Revisiting the internal absorption coefficient of the retinal pigment epithelium melanosome

Retinal pigment epithelial (RPE) cells of the vertebrate eye contain melanin packaged in structures called melanosomes. Previously, we reported that the bovine melanosome has a laser photodisruption threshold of 153.6 mJ/cm² at 25 °C and 532 nm (Glickman et al., Proc SPIE 2681:460-467, 1996), and an internal absorption coefficient of 2237 cm^-1 (Jacques et al., Proc. SPIE 2681:468-477, 1996). Those values used an estimate of melanosome density inferred from studies of skin laser effects (Jacques et al., J. Invest. Dermatol. 88:88-93, 1987). We now revisit that calculation using a density value obtained from density centrifugation analysis of bovine and baboon RPE melanosomes. Stepped-density gradients (60% to 80%) of the nonionic medium, Nycodenz, were formed, and samples of melanosomes were spun on the gradients in a swinging-bucket rotor at 10,000 rpm for 60 m. Bovine melanosomes formed two populations, one at the interface between 70% and 75%, and the other between 75% and 80%, corresponding to densities of ∼1.38 and ∼1.41 gm/cm³, respectively. Baboon melanosomes migrated to within the same density region. Using a density value of 1.41 gm/cm³, and assuming a water content of 52% for hydrated melanosomes, the internal absorption coefficient was calculated as 2339 cm^-1. Although this calculation uses an objective density measurement, the water content remains an estimate, and the actual value in situ may differ.