The presence of ascorbate
free radical (
AFR) reductase (
NADH:AFR
oxidoreductase, EC 1.6.5.4) in senile cataractous human
lenses was demonstrated by measuring spectrophotometrically
NADH oxidation in the presence of ascorbate plus
ascorbate oxidase. About 80-85% of the lens
AFR reductase was probably recovered in the supernatant of the lens homogenate. Michaelis constants of the
reductase were about 10 microM and less than 1 microM for AFR and
NADH, respectively. We also showed that
AFR reductase activities in the cataractous
lenses tended to decrease with increase of insoluble lens
protein contents, or showed rather the possibility that the
reductase activity may have decreased before the lens
protein aggregation. In the highest activity group (about 120-160 nmol
NADH oxidized/min/lens), it was roughly calculated that the
reductase in the lens could re-reduce immediately the total (or almost total) amount of AFR produced there by ascorbate oxidation even at a high rate of 600-800 microM/min, if
NADH concentration in the lens were sufficiently maintained. The above results suggested that
AFR reductase in the human lens plays important roles in ascorbate regeneration of its redox cycle, and that activity loss of
AFR reductase, as well as of
superoxide dismutase,
glutathione peroxidase and
glutathione reductase, may be responsible for the oxidative changes in
lens proteins with the development of senile
cataracts.