Oxidation of carotenoids by heat and tobacco smoke.

The stability to autoxidation of the polar carotenoids, lutein and zeaxanthin, was compared to that of the less polar carotenoids, beta-carotene and lycopene at physiologically or pathophysiologically relevant concentrations of 2 and 6 microM, after exposure to heat or cigarette smoke. Three methodological approaches were used: 1) Carotenoids dissolved in solvents with different polarities were incubated at 37 and 80 degrees C for different times. 2) Human plasma samples were subjected to the same temperature conditions. 3) Methanolic carotenoid solutions and plasma were also exposed to whole tobacco smoke from 1-5 unfiltered cigarettes. The concentrations of individual carotenoids in different solvents were determined spectrophotometrically. Carotenoids from plasma were extracted and analyzed using high performance liquid chromatography. Carotenoids were generally more stable at 37 than at 80 degrees C. In methanol and dichloromethane the thermal degradation of beta-carotene and lycopene was faster than that of lutein and zeaxanthin. However, in tetrahydrofuran beta-carotene and zeaxanthin degraded faster than lycopene and lutein. Plasma carotenoid levels at 37 degrees C did not change, but decreased at 80 degrees C. The decrease of beta-carotene and lycopene levels was higher than those for lutein and zeaxanthin. Also in the tobacco smoke experiments the highest autoxidation rates were found for beta-carotene and lycopene at 2 microM, but at 6 microM lutein and zeaxanthin depleted to the same extent as beta-carotene. These data support our previous studies suggesting that oxidative stress degrade beta-carotene and lycopene faster than lutein and zeaxanthin. The only exception was the thermal degradation of carotenoids solubilized in tetrahydrofuran, which favors faster breakdown of beta-carotene and zeaxanthin.

Corneal tissue ablation depth and the Munnerlyn formula.

PURPOSE: To determine whether the error in ablation depth produced by approximations inherent in the Munnerlyn formula are clinically significant when estimating residual corneal stromal depth for the evaluation before refractive surgery. SETTING: Department of Ophthalmology, University of Texas Medical Branch, Galveston, Texas, USA. METHODS: Using identical geometric assumptions, the exact ablation depth was calculated and compared to the approximate ablation depth predicted by the Munnerlyn formula. An adjustment factor was then derived for large optical zones and corrections. RESULTS: The exact ablation depth is always larger than the ablation depth predicted by Munnerlyn's formula. Analysis found the error in ablation depth varied as the fourth power of the optical zone and linearly with correction. The initial corneal radius had little effect on the difference. The ablation depth could be reasonably approximated by adding an adjustment factor for large optical zones and refractive corrections. CONCLUSIONS: In patients with large optical zones, it may be preferable to calculate tissue ablation depth using the exact formula. Alternately, the Munnerlyn formula can be used to calculate ablation depth and then an adjustment factor can be added.