Von Goethe had his Mephistopheles say “Blood is a juice of very special kind”. And not wrongly, since miesteries keep coming out. The hemoglobin gives blood its red color and ensures that the erythrocytes (red blood cells) can bind oxygen for breathing. This is managed by the hemoglobin-bound molecule heme, which is a complex of a porphyrin molecule bearing a central iron ion. When the blood pigment is broken down, “heme” is produced, which in turn can influence the protein cocktail in the blood. Researchers at the University of Bonn, led by Dr. Diana Imhof, have now discovered in complex detective work that the “activated protein C” (APC) can be commandeered by heme. At the same time, APC can also reduce the toxic effect of heme. Perspectively, the findings may provide the basis for better diagnostic and therapeutic approaches to blood diseases. The study has been published online in advance in the journal Antioxidants & Redox Signaling. The print version will be published soon.
As it can exert toxic effects in high concentrations, the body tries to keep the amount of heme in check. This usually happens through its iron atom that may trigger oxidative stress. However bilirubin, a final breakdown derivative of heme is toxic especially for brain cells. It has been known for quite some time that free heme affects the function of biomolecules. For example, as an endproduct of its own synthesis in the bone marrow, it represses by negative feedback the upstream enzyme ALA synthase. Heme can activate the Nrf-2 transcription factor, which regulates antioxidant enzymes and proteins production. Another transcription factor regulated by free heme is called Bach-1; this works as a repressor to enzymes needed to heme breakdown itself. The team around Diana Imhof has now discovered in meticulous detective work which of the many proteins in the blood is particularly under the control of heme.
She explained: “Over the last few years, our research group has established a large database of model peptides. The peptides are individual protein “snippets” from which the sometimes huge and complex structures are built. Instead of studying the giant molecules, the proteins, in their entirety, we first took a shortcut with the snippets. We did in a similar way to profilers in thrillers, who draw conclusions about the perpetrator’s behavior from crime scene traces, circumstantial evidence and the type of crime. The researchers used an algorithm to systematically search the database for protein fragments that might potentially interact with heme. Using these data, they were able to conclude that the “activated protein C” (APC) is a particular candidate for heme binding. This enzyme is known for its anticoagulant and clot-dissolving effect, but can also take over cell-protective and anti-inflammatory tasks.
So far, the impact of heme on the function of APC has been unknown. The researchers investigated the association with pure compounds in the test tube and by using blood plasma samples provided by the Institute of Experimental Hematology and Transfusion Medicine at the University Hospital Bonn. There, Prof. Dr. Bernd Pötzsch and Dr. Nasim Shahidi Hamedani also supported the pharmacists with know-how, APC samples, test systems and access to specific devices. They demonstrated that the enzymatic and anticoagulant activity of APC is reduced in the presence of heme. For example, if there is too little APC or its activity is restricted, the risk of a clot forming in the bloodstream increases, thereby causing thrombosis, heart attack or stroke. Indeed, hemolytic diseases with an increased incidence of labile heme, such as sickle cell disease, are often associated with thrombotic complications.
Molecular mapping led to discover that APC hs two regions potentially able to bind heme, corresponding to sequences 285-293 and 387-395 of the proten itself. For this reason, scientists think that the influence of heme on the enzyme APC is more significant than has probably been suspected so far. Furthermore, the team discovered that APC might protect the cells of the inner blood vessel wall like a bodyguard against the cytotoxic effect of heme. The researchers cultivated human endothelial cells and exposed them to heme. If APC was present at the same time, the toxic effect of heme on the cells was suppressed. They are convinced that this interaction between APC and heme is significant, because many other blood proteins they were looking for did not bind heme. It might be worthwhile to further investigate the impact of labile, regulatory heme on APC in order to also gain new diagnostic and therapeutically relevant insights regarding blood coagulation disorders that occur in hemolytic diseases.
- Edited by Dr. Gianfrancesco Cormaci, PhD, specialist in Clinical Biochemistry.
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