Michaelis, Leonor

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(b. Berlin, Germany, 16 January 1875: d. New York City, 8 October 1949). biochemistry.

Few twentieth-century scientists have contributed significantly in as many different areas of scientific endeavor as Michaelis. His contributions range from embryology to magnetochemistry and bespeak the extraordinary fertility of his mind. Although a central figure in the development of modern biochemistry, especially in the application of physical-chemical principles to biological problems, Michaelis did not attain a permanent academic post with adequate facilities for his work until he was appointed at the age of fifty-four to the staff of the Rockefeller Institute of Medical Research in New York. Even before the rise of Hitler, anti-Semitism was a fact of German university life, and Michaelis, a man who had already demonstrated his remarkable ability as a teacher and as an investigator, was denied the opportunity to participate fully in the education of future German scientists and physicians.

He was the son of Moriz Michaelis, who ran a small business; his mother’s maiden name was Hulda Rosenbaum. The educational program of the Berlin Koellnisches Gymnasium, from which he graduated in 1893, was directed to the liberal arts but offered to selected students the use of a chemical and physical laboratory for additional work. Michaelis thus acquired proficiency in Latin and Greek, modern languages, literature, and history, along with a desire to become a scientist. The broad scope of his cultural interests, especially his love of linguistics and music, may be traced to his school days. He chose to enter science through the study of medicine at the University of Berlin and received the M. D.degree there in 1897. Among his teachers were Wilhelm von Waldeyer in anatomy, Oskar Hertwig in histology and embryology, Emil du Bois–Reymond in physiology, and Emil Fischer in chemistry. Although Michaelis passed the premedical examination (Physikum) in 1895 with distinction in all subjects, his performance in the examination for a medical license (especially in surgery and gynecology) was only satisfactory; the latter was taken at Freiburg im Breisgau, where he spent the final semester of medical training in 1897.

Michaelis began his scientific research as a medical student and spent his free time in Hertwig’s laboratory. An investigation of the histology of milk secretion won Michaelis a prize awarded by the medical faculty, and his first publication (1896) dealt with the fertilization of the egg of the amphibian Triton taoniatus. For his M. D. thesis he studied the direction of the first cleavage in the frog’s egg and showed that the point at which the sperm fertilizes the egg does not determine the first cleavage furrow. His enthusiasm for communicating new knowledge to students, evident throughout his later life, led him to prepare the small book Kompendium der Entwicklungsgeschichte des Menschen, mit Berücksichtigung der Wirbeltiere, published in 1898. It was well received and went through many editions; the ninth appeared in 1921. By that time his scientific interests were far removed from embryology. Three further editions appeared between 1927 and 1931 under the authorship of Richard Weissenberg.

After completing his medical studies in Freiburg, Michaelis resumed work in Hertwig’s laboratory, where he observed that eosinophilic cells accumulate in the lactic gland of the guinea pig during the colostrum period. Paul Ehrlich happened to learn of this, asked Michaelis to show him the histological preparations, and thereupon offered him an appointment as his private assistant in his Institut für Serumforschung und Serumprüfung in Steglitz, a Berlin suburb. Michaelis’ assignment was to continue Ehrlich’s work on histological staining, which Ehrlich could not pursue because of his immunological research. This obliged Michaelis to delve deeply into the organic chemistry of the dyes that had been developed for the textile industry. He also studied intensively the new physical chemistry and the mathematics underlying its principles, in order to understand better the interaction of dyes with the chemical constituents of tissues. He remained an omnivorous student of these subjects the rest of his life. During his brief association with Ehrlich, Michaelis made the important histological discovery of the specific staining by Janus green of cellular granules later to be called mitochondria; this method of vital staining was widely employed, especially by Edmund Vincent Cowdry. Michaelis also wrote the book Einführung in die Farbstoffchemie für Histologen, published in 1902.

According to Michaelis’ personal account, his agreement with Ehrlich was that after one year in the latter’s laboratory, Michaelis would enter clinical medicine since, in Ehrlich’s view, “only a man of sufficient wealth should stay permanently in fundamental scientific research.”1 Consequently, in 1900 Michaelis joined the staff of Moritz Litten, the physician in chief of the Berlin municipal hospital. During the four years of his clinical service there, Michaelis managed to continue research on histological staining and also began to publish papers on immunology.

In 1903 Michaelis was appointed Privatdozent at the University of Berlin; his Habilitationsschrift dealt with protein precipitins. Among his findings was the observation that brief digestion (with pepsin) of serum albumin markedly altered its immunological properties. Later he entered the arena of controversy between Ehrlich and Svante Arrhenius regarding the application of the mass-action law to immunology. Michaelis’ other immunological contributions included the demonstration that aqueous extracts of normal livers can be used for the Wassermann test and that the complement-fixation test in the Wassermann reaction can be replaced by a direct precipitation test through the use of high-potency extracts of syphilitic fetuses.

