Membrane structure and the activity of phospholipase and sphingomyelinase D

Abstract

Lipid-modifying enzymes play a vital role in the regulation of lipids as mediators of cell function. One example is the hydrolysis of phospholipids through phospholipase D (PLD), which produces the signalling molecule phosphatidic acid (PA). These processes at lipid membranes can be observed in situ through the application of different biophysical techniques. Thus, the hydrolysis of phosphatidylcholines by PLD was investigated, showing that the enzyme is highly affected in its catalytic activity by the lipid membrane structure. Briefly, by using Langmuir monolayers as a model system, we revealed that PLD activity depends on the segregation of the hydrolysis product PA within the monolayer. Hence, we could describe how the structure of the PA-rich domains is decisive for the activation and inhibition of PLD. This study demonstrates how membrane structure influences the activity of PLD and regulates the concentration of the lipid messenger PA.The current research project is aiming at describing a toxic component of the venom of brown spiders (Loxosceles), which has a rare enzymatic activity termed sphingomyelinase D (SMD). SMD catalyzes the conversion of sphingomyelin (SM) into ceramide-1-phosphate (Cer-1-P). While the enzymatic substrate SM is an integral constituent of many cell membranes, especially in the vascular epithelium and red blood cells, the reaction product Cer-1-P occurs in very low concentrations. Cer-1-P is suggested to be a novel lipid second messenger in cellular signal transduction events. At present, the precise mechanism of venom action is incompletely understood, but preliminary results show the strong effect of SMD activity on the membrane structure of giant unilamellar vesicles.In summary, the presented work depicts the correlation between membrane structures and the activity of lipid-modifying enzymes. This implements new models for the regulation of cellular processes through distinct structures of biological membranes.