Metabolic Pathways of Carotenoid Metabolism: Insights into the Biochemical and Regulatory Pathways of Carotenoid Cleavage.
Consumption of a diet high in carotenoid-rich fruits and vegetables is strongly
associated with a decreased risk of many chronic and degenerative diseases. Carotenoids
are a class of lipophilic compounds found in many fruits and vegetables and have often
been targeted as imparting some of the beneficial effects associated with fruit and
vegetable consumption. However, our understand... read moreing of the biochemical and molecular
framework of carotenoid metabolism is incomplete. Investigating the biochemical and
molecular pathways of carotenoid metabolism are important to understanding their
biological functions. Carotene-15,15'-monooxygenase (CMO1) is involved in vitamin A
formation, while recent studies suggest that carotene-9',10'-monooxygenase (CMO2) may
have a broader substrate specificity than previously recognized. The first part of this
research investigated the in vitro cleavage activity of recombinant ferret CMO2 towards
the xanthophylls lutein, zeaxanthin and β-cryptoxanthin. Utilizing HPLC, LC-MS and
GC-MS, we identified both volatile and non-volatile apocarotenoid products including
3-OH-β-ionone, 3-OH-α-ionone, β-ionone,
3-OH-α-apo-10'-carotenal, 3-OH-β-apo-10'-carotenal, and
β-apo-10'-carotenal, indicating cleavage at both the 9,10 and 9',10' carbon-carbon
double bond. Kinetic analysis indicated the xanthophylls zeaxanthin and lutein are
preferentially cleaved over β-cryptoxanthin, indicating a key role of CMO2 in
non-provitamin A carotenoid metabolism. Furthermore, incubation of
3-OH-β-apo-10'-carotenal with CMO2 lysate resulted in the formation of
3-OH-β-ionone. In the presence of NAD+, in vitro incubation of
3-OH-β-apo-10'-carotenal with ferret hepatic homogenates resulted in the formation
of 3-OH-β-apo-10'-carotenoic acid. Since apo-carotenoids serve as important
signaling molecules in a variety of biological processes, enzymatic cleavage of
xanthophylls by mammalian CMO2 represents a potential new avenue of investigation
regarding vertebrate carotenoid metabolism and biological function. Several
well-implemented cohort studies have shown blood levels and dietary intake of
β-cryptoxanthin to be strongly associated with a decreased risk of lung cancer
independent of vitamin A. The objective of this study was to assess the regulation of
CMO1 and CMO2 expression in selected ferret tissues in response to varying doses of
β-cryptoxanthin supplementation. We first partially cloned the ferret CMO1 gene
and compared the relative abundance of CMO1 and CMO2 expression in various tissues.
Tissue-specific comparisons revealed significant differences in expression levels of
CMO1 and CMO2. CMO1 expression was significantly higher in the intestinal mucosa while
CMO2 expression was significantly higher in the lungs, visceral adipose and kidneys
compared to CMO1. Low-dose (7.5 μg/kg body weight per day) and high-dose (37.5
μg/kg body weight per day) β-cryptoxanthin supplementation for 9 weeks
resulted in a dose-dependent increase in β-cryptoxanthin concentrations with no
changes in retinyl palmitate selected tissues. There was a significant decrease in lung
CMO2 expression in both low- and high-dose supplementation groups with little changes in
CMO1 or CMO2 expression in other tissues. The relative abundance of CMO2 expression in
the lungs and down-regulation by β-cryptoxanthin supplementation indicates a
potential role of CMO2 in the biological activity of β-cryptoxanthin in human
Thesis (Ph.D.)--Tufts University, 2011.
Submitted to the Dept. of Biochemical and Molecular Nutrition.
Advisor: Xiang-Dong Wang.
Committee: Robert Russell, and Gregory Dolnikowski.
Keywords: Nutrition, and Biochemistry.read less