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RESEARCH PROFILES LD-MS detects malaria parasite
DAVID SULLIVAN, JR., LIRONG SHI, AND MARIA RIVAROLA/JHU
gions, and it can be laser-desorbed with cells, and thus begins the cycle of fever Derived from archaic Italian, the name low detection limits into a mass spectrommalaria stuck even after scientists discov- and remission. Historically, the key to understanding eter. In addition, the spectrum is pretty ered that the parasite Plasmodium—not and destroying malaria, heme is also crit- clean because contaminants “are not as the “mal’aria”, or “bad air”, in marshes good photoabsorbers as heme,” and and swamps—causes the disease. Unfor- ical for the new detection method. The association was first made in 1897, when heme has a specific MS signature. “Havtunately, the disease is just as persistent ing both the molecular ion weight and Ronald Ross found heme crystals in the as the name, so more sensitive and rapid the four fragments makes our method methods to detect Plasmodium are need- digestive tracts of mosquitoes. Now revery resistant to getting false positives,” searchers know that Plasmodium’s proed. In the July 15 issue of Analytical tein synthesis is contained in a food vac- he says. “Seeing all those five peaks from Chemistry (pp 3262–3266), physicists, something else and not chemists, medical doctors, our compound is almost and parasitologists at the impossible.” Applied Physics LaboratoThe “gold standard” ry and the School of Pubof malaria detection is oplic Health at Johns Hoptical microscopy of Giemkins University report an sa-stained blood smears. in vitro method that can “[Classical methods] need detect very low levels of 1 mL of blood and an Plasmodium in blood. hour to detect levels of Malaria parasites invade parasitemia on the order red blood cells to survive. of 100 parasites per microInfected humans get a fever liter of blood, [whereas] similar to that caused by our method—which takes influenza and may die An optical microscopy image of human red blood cells (pink) that contain the only a few minutes and 50 from complications. The most deadly malaria parasite, Plasmodium falciparum (purple), and intracelµL of blood—is sensitive World Health Organizalular heme crystals (red-brown) in different stages. A red blood cell is ~6 µm down to 10 parasites per tion reports that a young in diameter. microliter,” says Demirev. child dies in Africa every “So we are 10 times better 30 s from malaria. It was uole, where the heme crystallizes, because than the official technique.” In terms of virtually eliminated from countries with temperate climates in the mid-20th cen- “if the heme is left to float freely around diagnosis, the “advantage is we can make an array with 96 spots from 96 people; the cell, it would kill [Plasmodium],” tury, but 40% of the world’s population and in less than an hour, we can get data.” remains at risk—especially because malaria Demirev says. The most effective drugs The researchers are working out exactly parasites are becoming resistant to drugs, prevent this heme sequestration. how to multiplex the processing. Initially, the physicists had hoped to and the mosquitoes that play a role in The researchers recently validated use MALDI MS to find parasite proteins Plasmodium’s complicated life cycle are found only in infected humans, but they their method on a mouse model and developing resistance to insecticides. plan to conduct additional tests using couldn’t detect any useful differences. The parasite’s life cycle begins when various Plasmodium species. In initial the female Anopheles mosquito transmits The researchers’ next idea was to target parasitemia measurements that compare Plasmodium to humans through her sali- heme. After removing all of the uninva, explains lead author Plamen Demirev. fected red blood cells and residual heme classical with LD-MS methods, Demirev boasts, “The good news is that the meawhile keeping the parasite wall intact, After proliferating in the human liver, surements are tracking each other; and “we [could] rupture the parasite open immature parasites are released into the moreover, in day one, [the classical bloodstream to enter the red blood cells. and . . . see the heme by laser desorpmethods] cannot see any infection in tion,” Demirev explains. “To our great Here, the parasite breaks down hemothe blood, but we can.” Their goal now delight, the method worked.” globin—essentially the only protein is to get the sensitivity down to one parDemirev says laser desorption (LD) available—to synthesize its own proteins. asite per microliter and to see if the time-of-flight MS is a very efficient tool Ferriprotoporphyrin IX (heme) is the method may be useful for other microfor heme detection so “using [it] as a byproduct. After 48–72 h, the parasite biology applications. a biomarker worked very nicely.” Heme matures and multiplies. The new para—Rachel Petkewich absorbs in both the visible and UV resites attack more uninfected red blood A U G U S T 1 , 2 0 0 2 / A N A LY T I C A L C H E M I S T R Y
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