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Theorell, Axel Hugo Theodor


(b. Linköping, Sweden, 6 July 1903, d. Stockholm, Sweden, 15 August 1982), chemistry, biochemistry, structural chemistry, physical chemistry.

Theorell was awarded the Nobel Prize for Physiology or Medicine in 1955 for his discoveries concerning the nature and mode of action of oxidation enzymes. Enzymes are the workhorses of the never-ending process of chemical transformations that characterizes the world of living organisms. In this maelstrom of chemical reactions, the substrates are supplied in an activated form that makes them ready for further processing. A crucial role in their activation is played by so-called coenzymes and prosthetic groups. Theorell made major discoveries regarding the molecular structure of enzymes, coenzymes acting as activation groups and substrates, and the mechanisms of the interactions between them. His elucidation of spatial structures of molecules greatly contributed to closing the gap between biochemistry and subcellular morphology, which had persisted until the 1960s.

Family Life and Professional Career. Axel Hugo Theodor Theorell was born on 6 July 1903, the son of Thure Theorell, surgeon-major to the First Life Grenadiers, who practiced medicine in Linköping, and his wife Armida Bill. Hugo Theorell, often known as Theo, had two sisters. At the age of three, he suffered a severe poliomyelitis infection, which left his left leg permanently paralyzed. He later underwent a muscle transplant, which allowed him to walk with relative ease. Perhaps it was because of his disability that he developed strong arms and shoulders, giving him a muscular appearance.

Theo was educated at a state-run secondary school in Linköping for nine years, passing his matriculation exam there on 23 May 1921. In September 1921 he started studying medicine at the Karolinska Institute in Stockholm, where he graduated with a bachelor of medicine in 1924. In 1930 he obtained his MD degree, having written a thesis on the lipids of blood plasma, and was appointed lecturer in physiological chemistry at the Karolinska Institute. Theorell started to work under Einar Hammarsten, who at the time was only thirty-five. One year after obtaining his doctorate, Theo moved to the institute led by The Svedberg at Uppsala University in Sweden, where he was appointed associate professor of

medical and physiological chemistry in 1932. In 1933 a Rockefeller Foundation fellowship enabled him to go to Berlin-Dahlem to work with the biochemist and cell physiologist Otto Warburg.

It was through his interest in music that he met his future wife, harpsichordist Elin Margit Elisabeth Alenius, whom he married on 5 June 1931. They had a daughter Eva Kristina, who died of tuberculosis in 1935, shortly after the family had returned to Sweden from Berlin-Dahlem, and later had three sons. His great interest in music is apparent from the fact that he became a member of the Swedish Royal Academy of Music, as well as chairman of the Stockholm Symphony Society.

Upon his return to Sweden, Theorell once again took up a post at the Karolinska Institute, and in 1936 he was appointed head of the newly established Biochemical Department of the Nobel Medical Institute. This institute, which was officially opened in 1937, was located at the Karolinska Institute, which also took care of its administrative affairs. It was not until 1947 that the Nobel Medical Institute got its own building on the outskirts of Stockholm. The institute’s staff had no teaching duties, which suited Theorell, as he was not particularly interested in teaching. He was, however, a gifted speaker with a good command of several languages, and he was an inspired raconteur of humorous anecdotes. He continued to head the Biochemical Department until 1970.

During World War II, Sweden remained neutral, but became relatively isolated, and Theorell’s laboratory had very few guest researchers from abroad. Immediately after the war, however, a stream of prominent foreign biochemists came to work there for shorter or longer periods. Those among the postdoctoral students and guest researchers who worked under Theorell’s supervision were usually encouraged by him to publish their results without including Theorell as a coauthor. It was Hugo Theorell’s combination of creative imagination, critical accuracy, and technical skills that made him such an outstanding researcher and research supervisor.

The “Yellow Enzyme.”. In the 1930s, biochemists were taking an avid interest in oxidation and reduction reactions in living organisms. Warburg had studied the process of respiration in cells and the role of a catalytic iron compound in this process in the 1920s. He had called this enzyme Atmungsferment and had studied the effects of carbon monoxide and light on the respiratory process, research for which he was awarded the Nobel Prize for Physiology or Medicine in 1931, and more laboratories started to investigate biological oxidation-reduction reactions. The latter support the transduction of energy and the transformation of metabolites. In general, the enzymes which catalyze biological oxidation-reduction are organized in systems of enzymes, which function as a terminal respiratory chain, that is, a group of enzymes that transfers reducing equivalents to O2.

