Biochemist India: Metabolic Syndrome

What Lab Tests Should Be Used for Assessing Patients?

Seven years ago, the World Health Organization (WHO) published the first definition and criteria for metabolic syndrome. Even before the report was published, however, many clinicians had recognized a loose collection of risk factors that dramatically boost a person’s chances of developing potentially life-threatening illnesses. Syndrome X, insulin resistance syndrome, dysmetabolic syndrome, and multiple metabolic syndrome were all among the labels physicians initially used for a constellation of factors that promotes the development of atherosclerosis and increases the risk of cardiovascular events (CVEs).

Today, the most widely accepted definition and criteria for metabolic syndrome are those proposed in 2001 as part of the Third Report of the National Cholesterol Education Program (NCEP) Expert Panel on Detection, Evaluation, and Treatment of High Blood Cholesterol in Adults. Known as the Adult Treatment Panel III (ATP III), the report put new emphasis on this growing epidemic. The most recent National Health and Nutrition Examination Survey (NHANES), conducted by the Centers for Disease Control and Prevention, shows that as many as one in four American adults and 40% of adults age 40 or older have metabolic syndrome. In addition, there is an alarming increase in the prevalence of metabolic syndrome in children and adolescents.

Lipid of the week : Sphingomyelin

Not merely membrane building blocks, sphingomyelin species carry out important signaling roles as precursor molecules and by providing localized platforms.

Sphingomyelin species comprise a ceramide unit and a phosphorylcholine or phosphoethanolamine polar headgroup. Typically, sphingosine is the long-chain base component of ceramide, although other long-chain bases can be present; differences in the sphingoid base and the fatty acid components (which are very-long-chain saturated and monounsaturated) confer great variability on the composition of sphingomyelins¹.

As a major constituent of cell membranes, sphingomyelin is found at particularly high concentrations in the membranes of nerve cells (in the myelin sheaths) and red blood cells. It was previously thought to have a purely structural role, similar to the function of phosphatidylcholine, through intermolecular interactions mediated by the 2-amide group, the 3-hydroxy group and the 4,5-trans double bond of the sphingoid base. However, it is now appreciated that sphingomyelin has a high affinity for cholesterol and that these two lipids pack tightly into liquid-ordered domains among a liquid-disordered phase to form lipid rafts¹². These membrane microdomains are thought to function as signaling platforms that regulate the localization and interactions of proteins. But sphingomyelin does not just influence signaling as a component of lipid rafts — it is also a precursor to ceramides and other sphingolipid metabolites that comprise the sphingomyelin cycle or sphingolipid network.

Sphingomyelin biosynthesis begins in the Golgi but continues in the plasma membrane. Sphingomyelin synthase catalyzes the transfer of phosphorylcholine from phosphatidylcholine to ceramide. The consequent decrease in ceramide levels is accompanied by an increase in the levels of sphingomyelin and the release of the well-known signaling molecule diacylglycerol. Alternatively, ceramide can also first be converted to ethanolamine phosphorylceramide through the transfer of the head group from phosphatidylethanolamine followed by sequential methylation of the ethanolamine moiety to a choline group, although whether this pathway is physiologically relevant is unknown¹.

Conversely, sphingomyelin can be metabolized to ceramide (and further, to other biologically active sphingolipids such as sphingosine and sphingosine-1-phosphate) by sphingomyelinases. The function of ceramide as a proapoptotic molecule is well known, although the exact mechanism through which this lipid induces cell death is not fully understood. By contrast, a decrease in ceramide levels accompanies the accumulation of sphingomyelin in cells of the brain, spleen and liver that occurs as a consequence of a deficiency in sphingomyelinase in Niemann–Pick disease types A and B¹. Notably, levels of cholesterol are also elevated in Niemann–Pick disease, which is consistent with reports suggesting that the metabolism, as well as the distribution, of these two lipids is linked, which has clear implications for several other diseases.

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Metabolic Syndrome

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