Comparisons of the Structural and Chemical Properties of Melanosomes Isolated from Retinal Pigment Epithelium, Iris and Choroid of Newborn and Mature Bovine Eyes^¶

Abstract

Melanosomes were isolated from the retinal pigment epithelium (RPE), iris and choroid of mature (age >2 years) and newborn (age <1 week) bovine eyes. Scanning electron microscopy was utilized to analyze the morphology of the melanosomes, which were found to vary among different tissues and different ages. While the total content of amino acids differs slightly (ranging from 9% to 15% by mass), the distributions of the amino acids are similar. The pheomelanin content is low in the choroid and the RPE (0.1–0.5%), and moderate in the iris (<2%); therefore, the major melanin component of bovine eye melanosomes is eumelanin, independent of the shape of the melanosomes. The yields of pyrrole‐2,3,5‐tricarboxylic acid from melanosomes decrease in the following order: choroid > iris > RPE, and exhibit decreasing yields with age. 13C solid‐state nuclear magnetic resonance (NMR) spectroscopic analysis of iris and choroid melanosomes indicates the same trends. These observations suggest that the 5,6‐dihydroxyindole‐2‐carboxylic acid contents decrease in the following order: choroid > iris > RPE, and decrease with age. Moreover, the 13C solid‐state NMR spectra show (1) for the same age samples, the CH:Cq ratio for choroid is larger than that for iris melanosomes; and (2) an increase in the concentration of carbonyl groups with age within each type of melanosome.

Neuromelanins Isolated from Different Regions of the Human Brain Exhibit a Common Surface Photoionization Threshold

“…The small particles could be found in aggregates of varying dimensions and also seen on the surfaces of the larger granules. Such small‐scale substructure for melanin pigments has been previously reported for many natural melanins, including Sepia (20), human hair melanosomes (21), bovine ocular melanosomes (22) and human ocular melanosomes (23).…”

Characterization of melanin in human iridal and choroidal melanocytes from eyes with various colored irides

“…In another study on melanin content in uveal samples, Liu et al. (2005) performed chemical degradation analysis of bovine choroid and iris melanosome samples.…”

Insight into the Binding of Divalent Cations to Sepia Eumelanin from IR Absorption Spectroscopy

“…Natural eumelanins are associated with and have a considerable affinity for divalent and trivalent metal ions (5–8). It is believed that this pigment can serve as a reservoir of metal ions, such as Ca(II) (9), and as a trap of heavy metal ions, such as Cu(II) and Fe(III) (10,11).…”