membrane properties. Light of sufficient intensity to produce rapid photolysis of commonly used caged compounds induced changes in K + and Ca 2+ current, as well as changes in membrane capacitance. All responses to light showed a rapid timecourse, activating in a few ms and decaying within 10–50 ms after illumination ended. Experiments with radical scavengers and with inhibitors of cytochrome p450 and phospholipase A2 failed to block the light responses. These rapid responses to light emphasize that experiments employing ultra-violet light in the photorelease of physiological and pharmacological agents require special care for control of light artifacts.

Evaluation of Sunscreen Protection by Measurement of Epidermal DNA Synthesis

Different sunscreens were tested to determine their protection of epidermis from ultraviolet light effects. Ultraviolet light-induced changes in hairless mouse epidermal DNA synthesis were used for measurement of sunscreen protection. Visual assessment of erythema and edema was also performed. This initial study has evaluated sunscreens containing para-aminobenzoic acid (PABA) as the principle sunscreen chemical. These experiments were conducted using fluorescent sunlamp tubes and hydroxylapatite extraction of epidermal DNA. The ultraviolet light exposure was measured using a recording radiometer. The results showed that the sunscreens tested were able to partially prevent ultraviolet light induced changes in epidermal DNA synthesis. It may be possible to use this assay as one of the initial evaluations of potential ultraviolet light protectants.

Ultraviolet photoalteration of ion channels in voltage-clamped lobster giant axons.

An analysis of the ultraviolet light-induced changes in ionic conductances of lobster giant axon membranes has been carried out using the double sucrose gap voltage-clamp technique. The predominant effect of monochromatic light from a xenon arc source in the 255 to 305 nm region is an irreversible reduction in the magnitude of sodium conductance, without change in sodium channel activation or inactivation kinetics. A considerably smaller reduction in the magnitude of potassium conductance occurs, with some slowing of potassium channel activation kinetics. Leakage conductance is essentially not altered. The fall in sodium conductance follows an exponential time course toward a zero asymptote. The rate constant for conductance decrease was used as an assay for the wavelength dependence. The sodium conductance was maximally sensitive at 290 nm. It is suggested that individual sodium channels are closed upon absorption of single photons by aromatic amino acid residues in membrane proteins.

Ultraviolet light-induced changes in ribonuclease conformation

Ultraviolet light-induced inactivation of RNase A is accompanied by a decrease in the circular dichroism (CD) at the extrema observed at 239 nm and 275 nm. As inactivation progresses a new CD band centered near 325 nm also develops. For RNase maintaining up to one half of its original activity an isoelliptic point may be noted near 257 nm. These findings are consistent with initial normalization of a “buried” tyrosyl residue followed by chemical modification of the protein. Further chemical change and extensive conformational reorganization of the protein appear to accompany advanced inactivation.

Chronic solar dermatosis--an electronmicroscopic study of the epidermis.

Summary The epidermis from six cases of chronic solar dermatosis (three from the face, and three from the neck), has been examined with the electronmicroscope. There is electronmicroscopic evidence of reduced protein and lipid reserves in the keratinocytes, and of decreased keratin formation. These changes are believed to be caused by poor nutrition as a result of the ultraviolet light-induced changes in the dermis. There is no indication that the keratinocytes. are affected directly by the ultraviolet light. Some melanocytes are large and hyper-functional, probably due to ultraviolet light stimulation, but many are hypo-functional with few or no melanosomes, few cellular organelles, and frequently a deeply indented nucleus. It is considered likely that the ultraviolet light filtering ability of the epidermis is reduced in chronic solar dermatosis because there is less keratin in the stratum corneum and many melanocytes are incapable of producing melanin. Therefore, once chronic solar dermatosis is established, it would predispose to the development of ultraviolet light induced cancers.