Dependence of lipid peroxidation on pigmentation in porcine iris

Purpose: Melanin has been shown to act as antioxidant in lipid peroxidation studies. We have now investigated lipid peroxidation in dependence on stro mal pigmentation in isolated porcine irises. Methods: The same number of lightly pigmented and heavily pigmented porcine irises (visual selection) were homogenized in buffer (50 mmol/l Na2HPO4, 5 0 mmol/l Na2HPO4 and 4mmol/l sodium azide; 1:20 w/v). 500 mul homogenate we re incubated at 37 degreesC for 5, 10, 20 and 40 min in absence and presenc e of Fe2+ as inducer of lipid peroxidation. Lipid peroxidation was assayed by the thiobarbituric acid (TBA) test. Results are expressed as nmol of TEA reactive material produced (TBAR) per mg protein. Fe2+ concentration of th e supernatant was determined spectrophotometrically with phenanthroline. Results: 70 mu mol/l, 180 mu mol/l and 360 mu mol/l Fe2+ induced lipid pero xidation. A plateau region was reached after 20 min. Lipid peroxidation dif fered in dependence on stromal pigmentation in porcine irises by a factor o f 2.8. 180 mu mol/l Fe2+ induced 1.373 +/- 0.138 nmol TBAR/mg protein in li ghtly pigmented irises compared to 0.491 +/- 0.125 nmol TBAR/mg protein in heavily pigmented irises after 10 min incubation (p<0.0001, n=4). On the ot her hand, the content of Fe2+ in the supernatant was the same within error. Conclusions: There was a stronger induction of lipid peroxidation in lightl y pigmented porcine irises compared to heavily pigmented porcine irises. Th is effect may be related to the difference in stromal melanin content and i ts antioxidant activity.