Analyzing protein cocktails in a snap.

Analyzing protein cocktails in a snap

In less time than it takes to read an issue of SCIENCE NEWS, researchers now can use a mass spectrometer -- a sensitive molecule-weighing tool -- to detect and help identify dozens of different proteins in minuscule portions of even complex biological samples, such as breast milk.

The new technique measures the masses of huge protein molecules far more swiftly, accurately and easily than gel electrophoresis, the standard method for such analyses, say chemists Brian T. Chait and Ronald C. Beavis of the Rockefeller University in New York City. So far, the researchers have measured more than 160 proteins, some with molecular masses up to 350,000 daltons. One dalton equals the mass of a single hydrogen atom.

The new technique, which several research groups are developing, should prove helpful in basic biological and biochemical research, and in such quality-control operations as tracking differences in genetically engineered protein products intended for use in foods or drugs, Beavis says.

In the September PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES (Vol. 87, No. 17), he and Chait report successfully identifying proteins in such samples as egg whites, blood-cell extracts and human breast milk.

The Rockefeller researchers' mass spectrometer employs an electric field to accelerate charged gas-phase molecules or molecular fragments through a vacuum. The mass of these propelled molecules is proportional to the time it takes them to reach a detector about 1 meter away: The longer the wait, the heavier the particle.

To use mass spectrometry, scientists must convert sample molecules to a gas, usually by bombarding them with energetic electrons or ions. Until recently, however, attempts to volatilize proteins damaged the huge molecules beyond recognition. Then in 1988, researchers at the University of Munster in Germany reported successfully volatilizing proteins with a technique called "matrix-assisted laser desorption." The Munster chemists embedded sample proteins within a matrix of organic molecules, such as nicotinic acid. When they blasted the matrix with a laser, it disintegrated. Previously embedded proteins suddenly found themselves liberated, electrically charged, and streaming down the mass spectrometer's vacuum tube toward a detector.

At a meeting of the American Chemical Society in Washington, D.C., last month, one of the Munster researchers, Michael Karas, explained the volatilization mechanism by way of a metaphor: "The elephant at the 10th floor must fly if the building suddenly turns into fine grains of sand."

By using more precise detectors, which defense researchers originally developed for night vision devices, and different matrix molecules based on the perfume compound cinnamic acid, Beavis says he and Chait have refined the Munster team's mass-spectrometer technique into a routine protein-analysis tool. Beavis and Chait's matrix material, sinapinic acid, offers the additional bonus of allowing the researchers to use unpurified samples containing salts and pH-neutralizing buffers. Beavis says a Texas-based company expects to begin marketing such an instrument this fall.

Beavis and Chait "have made mass spectrometry into a tool for protein analysis," observes Peter Williams, a chemist at Arizona State University in Tempe, who is developing a similar method for analyzing nucleic acids such as DNA. Though mass spectrometric analysis of nucleic acids is proving a challenge, analytical chemists like Beavis and Williams nonetheless hope that as the Human Genome Project gains momentum, this technique will provide a door for their participation in the massive effort to sequence all the chromosomes in a human cell.