Scientists at Washington University in St. Louis say their study of an enzyme that breaks down plasmalogens could provide a breakthrough in understanding the molecular processes that occur during Alzheimer's and other diseases. Alzheimer's disease patients lose up to 60% of the plasmalogen from the membranes of the cells in their brains, but it's still not known how or why. 

The team’s research (“Cytochrome c Is an Oxidative Stress–Activated Plasmalogenase That Cleaves Plasmenylcholine and Plasmenylethanolamine at the sn-1 Vinyl–Ether Linkage”) will appear in the June 1 issue of the Journal of Biological Chemistry.

Plasmalogens are phospholipids critical for cell function and signaling that contain a vinyl–ether linkage at the  sn-1 position and are highly enriched in arachidonic acid (AA) at the sn-2 position. However, the enzyme(s) responsible for the cleavage of the vinyl ether linkage in plasmalogens has remained elusive.  Herein, we report that cytochrome c, in the presence of either cardiolipin (CL), O2 and H2O2, or oxidized CL and O2, catalyzes the oxidation of the plasmalogen vinyl ether linkage, promoting its hydrolytic cleavage and resultant production of 2-AA-lysolipids and highly reactive α-hydroxy fatty aldehydes. Using stable isotope labeling in synergy with strategic chemical derivatizations and high-mass accuracy mass spectrometry, we deduced the chemical mechanism underlying this long sought-after reaction. Specifically, labeling with either 18O2 or H218O, but not with H218O2, resulted in M+2 isotopomers of the α-hydroxyaldehyde while reactions with both 18O2 and H218O identified the M+4 isotopomer.  Furthermore. incorporation of 18O from 18O2 was predominantly located at the α-carbon. In contrast, reactions employing H218O yielded 18O linked to the aldehyde carbon.  Importantly, no significant labeling of 2-AA lysolipids with 18O2, H218O, or H218O2. was present. Intriguingly, phosphatidylinositol phosphates (PIP2and PIP3) effectively substituted for cardiolipin,” write the investigators.

“Moreover, cytochrome c released from myocardial mitochondria subjected to oxidative stress cleaved plasmenylcholine in membrane bilayers which was blocked with a specific mAb against cytochrome c.  Collectively, these results identify the first plasmalogenase in biology, the production of previously unanticipated signaling lipids by cytochrome c, and present new perspectives on cellular signaling during oxidative stress.”

“These molecules, plasmalogens, have been swept under the rug because nobody likes to think about them,” said Richard Gross, M.D., Ph.D., professor of chemistry at Washington University and professor of medicine and developmental biology at Washington University School of Medicine, who oversaw the new study. “(They're) hard to work with. They're susceptible to light, they're stable in only certain solvents, they have a limited lifespan after they're synthesized unless extreme precautions are taken, and they're expensive to make and synthesize.”

In the new study, Dr. Gross' team performed experiments to find the  mechanism by which plasmalogens are enzymatically degraded. Cytochrome c is typically found in mitochondria where it facilitates electron transport, but it is released into the cell under stressful conditions. Dr. Gross and his colleagues showed that cytochrome c released from the mitochondria can catalyze the breakdown of plasmalogens in the cell. Furthermore, the products of this reaction are two different lipid signaling molecules that were not previously known to originate from plasmalogen breakdown.

“That was one thing that surprised us,” Dr. Gross said of the signaling products. “The second thing that surprised us was the ease (with which the bond is broken)….The implication is that there is probably a lot of plasmalogen (breakdown) that's going on in conditions of oxidative stress.” 

The results tie in with another observation about the brain cells of Alzheimer's disease patients, which is that they often have dysfunctional mitochondria and a resultant release of cytochrome c. Dr. Gross is now interested in delving deeper into how and why plasmalogen loss occurs in Alzheimer's patients, particularly those who develop the disease in old age, not due to familial mutations. He speculates that as people age, the accumulation of reactive oxygen species leads to cytochrome c release, activation of its peroxidase activity, and plasmalogen breakdown in many membranes. 

The results also have implications for understanding disorders in the heart and other plasmalogen-rich tissues, integrating studies of mitochondria, cell membranes, and cell signaling under stressful conditions, according to Dr. Gross.

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