Cataract induction by 1,2-naphthoquinone. I. Studies on the redox properties of bovine lens proteins.

Conditions of oxidative stress may lead to cataract formation. Reaction of certain flavoproteins, the NADH: oxidoreductases, with different quinones is well known to form hydrogen-peroxide. This reaction was investigated to get more information on cataract induction by naphthalene and its quinone metabolites. Protein extracts from bovine lens cortex exhibit "diaphorase" activity, indicated as dye reduction in the presence of NADH and dichlorophenol-indophenol (DCPIP) or ferricyanide. Different redox cycling compounds are shown to be active in this "diaphorase" reaction by lens protein extract (LCE): Oxygen consumption can be detected in the presence of pyrroloquinoline quinone and juglone whereas 1,4-naphthoquinone, menadione and paraquat are no redox cyclists in this flavoprotein catalyzed reaction.

Cataract induction by 1,2-naphthoquinone. II. Mechanism of hydrogenperoxide formation and inhibition by iodide.

Naphthalene cataract is probably due to peroxide production through naphthoquinone (NQ) redox cycling and/or glutathione conjugation. Both mechanisms yield losses of essential SH-groups in cristallins and are thus probably involved in protein modification finally visible as lens opacity. 1,2-Naphthoquinone produces H2O2 in the presence of either ascorbate, glutathione, NADH or--to a lesser extend--by homogenates of lens protein preparations. In the presence of 1,2-naphthoquinone and the above reductive additions, both, oxygen uptake and H2O2 formation can be observed. Reductive oxygen activation in these systems are diminuated by iodide in a concentration-dependent manner. Since maleimide-treated proteins are less capable to activate oxygen by 1,2-naphthoquinone, a direct oxygen activation by the interactions of 1,2-naphthoquinone with protein-SH is indicated. Catalysis of "diaphorase"-type (dia) enzymes via NADH--dia--1,2-NQ--O2 seems not to operate in hydrogenperoxide production during 1,2-naphthoquinone lens toxicity.

[New biochemical models for cataract research]. / Neu biochemische Modelle zur Kataraktforschung.

In recent years, it has been suggested that reactive oxygen species are involved in the genesis of cataracts. To elucidate the basic reaction mechanisms underlying these processes and the influence of drugs, we developed simple biochemical model reactions. The purpose was to simulate cataractogenic processes and to document the effects of potential "anticataractous" drugs in vitro. Application tests allowed us to quantify the penetration rates and the enrichment processes of iodidecontaining drugs. Our results document the ability of KI to inhibit photodynamic reactions and related cataractogenic processes, such as lipid and sulfhydryl oxidation, as well as the structural changes of the lens proteins.

[Investigations of drug-related sulfhydryl oxidation in isolated rabbit lenses]. / Untersuchungen zur medikamentösen Beeinflussung der Sulfhydryloxidation in isolierten Kaninchenlinsen.

The appearance of cataract in human lenses mainly consists in the formation of a protein network initiated by oxidative processes. Two types of covalent bondings are produced in the cristallins during cataractogenesis: The formation of C-N bonds produced from amino groups of the cristallin proteins and aldehyd or keto groups derived from the oxidation of sugars or unsaturated fatty acids. The formation of S-S bonds from SH groups specially in the gamma-cristallins. These disulfid bonds are scarcely reducible by reductants such as ascorbic acid or glutathione. Experimentally these SS bonds can be produced in rabbit lenses by incubation with riboflavin in the light. The presence of Pherajod during the incubation period prevents the oxidation of SH groups.

Biochemical model reactions for cataract research.

There are several experimental indications that cataract formation is induced and/or enhanced by activated oxygen species including hydrogen peroxide, superoxide radical anion, singlet oxygen and hydroxyl radical. These species can be generated chemically, enzymatically or photodynamically. Taking advantage of endogenous photodynamic compounds in isolated lens, aqueous humor or vitreous preparations in the presence of S-methyl-alpha-ketobutyric acid (KMB), ethylene formation can be monitored for at least 2 h of light-dependent KMB degradation. This reaction is extremely sensitive and can be inhibited by potassium iodide in low concentrations. This model reaction might be useful for studying possibly inhibiting substances or stimulating processes involved in cataract formation.

[Kinetics of uptake of Pherajod by the rabbit eye]. / Kinetik der Aufnahme von Pherajod in das Kaninchenauge.

The aqueous humor and lenses of rabbit eyes contain inhibitors of photodynamic destructions. Preparations of vitreous, however, more likely seem to contain stimulatory compounds. The anticataractic drug Pherajod significantly blocks the photodynamic formation of destructive oxygen species. Inhibition by 65% of photodynamic destructions, measured as ethylene release from S-methyl-a-ketobutyric acid in the riboflavin-driven oxygen photoactivation, is observed with aqueous humor preparations isolated 5 min after start of incubation of rabbit eyes in Pherajod solution.

[The uptake of potassium iodide and its effect as an antioxidant in isolated rabbit eyes]. / Die Aufnahme von Kaliumjodid und seine Wirkung als Antioxidans in isolierten Kaninchenaugen.

Potassium iodide (KI) passes the cornea of isolated rabbit eyes with kinetics of approximately 0.25 mumol/h/ml aqueous humor. In photodynamic reactions, simulated as light-dependent decay of S-methyl-alpha-ketobutyric acid in the presence of riboflavin, KI acts as an antioxidant cooperating with internal scavengers such as ascorbate. With the simple model reactions applied it may be possible to study mechanism and functions in vivo of eye-protecting factors or combinations of compounds.