Effect of UVA light on the activity of several aged human lens enzymes.

Abstract

To determine the effect of UVA irradiation on the specific activities of several protective and metabolic enzymes in aged human lenses. Intact human lenses (ages 55-75) in artificial aqueous humor were irradiated in a quartz cuvette with UVA light (925 J/cm(2) per hour) at +20 degrees C. The lenses were homogenized and the activities of enzymes in the water-soluble (WS) fraction were measured in irradiated and nonirradiated lenses. One hour of UVA photolysis of human lens resulted in a 70% loss in glutathione reductase (GR)-specific activity and a 24% loss in glyceraldehyde-3-phosphate dehydrogenase (G3PD)-specific activity. At the same time, glutathione peroxidase (GPx) and superoxide dismutase (SOD) showed little or no loss in specific activity. GR and G3PD showed similar losses when human lenses were photolyzed with the same dose of UVA light delivered to the lens over 8 hours, using a 12.5% neutral-density filter (ndf), or over 24 hours with a 4.25% ndf. One hour photolysis of the human lens WS fraction under anaerobic conditions yielded an almost complete inactivation of GR, but only an 18% loss of G3PD activity. Under aerobic conditions, however, both enzyme activities were almost completely lost. Clear fetal bovine lenses, photolyzed under the identical conditions, displayed essentially the same loss of GR activity. UVA light causes inactivation of GR in human and fetal calf lenses under both anaerobic and aerobic conditions. This suggests that flavine adenine dinucleotide (FAD), the prosthetic group of GR, may be responsible for the enzyme's self-sensitizing properties. WS proteins from aged human lens generate reactive oxygen species (ROS) during UVA irradiation, which may be responsible for the inactivation of G3PD.