In the respiratory chain, oxidation takes place through a series of cytochromes, which receive and process the activated hydrogen atoms from the substrate. What makes the respiratory chain such an excellent research model is that the redox status of the cytochromes can be followed spectroscopically. At Cambridge, David Keilin was doing outstanding research in this field. Within the cytochrome system, it is molecules such as pyridine nucleotides and flavoproteins that are involved in the oxidation reactions, that is, in the transfer of hydrogen atoms. Many specific steps in the redox reactions could be followed experimentally as the oxidized state had different physico-chemical properties than the reduced state. Warburg’s laboratory was to become world-famous for this type of cell physiology research; its status is reflected by the number of Nobel Prizes won by Warburg’s pupils: Otto F. Meyerhof in 1922, Hans A. Krebs in 1953, and Hugo Theorell in 1955.

Warburg found a so-called yellow enzyme, whose color faded upon reduction and returned upon oxidation. In view of this enzyme’s remarkable properties, several prominent researchers started studying it, including its discoverer Warburg at Berlin, Richard Kuhn at Heidelberg, and Paul Karrer at Zürich. It turned out that a prosthetic group—a yellow pigment—acted as the hydrogen carrier. The pigment was isolated and was referred to as lactoflavin, riboflavin, or vitamin B2. The phenomenon of the reversible redox reactions occurred only when it was bound to the high molecular weight protein part of the enzyme.

The receipt of a Rockefeller Foundation grant in 1933 allowed Theorell to work at Warburg’s laboratory in Berlin-Dahlem for eighteen months. He spent this time researching the yellow enzyme’s redox reactions in more detail. As part of his dissertation research, he had developed an electrophoresis machine to separate various lipid components, some time before Arne Tiselius developed a similar machine. When he left for Berlin, he took two of his electrophoresis machines with him, one for analytical and one for preparative uses, and used them to study and isolate the yellow enzyme.

Using electrophoresis at pH 4.2–4.5, Theorell was able to remove polysaccharide contaminants from Warburg’s yellow enzyme preparation. Studies with Svedberg’s centrifuge at Uppsala subsequently revealed that the purified product was a protein with a molecular weight of 75,000. This was the first time that anyone had been able to separate an active enzyme into components—a protein and a prosthetic group—that were pure. Individually they were inactive, but when combined, the full enzymatic activity was produced. The protein and the prosthetic group—or coenzyme—were obtained in stoichiometric proportions. In Theorell’s electrophoresis machine, the flavin moved to the anode, making it unlikely that it was only neutral flavin. He was later able to show that it was actually lactoflavin phosphoric acid ester, in which the pigment was bound to the protein via phosphoric acid. The coenzyme was eventually named FMN (flavin mononucleotide).

Theorell’s chemical and physical examinations of the yellow enzyme put him at the forefront of research into vitamins, coenzymes, and prosthetic groups. This was a very fruitful research area, as was evident from the Nobel Prizes for Chemistry that were awarded to Walter N. Haworth and Paul Karrer in 1937 and to Richard Kuhn in 1938 for their work on vitamins, carotenes, and flavins. These awards were, however, mostly based on studies of the chemical structure of the compounds involved, rather than the interaction between protein and coenzyme. In the years after his return to Sweden, Theorell focused on the specific chemical interactions between heme as a coenzyme and proteins, as in cytochrome c. Painstaking research revealed that the heme component was bound to the protein by two cysteine-S bridges and that the iron atom in the heme was bound to a histidine in the protein.

