Milk, revealed “silent” chemistry: new mode of cycloretinal synthesis
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Bovine milk is by far the most commonly consumed milk in the western world. The protein composition in milk consists of casein and whey proteins, of which β-lactoglobulin (BLG) is the principal constituent of the latter. Here we provide biochemical evidence that this milk protein, in purified form and in pasteurized store-bought milk, promotes the formation of cycloretinal (all-trans retinal dimer), and a variety of other cycloterpenals of biological relevance [Fishkin et al., Proc. Natl. Acad. Sci. U. S. A., 2005, 102, 7091-7096; Fishkin et al., Chirality, 2004, 16, 637-641; Kim et al., Proc. Natl. Acad. Sci. U. S. A., 2007, 104, 19273-19278]. Cycloretinal is an eye metabolite and among several toxic byproducts of the visual cycle firmly established to cause age-related macular degeneration. Experiments in rabbits further demonstrate that BLG/milk can survive the digestive system and promote this reaction in vivo [Caillard et al., Am. J. Physiol., 1994, 266(6), G1053-G1059]. Proteomic studies on age-related macular degeneration patients have detected BLG in the eye of these patients further suggesting that this milk protein could contribute to disease progression [Crabb et al., Proc. Natl. Acad. Sci. U. S. A., 2002, 99(23), 14682-14687].
author list (cited authors)
Bench, B. J., Foulke-Abel, J., & Watanabe, C.