Characterization of the Role of ß-Carotene 9,10-Dioxygenase in Macular Pigment Metabolism.

A family of enzymes collectively referred to as carotenoid cleavage oxygenases is responsible for oxidative conversion of carotenoids into apocarotenoids, including retinoids (vitamin A and its derivatives). A member of this family, the ß-carotene 9,10-dioxygenase (BCO2), converts xanthophylls to rosafluene and ionones. Animals deficient in BCO2 highlight the critical role of the enzyme in carotenoid clearance as accumulation of these compounds occur in tissues. Inactivation of the enzyme by a four-amino acid-long insertion has recently been proposed to underlie xanthophyll concentration in the macula of the primate retina. Here, we focused on comparing the properties of primate and murine BCO2s. We demonstrate that the enzymes display a conserved structural fold and subcellular localization. Low temperature expression and detergent choice significantly affected binding and turnover rates of the recombinant enzymes with various xanthophyll substrates, including the unique macula pigment meso-zeaxanthin. Mice with genetically disrupted carotenoid cleavage oxygenases displayed adipose tissue rather than eye-specific accumulation of supplemented carotenoids. Studies in a human hepatic cell line revealed that BCO2 is expressed as an oxidative stress-induced gene. Our studies provide evidence that the enzymatic function of BCO2 is conserved in primates and link regulation of BCO2 gene expression with oxidative stress that can be caused by excessive carotenoid supplementation.

Comparison of dietary supplementation with lutein diacetate and lutein: a pilot study of the effects on serum and macular pigment.

UNLABELLED: The responses of subjects taking a 20 mg/day lutein diacetate supplement were compared with that for a 20 mg/day crystalline lutein or a placebo. Ten subjects, assigned to each of three groups, lutein diacetate (group 1), lutein (group 2), and a placebo (group 3), were supplemented for 24 weeks. Groups 1 and 2 consumed a dose equivalent to 20 mg per day of free lutein. Serum samples, collected at baseline, and at weeks 6, 12, 18, and 24 were analyzed by HPLC. Macular Pigment Optical Density (MPOD) was obtained by heterochromatic flicker photometry at baseline and weeks 6, 12, 18 and 24. RESULTS: The average serum lutein concentrations for weeks 6 to 24 expressed as a ratio to the baseline value (±S.D.) were 5.52 ± 2.88 for group 1, 4.43 ± 1.61 for group 2, and 1.03 ± 0.25 for group 3. The median rate of macular pigment increase (milli-absorbance units/week) for groups 1, 2, and 3 were 2.35, 1.55, and 0.19 mAU/wk, respectively. P-values for these serum and MPOD increases are both highly significant when compared to placebo. The average serum response was about 25% higher for group 1 compared with group 2 and, the median MPOD response was 52% higher for group 1 than group 2. P-values calculated for the differences in these increases were, p = 0.066, marginally significant, for serum, and p = 0.09 approaching significance, for MPOD.

Mammalian carotenoid-oxygenases: key players for carotenoid function and homeostasis.

Humans depend on a dietary intake of lipids to maintain optimal health. Among various classes of dietary lipids, the physiological importance of carotenoids is still controversially discussed. On one hand, it is well established that carotenoids, such as ß,ß-carotene, are a major source for vitamin A that plays critical roles for vision and many aspects of cell physiology. On the other hand, large clinical trials have failed to show clear health benefits of carotenoids supplementation and even suggest adverse health effects in individuals at risk of disease. In recent years, key molecular players for carotenoid metabolism have been identified, including an evolutionarily well conserved family of carotenoid-oxygenases. Studies in knockout mouse models for these enzymes revealed that carotenoid metabolism is a highly regulated process and that this regulation already takes place at the level of intestinal absorption. These studies also provided evidence that ß,ß-carotene conversion can influence retinoid-dependent processes in the mouse embryo and in adult tissues. Moreover, these analyses provide an explanation for adverse health effects of carotenoids by showing that a pathological accumulation of these compounds can induce oxidative stress in mitochondria and cell signaling pathways related to disease. Advancing knowledge about carotenoid metabolism will contribute to a better understanding of the biochemical and physiological roles of these important micronutrients in health and disease. This article is part of a Special Issue entitled Retinoid and Lipid Metabolism.