The fact that Theorell received the Nobel Prize for Physiology or Medicine in 1955, twenty years after his pioneering work on the yellow enzyme at Warburg’s laboratory, allows two conclusions. The first is that this concerned a different type of work from the straightforward chemical characterization of biologically active compounds by Haworth, Karrer, and Kuhn: Theorell’s work was concerned with physiology, the chemistry of dynamic processes. The second is that Theorell’s use of new physico-chemical techniques—in combination with comparative studies—meant a revolutionary new experimental approach. In Warburg’s letter of recommendation to the Nobel Committee, the German cell physiologist wrote that the separating and reuniting of protein and coenzyme continued to provide the foundation for the study of enzyme action: “immer sind es die Experimente Theorells, auf denen das Gebäude man kann fast sagen der modernen Biochemie ruht” (Werner, 1991, p. 266).

One year before Theorell was awarded his Nobel Prize, Linus Pauling had delivered his 1954 Nobel Lecture. In this lecture, he presented newly developed three-dimensional chemical models, in which the nature of the bonds, possible hydrogen bridges, and steric hindrance determined the hypothetical molecular structures. Pauling had made a gift of these models to Theorell, and they featured for the second time in a Nobel lecture when Theorell accepted his prize in 1955. The meccano-type models allowed Theorell to discard a number of steric molecular configurations for cytochrome c as improbable on the basis of the distances between the atoms and the valence angles. He ended up with twenty alternatives, one of which had the highest probability. It was this option that he displayed at the Nobel ceremony. The model featured an alpha helix, which Pauling had proved to be a stable protein configuration. The presence of alpha helices was confirmed by x-ray crystallography but instead of being left-handed as suggested by Theorell in his Nobel Lecture, it was later shown to be right-handed (Dalziel, 1983, p. 596).

From Chemistry to Physiology. The strength of the bonds between the prosthetic group and the protein differs between enzymes. Cytochrome c is an example of an enzyme with an immobile prosthetic group that remains bound even if the protein is denatured. The yellow enzyme is a conjugated protein with a coenzyme that can be dissociated but is then nonfunctional, whereas liver alcohol dehydrogenase has a coenzyme that remains functional after dissociation. These differences offer opportunities for comparative studies into the mode of action of enzymes and the involvement of coenzymes. The key concept in Theorell’s views was that the wide variety of properties of, for instance, various iron-containing enzymes had to be caused by the way in which the iron is bound to the protein.

As a result, Theorell’s research program was based on studying enzymes within a particular category, and involved isolating, purifying, and characterizing as many different enzymes from such a category as possible, after which comparative studies provided insights into their physiology. He focused mainly on enzymes having iron porphyrins as coenzymes, a category that includes a wide range of enzymes, such as hemoglobin, cytochromes, catalases, and peroxidases. After cytochrome c had been discovered by Keilin, Theorell was the one to purify and characterize it as a heme protein with a molecular weight of 13,000. The enzymes were isolated using standard procedures from biochemistry, such as fractionated precipitation with ammonium sulfate, further differentiation using a range of pH values, and denaturation of the byproducts. Of course, Theorell also applied methods from physical chemistry, such as electrophoresis, and he used ultracentrifugation to check the purity of the enzymes and determine their molecular weights. The fact that Svedberg and Tiselius—the world’s most renowned experts on ultracentrifugation—were working at Uppsala was obviously very helpful.

The comparative studies showed in how many different ways enzymes can use coenzymes. Inspired by his own love of music, Theorell used a musical metaphor for the relation between enzyme and coenzyme. In his words, protein entities control the prosthetic groups “etwa wie ein Virtuose sein Instrument beherrscht” (Theorell, 1944, p. 96). While the enzyme is able to play the same melody on different instruments, the iron in the coenzyme is limited by the expressive opportunities offered by the instrument. All this, Theorell stated, highlights the delicate differentiation of enzymes and proteins in biology.

This field obviously yielded a wide-ranging research program, which used rather laborious methods, so it is not surprising that Theorell’s laboratory employed about twenty permanent staff, including technicians and mechanics. The research also required great experimental dexterity, and in this respect Theorell was able to rely on the valuable assistance of Åke Åkeson; it is not unusual to find that behind a great chemical researcher there is a great technician. The postwar period saw the arrival of a whole series of guest researchers, who were made very welcome, notwithstanding the somewhat primitive working conditions at the Karolinska Institute because of deprivation during the war and overcrowding of laboratory space. The researchers visiting Theorell’s lab shortly after the war included top names such as Britton Chance, Ralph Holman, Christian Anfinsen (winner of the 1972 Nobel Prize for Chemistry), and J. M. Buchanan from the United States; Christian de Duve (winner of the 1974 Nobel Prize for Medicine) from Belgium; Andreas Maehley from Switzerland; and Elèmer Mihalyi from Hungary.

