Proving That Enzymes Are Proteins

Northrop, John
Courtesy of the Rockefeller Archive Center

Enzymes catalyze the chemical reactions that take place in living cells. By the beginning of the 20th century researchers had studied the effects of enzymes for decades—fermentation was known to be an enzymatic process, for example, and pepsin had been identified as the agent in the stomach necessary to digest meat. But scientists debated whether chemical processes in living things differed from inorganic reactions, and they did not have methods to determine the chemical nature of enzymes. Many researchers agreed with the influential German chemist Richard Willstatter (Nobel Prize, 1915), who concluded that enzymes were an as-yet undescribed special class of compounds.

Then, in 1926, James Sumner crystallized the enzyme urease. Crystallization was the standard of the day for obtaining a pure sample of a molecule, and after further tests Sumner concluded that urease was a protein. His results were met with skepticism; however, they prompted Rockefeller's John Northrop (1891-1987) to attempt to obtain crystals of pepsin. In 1929 Northrop succeeded, and the next year he provided extensive evidence of pepsin's protein nature. In 1946 Northrop shared the Nobel Prize with Sumner (1887-1955), of Cornell University, and Rockefeller virologist William Stanley (1904-1971), who used Northrop's techniques to crystallize tobacco mosaic virus.

Northrop went on to work with others in his laboratory, notably Moses Kunitz (1887-1978), to isolate and crystallize trypsin, chymotrypsin, carboxypeptidase, and hexokinase, as well as some of their precursors. Their work put to rest any doubt that enzymes were proteins, and opened the door to studying the chemistry of enzyme action. In addition, Kunitz crystallized ribonuclease and deoxyribonuclease, providing valuable tools for studying nucleic acids at a time when the functions of these molecules were just beginning to be understood.

Northrop also was interested in viruses, and Rockefeller in the 1930s was a hub of research in virology: Richard Shope, for example, was working to demonstrate the viral nature of swine influenza, and Wendell Stanley was exploring the essential nature of viruses with his research on tobacco mosaic virus (TMV). Northrop, with his coworker Albert Krueger, developed a method to assay bacteriophage infections of Staphylococcus cultures. In 1936 Northrop isolated and purified ribonucleic acid from his phage preparations. This discovery lent support to Stanley's controversial findings on the chemical nature of TMV, and also established that phage are viruses. Later in his career Northrop focused on isolating and crystallizing antibodies, including diphtheria antitoxin and type 1 pneumococcal polysaccharide antibody.

Crystals of diphtheria antitoxin. From J Gen Physiol, 1942, 25: 465-485

John H. Northrop received the BS (1912), the MS (1913), and the PhD (1915) from Columbia University. In 1915 he joined the Rockefeller laboratory of physiologist Jacques Loeb (1859-1924), one of the great experimentalists of the early 20th century. After Loeb's death in 1924, Northrop became head of his own laboratory. He set up his research group at the campus of the Rockefeller Institute, as the University was then known, in Princeton, New Jersey. In the late 1940s the Princeton facility was closed, and in 1948 Northrop moved to the University of California at Berkeley, where he was visiting professor of bacteriology and biophysics while retaining his appointment at Rockefeller. He became emeritus at Rockefeller in 1962, and continued to work in his Berkeley laboratory until 1968. In addition to the Nobel Prize, Northrop's achievements were recognized with the Stevens Prize (Columbia, 1931), the Chandler Medal (1936), the Daniel Giraud Elliot Medal (1939), the President's Certificate of Merit (1948), and the Alexander Hamilton Medal (1961). He received honorary degrees from Harvard, Columbia, Yale, Princeton, Rutgers, and the University of California. Northrop was an elected member of the U.S. National Academy of Sciences (1934), an Honorary Fellow of the Chemical Society (London), and a member of many other scientific societies.

Selected Publications

Northrop JH. Crystalline pepsin. Science, 1929, 69: 580

Northrop JH. Crystalline pepsin. I. Isolation and tests of purity. J Gen Physiol, 1930, 13: 739-766

Northrop JH and Kunitz M. Crystalline trypsin. I. Isolation and tests of purity.  J Gen Physiol, 1932, 16: 267-294

Kunitz M and Northrop JH. The isolation of crystalline trypsinogen and its conversion into crystalline trypsin. Science, 1934, 80: 505-506

Herriott RM and Northrop JH. Isolation of crystalline pepsinogen from swine gastric mucosae and its autocatalytic conversion into pepsin. Science, 1936, 83: 469-470

Northrop JH. Concentration and partial purification of bacteriophage. Science, 1936, 84: 90

Northrop JH. Purification and crystallization of diphtheria antitoxin. J Gen. Physiol, 1942, 25: 465-485

Northrop JH and Goebel WF. Crystalline pneumococcus antibody. J Gen Physiol, 1949, 32: 705-724

Further Reading

Northrop JH. Crystalline Enzymes: The Chemistry of Pepsin, Trypsin, and Bacteriophage. New York: Columbia Univ Press, 1939

Herriott RM. John Howard Northrop, 1891-1987: A Biographical Memoir. Washington, DC: U.S. National Academy of Sciences, 1994, pp. 421-450

Herriott RM. Moses Kunitz, 1887-1978: A Biographical Memoir. Washington, DC: US National Academy of Sciences, 1989, pp. 303-317


The Nobel Prize in Chemistry, 1946