Human RPE Melanosomes Protect from Photosensitized and Iron-Mediated Oxidation but Become Pro-Oxidant in the Presence of Iron upon Photodegradation
Bartosz K. Rozanowski 1,
Janice M. Burke 2,
Michael E. Boulton 3,
Tadeusz Sarna 4,
and
Malgorzata B. Rozanowska 5*
1 Genetics and Cell Biology, Institute of Biology, Krakow, Poland
2 Ophthalmology, Medical College of Wisconsin, Milwaukee, Wisconsin, United States
3 Ophthalmology and Visual Sciences, The University of Texas Medical Branch, Galveston, Texas, United States
4 Biophysics, Jagiellonian University, Krakow, Poland
5 Optometry and Vision Sciences, Cardiff University, Redwood Building, Cardiff, CF10 3NB, United Kingdom
* To whom correspondence should be addressed. E-mail: rozanowskamb{at}cardiff.ac.uk.
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Abstract |
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PURPOSE: To determine the effects of human retinal pigment epithelial cell (RPE) pigment granules on photosensitized and iron ion-mediated oxidation, and the effect of photodegradation of melanosomes on their antioxidant properties.
METHODS: RPE cells were isolated from human and bovine eyes; pigmented and non-pigmented bovine RPE were isolated separately. Melanosomes, melanolipofuscin and lipofuscin granules were isolated from human RPE donors over 60 years old. Melanosomes were photodegraded by exposure to blue light. Oxidation of RPE cells or of linoleate was induced by iron/ascorbate in the presence and absence of pigment granules. The photosensitized oxidation of histidine was induced by blue light irradiation of cationic porphyrin. Progress of oxidation was monitored by electron spin resonance oximetry.
RESULTS: Iron/ascorbate induced rapid oxidation in suspensions of non-pigmented bovine RPE cells. The rates of oxidation were diminished about 4 times in suspensions of pigmented bovine RPE cells. Addition of bovine melanosomes or synthetic melanin to non-pigmented bovine RPE cells resulted in a concentration dependent decrease in the rate of oxidation to levels similar to those of pigmented bovine RPE. Human melanosomes exerted a concentration dependent inhibitory effect on photosensitized and iro-mediated oxidation. Photodegradation of human melanosomes led to loss of the inhibitory effect on iron-mediated oxidation, while their ability to inhibit photosensitized oxidation was enhanced.
CONCLUSIONS: Human melanosomes act as effective antioxidants by preventing iron ion-induced oxidation. Photodegradation of melanosomes results in the loss of these antioxidant properties, while still preserving their ability to deactivate cationic photosensitizers.
Key Words:
oxidative damage, antioxidants, retinal pigment epithelium, light damage, melanin, iron