In 1947 the Nobel Medical Institute moved into new premises at Solnavägen on the outskirts of Stockholm. The new laboratory occupied three floors; its equipment has been cofinanced by the Rockefeller Foundation and the U.S. National Institutes of Health. Four years later, a further wing was added: the Wallenberg Foundation Laboratory for Physiological Chemistry, as well as a wing with guest accommodations. From the second half of the 1940s, many new instruments were acquired, including reliable photoelectric quartz spectrophotometers (such as the Beckman DU instrument), which removed the need to use complex methods based on Warburg manometers. In 1956 a new technique to purify proteins, ion-exchange chromatography, became available.

The Fourth Dimension of Biochemistry. Successful biochemical research is determined to a considerable extent by a felicitous choice of experimental model. After the war, Theorell’s staff took up the subject of alcohol dehydrogenase, or ADH, while continuing their work on heme-based enzymes. The enzyme ADH had first been crystallized from yeast in 1937. In 1948 Theorell succeeded in crystallizing it also from horse liver, and subsequently by x-ray diffraction studies. The enzyme has a specific coenzyme, NAD+, which is reduced to NADH. These reduction-oxidation processes are accompanied by many changes in light absorption and fluorescence, allowing them to be followed and analyzed with the right instruments.

Theorell and his staff started elaborate studies on the enzyme’s kinetics, equilibriums, reaction mechanisms with various substrates and inhibitors, and so on. They found that the formation of the complex of enzyme and coenzyme and the ensuing reactions led to changes in the conformation of the enzymatic protein. These phenomena attracted great interest among biochemical researchers in the 1950s and 1960s. Various names were coined for them, such as “conformational adaptability,” “rack mechanism,” “induced fit,” and finally “allosteric effects,” the name that stuck. From his work on ADH, Theorell drew the important conclusion that biological macromolecules possessed an extra dimension. Proteins are not just the three-dimensional static structures that are found by x-ray diffraction studies; a macromolecule’s conformation also depends on its environment and can change in time.

Theorell concluded that active enzymes “have, as ‘a fourth dimension,’ their conformational adaptability” (Theorell, 1967, p. 40).

The results of Theorell’s studies of ADH kinetics were later applied in quantitative assessment of blood alcohol levels. The ADH method proved more sensitive than previous techniques, and was also highly specific for ethanol, so there was no interference from other alcohols. Hence, forensic chemists in Sweden and Germany adopted an ADH-based method.

Executive Positions. In 1967 Hugo Theorell was elected president of the Swedish Academy of Science. In addition, he was president of the International Union of Biochemistry from 1967 to 1973. An important achievement was the building of the Wenner-Gren Center in Stockholm. Axel Wenner-Gren, owner of companies such as Electrolux, had already founded the Wenner-Gren Foundation for the Support of Scientific Research in 1937. In 1954 Swedish scientists approached Wenner-Gren with the proposal to establish a center for scientific conferences. The center was opened in 1961, and Theorell was president of its board until 1976. A Wenner-Gren symposium was held there in Theorell’s honor in 1970, on the occasion of his retirement.

In 1971, while Theorell was chairman of the board of the Wenner-Gren Foundation, a case of fraud at the foundation came to light. Several members of the foundation, including Theorell, were indicted for fraudulent administration but only the executive director was found guilty (Theorell’s case was dismissed because of his ill health). This affair came as a great shock to Theorell, as his complete confidence in his staff had been shaken. In 1981 he suffered a severe case of bronchitis, which hospitalized him for weeks. He died in the summer of 1982.


A complete bibliography of Hugo Theorell’s publications is included in Dalziel’s biography.


“Über die Wirkungsgruppe des Gelben Ferments.” Biochemische Zeitschrift 275 (1934): 37–38. Translated and reprinted in Herman M. Kalckar, Biological Phosphorylations: Development of Concepts. Englewood Cliffs, NJ: Prentice-Hall, 1969.

