Sphingomyelin metabolism in erythrocyte membrane in asthma

Sphingomyelin (SM), a major lipid constituent of outer leaflet of plasma membranes, with cholesterol, constitutes microdomains, which are termed as lipid rafts. These rafts provide support to proteins, receptors, enzymes, and so on and organize and orient them to conduct cellular functions including transmembrane signaling to substances in external milieu. The SM contents are regulated by its metabolism, changes in which may affect the composition of lipid rafts and cell response to the triggers of asthma which may lead to the pathophysiology. For studying changes in membranes, erythrocytes, which contain lipid rafts, are considered to be the best cell type. Hence, this study was conducted on plasma membrane of erythrocytes of asthmatic patients.

The objective is to understand the changes in SM metabolism in asthma.

The study included 50 subjects (25 asthmatics and 25 healthy subjects). Erythrocytes were isolated from the peripheral blood and membrane prepared. This was followed by determination of total cholesterol, phospholipids, SM, and sphingomyelinase activity. P < .05 was considered significant.

In asthmatics, there was a significant decrease in cholesterol contents (p < .05), decrease in total phospholipid contents (p < .005), increase in SM (p < .01), decrease in cholesterol: SM ratio (p < .001) and increase in sphingomyelinase activity (p < .001) in erythrocyte membranes. We conclude that in asthma, the increase in SM contents is associated with increased sphingomyelinase activity which shows an imbalance in SM metabolism, directed toward its accumulation. The ratio of cholesterol to SM, critical for maintenance of lipid rafts, was significantly lower in asthmatics. This indicates changes in structure of lipid rafts which may lead to the pathophysiology and development of asthma. Regulation of SM metabolism may help in disease regulation and its control.