Do you know your blood type? No matter what kind, it ends in either a "positive" or "negative," which refers to which kind of so-called Rhesus factor you carry. Although scientists have been typing blood for decades, no one knew the purpose of Rh proteins. Now a study shows that the proteins transport ammonium, a toxic by-product of energy metabolism, and possibly keep the blood's pH level balanced.
Until now, the Rh proteins that dot the surface of red blood cells were known for the problems they can cause for blood transfusions and pregnancy. If a patient receives a transfusion with mismatched Rh proteins, antibodies build up and attack the new blood. Likewise, an Rh-positive fetus can trigger an immune response in an Rh-negative mother that, if untreated, can injure or kill the fetus.
The link between Rh proteins and ammonium transport was unexpected. Several years ago, a team of yeast researchers led by Bruno André of the Free University in Brussels, Belgium, identified yeast proteins that transport ammonium. They entered the sequence of the gene that encoded the transporters in a genome data bank and saw that it closely matched the genes for Rh proteins in humans. To find out whether Rh proteins perform a similar function, the team knocked out ammonium transporter genes in yeast and replaced them with human Rh protein genes. The Rh proteins pumped ammonium out of the cells, they report in the November issue of Nature Genetics.
"This is the first strong evidence for a biological role" for Rh proteins, says hematologist Peter Agre of Johns Hopkins University. André and his team suggest that red blood cells pick up excess ammonium in the bloodstream and deliver it to the kidneys, which filter it out. This system would clear the blood of toxic ammonium and maintain pH, they claim.