Enzymatic activity of quinonoid dihydropterin reductase and tetrahydropterin content in human ocular tissues and senile cataracts.

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.