“Ein neuer Kataphoreseapparat für Untersuchungszwecke.” Biochemische Zeitschrift 275 (1935): 2–10.

“Das gelbe Oxydationsferment.” Biochemische Zeitschrift 278 (1935): 263–290.

“Konstitution und Wirkung einiger Häminproteide.” In Zur Chemie, Physiologie und Pathologie des Eiweisses: Eine Vortragsreihe veranstaltet von der Berner Biochemischen Vereinigung, 1943, edited by R. Signer et al. Bern: Verlag Paul Haupt, 1944.

“Nobel Lecture, December 12, 1955: The Nature and Mode of Action of Oxidation Enzymes.” Nobel Lectures Including Presentation Speeches and Laureates’ Biographies: Physiology or Medicine, 1942–1962. Amsterdam: Elsevier Publishing, 1964.

“Function and Structure of Liver Alcohol Dehydrogenase.” Harvey Lectures 1965–1966 61 (1967): 17–41. An honorary lecture delivered at the end of Theorell’s career, reflecting his lifelong interest in the chemistry and structural chemistry of enzymes.

“My Life with Proteins and Prosthetic Groups.” In Proteolysis and Physiological Regulation: Miami Winter Symposia Volume 11, edited by Douglas W. Ribbons and Keith Brew. New York: Academic Press, 1976.

Växlande Vindar. Stockholm: Natur och Kultur, 1977. Biographical memoir.


Beinert, Helmut, and Peter Hemmerich. “Flavins and Flavoproteins.” In The Encyclopedia of Biochemistry, edited by Roger J. Williams and Edwin M. Lansford Jr. New York: Reinhold, 1967. A concise and accurate overview of the chemistry and biological significance of flavins (including their role as coenzymes).

Buchanan, John Machlin. “A Backward Glance.” In Selected Topics in the History of Biochemistry: Personal Recollections II, Comprehensive Biochemistry. Vol. 36, edited by Giorgio Semenza. Amsterdam: Elsevier Science Publishers, 1986.

Dalziel, K. “Axel Hugo Theodor Theorell, 6 July 1903–15 August 1982.” Biographical Memoirs of Fellows of the Royal Society 29 (1983): 583–621.

Szent-Györgyi, Albert, et al. “Papers Dedicated to Hugo Theorell on His 60th Birthday, 6 July 1963, Copenhagen, 1963.” Acta Chemica Scandinavica 17, supp. 1 (1963): 1–352.

Werner, Petra, ed. Ein Genie irrt seltenerOtto Heinrich Warburg: Ein Lebensbild in Dokumenten. Berlin: Akademie Verlag, 1991. Includes correspondence between Hugo Theorell, Otto Warburg, and others.

Ton Van Helvoort

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Axel Hugo Theodor Theorell

Axel Hugo Theodor Theorell

The Swedish biochemist Axel Hugo Theodor Theorell (1903-1982) was awarded the Nobel Prize in Physiology or Medicine for his discoveries concerning the nature and mode of action of oxidation enzymes.

Hugo Theorell, the son of Ture Theorell, a medical practitioner, was born at Linköping on July 6, 1903. From 1921 he studied medicine at the Karolinska Institute at Stockholm and graduated as a bachelor of medicine in 1924. He soon became an assistant in the Institute of Medical Chemistry at Stockholm, and there he was temporary associate professor in 1928-1929. In 1930 he graduated as a doctor of medicine and was appointed lecturer in physiological chemistry at the Karolinska Institute.

In 1931, while working in Svedberg's Institute of Physical Chemistry in Uppsala, Theorell was the first to obtain crystalline myoglobin, and he determined its chemical and physical properties. In 1932 he became associate professor of medical and physiological chemistry in the University of Uppsala; but from 1933 to 1935 he worked, as a Rockefeller Fellow, in the laboratory of Otto Warburg at Berlin-Dahlem. There he began his work on oxidation enzymes.

