Lysine Reacts with Cholesterol Hydroperoxide to Form Secosterol Aldehydes and Lysine-Secosterol Aldehyde Adducts

Abstract

Two cholesterol secosterol aldehydes, namely, 3β-hydroxy-5-oxo-5,6-secocholestan-6-al (secosterol A) and its aldolization product 3β-hydroxy-5β-hydroxy-B-norcholestane-6β-carboxyaldehyde (secosterol B), are highly bioactive compounds which have been detected in human tissues and potentially contribute to the development of physiological dysfunctions such as atherosclerosis, Alzheimer’s disease, diabetes, and cancer. They were originally considered to be exclusive products of cholesterol ozonolysis and thus to be evidence for endogenous ozone formation. However, it was recently postulated that primary amines such as lysine may catalyse their formation from cholesterol-5α-hydroperoxide (Ch-5α-OOH), the main product of the oxidation of cholesterol with singlet oxygen. This involves cyclization of Ch-5α-OOH to an unstable dioxetane intermediate, which decomposes to form secosterol aldehydes with triplet carbonyl groups, whose return to the singlet state is at least partly coupled to the conversion of triplet molecular oxygen to singlet oxygen. Here, we subjected cholesterol to photosensitized oxidation, which predominantly produces Ch-5α-OOH and minor amounts of the 6α- and 6β-hydroperoxides, exposed the hydroperoxide mixture to lysine in the presence of the antioxidant 2,6-ditertiary-butyl-4-hydroxytoluene (BHT), and analysed the reaction mixture by liquid chromatography-electrospray ionization-mass spectrometry. Consistent with the postulated lysine-catalysed formation of secosterol aldehydes, we detected formation of the latter and several types of their lysine adducts, including carbinolamines, Schiff’s bases, and amide-type adducts. We propose that the amide type adducts, which are major biomarkers of lipid oxidation, are mainly formed by singlet oxygen-mediated oxidation of the carbinolamine adducts.

The Chemical Reactivity of Membrane Lipids

“…462 Amides such as 94 have been detected during the photosensitized oxidation of cholesterol and are proposed to form via oxidation of the hemiaminal intermediate formed by the reaction of the terminal amino group of lysine with the aldehyde group of 93. 437 Formation of Schiff bases between cholesterol aldehydes and proteins leads to changes in protein folding that are potentially important in the development of disease. 453,463 Ketosterols are generally unreactive toward Schiff base formation.…”

“…These can be formed by a number of routes, including ozonolysis, reaction of cholesterol with 1 O 2 , and Hock rearrangement of the α-isomer of 85 . Amides such as 94 have been detected during the photosensitized oxidation of cholesterol and are proposed to form via oxidation of the hemiaminal intermediate formed by the reaction of the terminal amino group of lysine with the aldehyde group of 93 …”

“…436 A number of other experiments have provided secondary evidence for the presence of aminemodified PE lipids, such as the development of fluorescent chromophores following peroxidation of both model lipid mixtures and biological membranes in vitro (reviewed in ref 424). Oxidation of hemiketal intermediates formed by the reaction of amino groups with reactive aldehydes formed from cholesterol 437 and fatty acids 438 have recently been described, leading to the formation of amides. It is reasonable to expect that the amino groups of PE lipids may undergo similar reactions.…”

Mechanisms of the Regulation and Dysregulation of Glucagon Secretion

“…Although it has been suggested that such carbonylation involves hydroxyl radicals generated by the Fenton reaction between hydrogen peroxide and ferrous ions [ 48 ], this is unlikely due to the very high reactivity of hydroxyl radicals, which makes them react unselectively [ 50 ]. Singlet oxygen ( 1 O 2 ) is a more selective ROS, which can be formed by the reaction of hydrogen peroxide with glucose [ 51 ], oxidizes amines [ 52 ], and such oxidation was recently suggested as being involved in the formation of biologically relevant amide-type adducts such as N ε -(hexanoyl) lysine [ 53 ]. Thus, it is proposed that the carbonylation of Na + /K + ATPase may be mediated by singlet oxygen according to Figure 3 .…”