The St. Jude investigators showed that ABCB6 is lodged within the outer membrane of the cell's energy powerhouse called the mitochondria; and that it ferries into mitochondria a type of molecule called a porphryin, a molecule essential for life. A report on these results appears in the advanced online publication of Nature.
Inside the mitochondrion, porphyrins are converted to heme. Heme is the oxygen-carrying part of the red blood cell molecule called hemoglobin, as well as a critical part of certain liver enzymes that break down toxins, and so-called "respiratory chain" enzymes in mitochondria that use oxygen to produce energy-rich molecules.
The discovery of the location and function of ABCB6 solved the long-standing riddle of how porphyrins get into mitchondria so they can be used to make heme, said John Schuetz, PhD, a member of St. Jude Pharmaceutical Sciences. "Porphyrins are negatively charged and so is the inside of mitochondria," Schuetz said. "So it wasn't clear how a negatively charged molecule could get into a negatively charged environment when it should have been repulsed--like two negative poles of a magnet pushing each other apart. Our work showed that ABCB6 overcomes that problem by simply grabbing porphyrin molecules and pulling them in.
"We showed that there is a dynamic relationship between ABCB6 and the levels of heme and porphyrin that helps to regulate the production of heme," Schuetz said. The team found that the increase in ABCB6 in mitochondrial membranes caused the cells to make more porphyrin, the building block of heme. "Any disruption of this relationship can cause serious problems in the cell by altering leve ls of heme or porphyrins," Schuetz said.
Armed with this new information, researchers will be better able to study certain diseases at the molecular level and learn more about their causes and perhaps how to prevent or treat them. One such group of diseases is cutaneous porphyria, a buildup of porphyrins that causes the skin to blister and swell in sunlight; and acute porphoryia, a nerve problem that triggers pain in the chest, abdomen, limbs or back, as well as paralysis, cramping and personality changes or mental disorders. "Also, people with porphyria can suffer sudden, acute attacks if they take certain tranquilizers and other drugs that interfere with this system," Schuetz said. "So our work contributes to the overall picture of how disruption of the biochemical pathway of heme production can cause a variety of serious diseases."