Cholesterol is an essential archetypical sterol of cell membranes throughout all organisms. The metabolism of cholesterol starts from its oxidation to oxysterols. The precursors of cholesterol can also be oxidized to oxysterols and be metabolized. Therefore, oxysterols are the oxidized forms of cholesterol molecules, with one or more additional carbonyl, hydroxyl or carboxylic acid groups in the ring or the side-chain of the cholesterol skeleton.
Oxysterols have long been known as oxidized transportation forms of cholesterol and the intermediates of bile acids synthesis. Acting as agonists to the G protein-coupled receptor, the liver X receptors and Epstein-Barr virus-induced receptor 2, oxysterols play an important role in cell signaling. Oxysterols synthesis takes part in the immune response to pathogens. For example, when invaded by bacteria or viruses, macrophages, one type of white blood cells with critical role in immunity, can up-regulate cholesterol 25-hydroxylase and create high level of 25-hydroxycholesterol (25-HC). Some reports show that 25-HC, act as feedback, can negatively regulate the production of interleukin 1 (IL-1) family cytokine. Most recently, through their reactions with known receptors of oxysterols— the liver X receptors (LXRs), it is implicated that oxysterols play a role in neurogenesis and cholestenoic acids, relatives of oxysterols, is vital for the survival of motor neurons.
As biomarkers of oxidative stress, oxysterols are a group of molecules with potent pharmacological properties, which perhaps take part in the progression of atherosclerosis, neurodegenerative and inflammatory diseases. Because of this, interest in these compounds has grown dramatically.
A large number of sterol hydroxylases involve in biosynthesis of oxysterols from cholesterol. Most of these members are of the cytochrome P450 (CYP) family. For example, in the neutral pathway of bile acid synthesis, the first step is CYP7A1 converting cholesterol to 7α-hydroxycholesterol, 7α-HC. Also, in acidic pathway of bile acid synthesis, in the first step, CYP27A1 acts as catalyzing enzyme in the formation of (25R)26-hydroxycholesterol, 26-HC.
Endogenous oxysterols are rather low in human plasma, ranging from pg/mL to ng/mL. Besides, oxysterols lacks chromophore, which can make them easily detectable with colorimetric method. What’s more, oxysterols can be poorly ionized. All these characteristics make the analysis of oxysterols rather challenging. GC-MS has long been regarded as the ‘gold standard’ analytical method for oxysterols analysis. Recently, LC-MS emerges as the more sensitive and reliable analytical method for oxysterols analysis. Using solid-phase extraction (SPE), prompt separation of cholesterol from oxysterols at the earliest possible point during the sample preparation can avoid artifacts derived from cholesterol.
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