Regiospecific oxygenation of alkenones in the benthic haptophyte Chrysotila lamellosa Anand HAP 17

Abstract

Two groups of previously unidentified C 37–C 39 epoxyalkenones and alkenediones were detected in late stationary phase cultures of the haptophyte microalga Chrysotila lamellosa. The formation of these compounds is attributed to the involvement of enzymatic processes acting specifically on the C-21 or C-22 allylic carbon and the ω15 double bond of methyl and ethyl alkenones respectively. Thus, the epoxyalkenones appear to be derivatives of alkenones where the ω15 double bond is oxidized to the epoxide. These epoxyalkenones disappear as the cells age to be replaced by a series of alkenediones. The structures of these compounds indicate that they are derivatives of methyl and ethyl alkenones with an additional carbonyl group on the C-21 or C-22 carbon respectively and without the ω15 double bond. We propose that these compounds are formed by an initial regiospecific lipoxygenase-catalyzed peroxidation of methyl and ethyl alkenones on their C-21 or C-22 allylic carbon, respectively. Lipohydroperoxidase-catalyzed homolytic cleavage of the O–O bond could then result in the formation of conjugated ketones which may then undergo a saturation reaction to form the diketones identified. This work demonstrates that alkenones can be degraded by enzymatic reactions in senescent cells, and by implication this could also occur in the natural environment.