Abstract
Retinoids (vitamin A) serve two distinct functions in higher animals: light absorption for vision and gene regulation for growth and development. Cigarette smoking is a contributing factor for diseases that affect vision such as age-related macular degeneration and increases the risk of birth defects; however, altered retinoid homeostasis has received little attention as a potential mechanism for smoking-associated toxicities. Herein, we demonstrate that nornicotine, a nicotine metabolite and component of cigarette smoke, catalyzes the Z-to-E alkene isomerization of unsaturated aldehydes and ketones, including retinals. Despite the recent explosion in the use of organic compounds as chemical catalysts, minimal effort has been devoted to biologically relevant organocatalysis. Our study demonstrates a system in which a lowest unoccupied molecular orbital-lowering intermediate similar to the endogenous protein rhodopsin effectively catalyzes isomerization under biologically relevant conditions. The product of retinal isomerization is all-E-retinal, which in the eye is a biosynthetic precursor to N-retinylidene-N-retinylethanolamine, a hallmark of age-related macular degeneration. Furthermore, 9-Z- and all-E-retinal isomers are biosynthetic precursors to 9-Z- and all-E-retinoic acids, ligands that mediate specific cellular responses by binding to transcriptional regulatory proteins critical in growth and development. Strict maintenance of retinal isomer composition is essential for proper transcriptional regulation. Nornicotine-catalyzed retinal isomerization implies an underlying molecular mechanism for age-related macular degeneration, the birth defects associated with smoking, and other smoking-associated abnormalities that stem from disruption of retinoid metabolism.