**Background:** The visual cycle involves the constant trafficking of retinoids (retinol, retinaldehyde, and retinyl palmitate) in the retina, exposing them to oxidative threats from light, high oxygen tension, and polyunsaturated fatty acids. Oxidation of these retinoids can form cation radicals, which are damaging to amino acids and proteins and may contribute to retinal diseases like age-related macular degeneration (AMD) and Stargardt's disease. The retina contains several antioxidants, including lutein, zeaxanthin, taurine, melanin, ascorbate (vitamin C), and α-tocopherol (vitamin E), but their relative effectiveness in scavenging retinoid cation radicals was not fully characterized. This study aimed to determine the bimolecular rate constants for scavenging cation radicals of all three visual cycle retinoids by these antioxidants.
**Methods:** Retinoid cation radicals were generated by pulse radiolysis of nitrous oxide-saturated benzene (for retinol, retinaldehyde, and retinyl palmitate) or aqueous buffered solutions of potassium bromide with retinoids solubilized in Triton X-100 micelles (for retinyl palmitate). The decay of retinoid cation radicals was monitored by transient absorption spectroscopy in the presence of varying concentrations of antioxidants (lutein, zeaxanthin, taurine, dopa-melanin, ascorbate, and α-tocopherol). Bimolecular rate constants were calculated from the concentration-dependent acceleration of decay. For lutein and zeaxanthin, formation of their respective cation radicals was also monitored.
**Key Results:**
- **Retinyl palmitate cation radicals:** Scavenged by lutein (k = (8.85 ± 0.25) × 10^9 M^−1s^−1), zeaxanthin (k = (6.39 ± 0.02) × 10^9 M^−1s^−1), α-tocopherol (k = (2.7 ± 0.3) × 10^7 M^−1s^−1), and ascorbate (k = (5.8 ± 0.1) × 10^8 M^−1s^−1). No scavenging by taurine or dopa-melanin was observed (upper limit < 2 × 10^7 M^−1s^−1).
- **Retinol cation radicals:** Scavenged by lutein (k = (1.26 ± 0.04) × 10^10 M^−1s^−1) and zeaxanthin (k = (7.9 ± 0.3) × 10^9 M^−1s^−1). Taurine showed no effect (upper limit < 2 × 10^6 M^−1s^−1). Dopa-melanin scavenged with k = (5.1 ± 0.1) × 10^6 M^−1s^−1.
- **Retinaldehyde cation radicals:** Scavenged by lutein (k = (1.15 ± 0.14) × 10^10 M^−1s^−1) and zeaxanthin (k = (6.48 ± 0.29) × 10^9 M^−1s^−1). Taurine showed no effect (upper limit < 1 × 10^7 M^−1s^−1). Dopa-melanin scavenged with k = (1.6 ± 0.8) × 10^7 M^−1s^−1.
- **Physiological relevance:** Based on estimated retinal concentrations, ascorbate (1–10 mM in neural retina/RPE) likely provides the greatest overall protection, especially in the periphery. In the macula, lutein and zeaxanthin (up to 172 and 133 µM, respectively, with supplementation) can contribute substantially. α-Tocopherol (115 µM average) is relatively inefficient for retinol and retinyl palmitate but may protect retinaldehyde. Melanin's high local concentration in RPE melanosomes (up to 398 mM) suggests effective scavenging of retinol and retinaldehyde cation radicals in proximity.
**Clinical Implications:** The study supports the rationale for antioxidant supplementation in AMD, as seen in the AREDS2 trial, by demonstrating that lutein, zeaxanthin, ascorbate, and α-tocopherol can scavenge damaging retinoid cation radicals. The high effectiveness of lutein and zeaxanthin in the macula underscores their importance in protecting the central retina. The lack of taurine activity suggests it does not directly protect retinoids from cation radical damage. The potential for melanin to scavenge retinoid cation radicals in the RPE may be relevant to aging and AMD, where melanin content decreases. These findings highlight the synergistic potential of combining carotenoids with vitamins C and E for retinal health.