The Use of Quinolizidine Derivatives of Coumarin in the Studies of the Mechanisms of Action of the Cytochrome c–Cardiolipin Complex

The cytochrome c–cardiolipin complex plays a key role in triggering apoptosis via the mitochondrial pathway due to lipoperoxidase and quasi-lipoxygenase activities of cytochrome c. As a result of the formation of this complex, the conformation of cytochrome c changes, which acquires the properties of peroxidase enzymes capable of triggering lipid peroxidation reactions. The functions of the cytochrome c–cardiolipin complex are usually studied using the recording of enhanced (activated) chemiluminescence. The chemiluminescence enhancer or activator increases the luminescence intensity due to the migration of the electron excitation energy from the excited lipid peroxidation products to the activator molecules, followed by its chemiluminescence with a high quantum yield. It is advisable to use in such studies activators that enhance luminescence without a chemical reaction with the components of the system under study and keep their concentration unchanged during the reaction time. At the end of the last century, it was shown on the Fe²⁺-induced lipid peroxidation system that quinolizidine derivatives of coumarin are such compounds. The ideas about the immutability of their concentration were transferred without additional studies to systems in which lipid peroxidation is triggered by peroxidase. However, it was found in this study by spectrophotometry using a reaction catalyzed by the cytochrome c–cardiolipin complex as an example that quinolizidine derivatives of coumarin, coumarin-314 (quinolizidine[5,6,7-gh]3-ethoxycarbonylcoumarin) and coumarin-334 (quinolizidine[5,6,7-gh]3-acetylcoumarin), are direct participants in the enzymatic lipoperoxidase reaction. Based on a comparison of changes in the concentration of coumarin derivatives in the presence and absence of phosphatidic acid, we found that coumarin derivatives are predominantly substrates of the second reaction of the peroxidase catalytic cycle, that is, the reduction of a peroxidase ferriform with two oxidized equivalents (compound 1) to a peroxidase ferriform with one oxidized equivalent (compound 2). It was also shown that during the catalysis of a quasi-hypoxygenase reaction (in the absence of H₂O₂ in the system), peroxidase passes through a catalytic cycle by the mechanism of one-electron oxidation followed by reduction, while there is no ferriform stage of peroxidase with two oxidized equivalents (compound 1). The rate constants of the first-order reaction of the decomposition of coumarin derivatives during the enzymatic lipoperoxidase reaction were determined, and based on them, functions were derived for calculating correction coefficients that take into account the decomposition of coumarin derivatives for correcting chemiluminograms obtained in the study of the cytochrome c–cardiolipin complex.