The
quinonoid dihydropterin reductase (DHPR) activity and
tetrahydropterin content were determined in human ciliary body--iris, retina, normal lens and senile
cataracts. The DHPR activity was higher in the retina [120.56 +/- 12.46 nmol
NADH oxidized min-1 (mg soluble
protein)-1] than in the ciliary body--iris [46.10 +/- 7.46 nmol
NADH oxidized min-1 (mg soluble
protein)-1] and lens [2.79 +/- 0.15 nmol
NADH oxidized min-1 (mg soluble
protein)-1]. In the distribution of DHPR activity in the lens, the
capsule-epithelium showed 1.5 and 10 times more activity than the cortex and nucleus, respectively. The apparent Km values for each of the substrates of DHPR activity in lens were obtained by Lineweaver--Burke plots. The plots. The
tetrahydropterin content was found to be higher in the retina [826 +/- 76 pmol (
g protein)-1] than in the ciliary body--iris [584 +/- 48 pmol (
g protein)-1] and lens [82 +/- 16 pmol (
g protein)-1]. The DHPR activity and
tetrahydropterin content were decreased significantly in senile
cataracts as compared with the values of age-matched clear
lenses. The importance of the DHPR activity in the maintenance of
tetrahydropterin in its reduced form in ocular tissues is discussed.