Studies on the mechanism of the UVA light-dependent loss of glutathione reductase activity in human lenses.

Purpose: To determine the mechanism that leads to the UVA light-dependent loss of glutathione reductase (GR) activity in human lens (HL).

Methods: Both the HL water-soluble (WS) fraction and yeast GR were irradiated with UVA light (200 mW/(cm(2). h) for 1 hour at +20 degrees C, and the specific activity (SA) was observed.

The effect of UVA light on the anaerobic oxidation of ascorbic acid and the glycation of lens proteins.

Purpose: To determine whether UVA-excited human lens chromophores can cause the oxidation of ascorbic acid in the absence of oxygen, and whether these oxidation products are capable of glycating lens proteins.

Methods: The oxidation of ascorbic acid, mediated by UVA irradiation in the presence of aged human lens proteins, was measured in the absence of oxygen by the decrease in absorbance at 265 nm in vitro. An action spectrum from 320 to 400 nm was determined for both ascorbate oxidation and the photobleaching of the lens yellow pigments at lambda = 350 nm.

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

Purpose: To determine the effect of UVA irradiation on the specific activities of several protective and metabolic enzymes in aged human lenses.

Methods: 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.

Studies on singlet oxygen formation and UVA light-mediated photobleaching of the yellow chromophores in human lenses.

The protein-bound chromophores, which increase with aging in the human lens, act as UVA sensitizers, producing almost exclusively singlet oxygen in vitro. Direct irradiation of whole, aged human lenses with high intensity UVA light (200 mW cm(-2) for 24 hr), however, failed to produce singlet oxygen damage, as evidenced by the lack of either His or Trp photodestruction. Total homogenates of human lenses prepared in a cuvette under air did show destruction of His and Trp residues by UVA light, but no destruction was seen when equivalent homogenates were prepared under argon.