Stable isotopes in carotenoid biochemistry

The use of stable isotopes in studies of carotenoid biosynthesis is reviewed, beginning with early work with l8 which defined the origin of oxygen in a range of xanthophylls. A series of experiments from our own laboratory is then described, in which 2H and 13C have been used to investigate the stereochemistry of cyclization to form C40 and C50 and i-ring carotenoids. Novel procedures involving 2H labelling from 2H20 have been used to demonstrate carotenoid transformations during biosynthesis and chloroplast development and to define the stereochemistry of the H attack at C-2 which initiates carotenoid cyclization. Procedures by which microorganisms are allowed to metabolize ['HI-glucose, in 2H20, or [13C6]glucose to make endogenous specifically labelled substrates are described. These methods allow the carotenoid C-I methyl substituents to be distinguished even when incorporation of exogenous labelled substrates is not possible, and have been used to show that the stereochemistry of cyclization to form the p and C-rings of the C40 carotenoids zeaxanthin and lutein is the same.