Warburg's "Yellow Enzyme"

In 1932 Warburg and his coworker Walther Christian discovered in yeast a bright yellow enzyme. Warburg showed that it consisted of a yellow pigment and a carrier substance. When Theorell began working at Berlin, it was known that the pigment part was a flavin, called lactoflavin. It was later named riboflavin and was identified with vitamin B2. When separated from the carrier substance, which was itself inactive, the pigment part lost its enzymatic activity. At Berlin, Theorell, using his own electrophoretic methods, had by 1934 purified and crystallized the enzyme and had separated the pigment from the colorless protein carrier. He also showed that the pigment part—the coenzyme or prosthetic group—was a protein. He determined its constitution—a lactoflavin phosphoric ester—and named it flavin monouncleotide (FMN).

After his return to Sweden, Theorell was appointed in 1937 director of the new Biochemical Department of the Nobel Medical Institute in Stockholm. The department was transferred to a new Biochemical Institute in 1947.

Cytochrome c

Cytochrome was rediscovered independently in 1925 by David Keilin, who soon isolated cytochromes a, b, and c and showed that cytochrome c was fundamental in cell respiration. In 1938 Theorell showed that the heme nucleus of cytochrome c was linked to the protein in two different ways. This was the first occasion on which the nature of such linkages had been demonstrated in any enzyme.

In 1941 Theorell, working with Å . Åkesson, studied cytochrome cextensively, especially the mode of linkage of the iron to the protein and its stereochemical structure. In 1943 Theorell suggested that there were two steps in its reduction. In 1955 Theorell and Anders Ehrenberg showed that the core of the cytochrome c molecule consisted of an iron atom at the center, within a porphyrin disk bearing histidine side chains. The core was surrounded by helical peptide chains. Theorell thought the structure was designed to protect the iron from oxidizing agents—his theory of the "embedded heme." He was able to construct a model of the core.

Peroxidases and Alcohol Dehydrogenases

In 1941 Theorell and his coworkers crystallized for the first time a peroxidase, found in horseradish, and in 1943 they isolated the lactoperoxidase in milk.

The alcohol dehydrogenases, oxidative enzymes consisting of a protein linked to diphosphopyridine nucleotide (DPN), are found especially in liver and in yeast. In 1948 the enzyme from horse liver was first crystallized in Theorell's institute, and in 1950 the velocity constant for the very rapid action of the liver dehydrogenase was determined. In succeeding years, Theorell and his coworkers elucidated the complicated action of the alcohol dehydrogenases. They found that the liver enzyme oxidized alcohol to aldehyde, whereas the yeast enzyme reduced aldehyde to alcohol. The later work of Theorell and his coworkers on these complex enzyme systems led to their development of a practically specific test for ethyl alcohol, used officially in medicolegal work.

For his discoveries on oxidation enzymes Theorell was awarded the Nobel Prize in 1955. In 1957 he was elected a Foreign Associate of the National Academy of Sciences in Washington and in 1959 a Foreign Member of the Royal Society. From 1967 to 1973, he was president of the International Union of Biochemists and in 1967-1968 president of the Swedish Academy of Sciences. A member of many foreign learned societies, he held honorary doctorates from seven universities. In 1960, he received the 150-Year Jubilee Medal of the Karolinska Institute and in 1965 the Paul Karrer Medal. In 1971, he received the Ciba Medal and the Semmelweiss Medal.

Interested in music all his life, Theorell was a member of the Swedish Royal Academy of Music and chairman of the Stockholm Symphony Society. His wife, Margit, was a professional pianist, and Theorell was a very accomplished violinist. Together, they were at the forefront of the musical life of Stockholm.

Theorell retired as director of the Nobel Medical Institute in 1970, after 33 years of service, although he continued to do research. Much of his work was done with long-time companion A. Akeson (1914-1988). Theorell died in Stockholm on Aug. 15, 1982.

Further Reading

There is a biography of Theorell in Nobel Lectures, Physiology or Medicine, 1942-1962 (1964), which also includes his Nobel Lecture. For references to his work see C. W. Carter, R. V. Coxon, D. S. Parsons, and R. H. S. Thompson, Biochemistry in Relation to Medicine (1959); M. Dixon and E. C. Webb, Enzymes (2d ed. 1964); and Dorothy M. Needham, Machina Carnis (1971). □

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