Among his other publications from Litten’s medical division was one on iron-containing inclusions in tumors of the urinary bladder. This paper led Ernst von Leyden to invite Michaelis in 1904 to become a research assistant at the newly created section for cancer research in the First Medical Clinic (headed by Leyden) at the Charite Hospital in Berlin. Michaelis’ contributions to cancer research were not striking, but his finding that various strains of mice differ in their susceptibility to transplantation with Jensen’s mouse carcinoma merits mention. For his later work, of greater impact was his introduction in 1904 to the ultramicroscope invented by Henry Siedentopf and Richard Zsigmondy and made commercially available by the Zeiss Optical Works. This instrument figured in the subsequent development of colloid chemistry, a field in which Michaelis became active shortly afterward.

Michaelis married Hedwig Philipsthal in 1905; they had two daughters. The same year he married, Michaelis received the title of ausserordentlicher Professor at the university of Berlin, a post without salary, laboratory, or funds for research. This dubious distinction convinced him that his chances for an established academic post were negligible, and in that year he accepted a newly created position of bacteriologist at the Berlin municipal hospital “am Urban.” Except for the years of World War I, when Michaelis served in various hospitals (tuberculosis, gastrointestinal diseases, infectious diseases), he remained at “am Urban” until 1921, and there made many of his most distinguished contributions to biochemistry. Together with his friend Peter Rona, the chemist at the hospital, he established a small laboratory from which there emerged over the years a series of studies on physical-chemical aspects of biology. The following account of Michaelis’ achievements during the period 1905–1921 is not given in chronological order, but rather in relation to the various fundamental problems that he attacked, often concurrently. These problems included (1) the role of the hydrogen ion concentration in determining the properties of solutions of proteins and enzymes; (2) the mode of enzyme-substrate interaction in catalysis; (3) the adsorption of small molecules by colloidal substances.

Priority for some of Michaelis’ work on hydrogen ion concentration belongs to the Danish chemist Søren P. L. Sørensen, whose memorable 1909 paper2 presented a detailed description of the electrometric method for the determination of the hydrogen-ion concentration, the introduction of the term pH (negative logarithm of the hydrogen ion concentration), the preparation of buffer solutions of fixed pH, the use of dyes for the colorimetric determination of pH, and studies on the effect of pH on the catalytic activity of several enzymes.

According to Michaelis, Sørensen’s paper appeared when his own work on the use of the hydrogen electrode to determine the effect of hydrogen-ion concentration on enzyme activity was nearing completion. Michaelis extended the scope of these studies, notably by showing in 1911 (with Heinrich Davidsohn) that the pH dependence of enzymatic catalysis resembles that of the dissociation of a weak acid. Moreover, he developed the theory of the dissociation of amphoteric electrolytes, such as amino acids and proteins, and gave quantitative formulation to the concept of the isoelectric point, introduced in 1900 by William Bate Hardy. In a series of experimental papers Michaelis demonstrated that the isoelectric point of proteins is not only an index of change in electrical charge but also a minimum or maximum for other properties, such as solubility or viscosity. For the determination of the isoelectric point of proteins, he developed an electrophoretic method in which the pH was kept constant during the flow of the current; data on the isoelectric points of many proteins were obtained and were found to agree with the results on the effect of pH on their solubility properties. In 1914 Michaelis published his influential book Die Wasserstoffionenkonzentration: Ihre Bedeutung für die Biologie und ihre Messung; there was a second edition in 1923 and an English translation (by William A. Perlzweig) in 1926.

The studies by Sørensen and Michaelis on the role of pH in the catalytic activity of enzymes led to important work in Michaelis’ laboratory on the effect of substrate concentration on the initial rates of enzyme action. Such measurements, but without control of pH, had been made earlier by several investigators, notably in 1903 by Victor Henri, who proposed that an intermediate enzyme-substrate complex is formed in the catalytic process. The validity of Henri’s data and his theory were questioned, however, in particular by William Maddock Bayliss3. Michaelis’ work, initially conducted with the help of a Canadian guest, Maud Leonora Menten, began with the enzyme invertase, which catalyzes the hydrolysis of sucrose. Their 1913 paper not only removed many of the objections to Henri’s data but also provided the definition of a constant for the affinity of an enzyme for its substrate. Later authors termed this dissociation constant of the assumed enzyme-substrate complex the Michaelis constant (or Michaelis-Menten constant), and the symbol KM to denote it has become a permanent component of the language of modern biochemistry. It should be noted, however, that this did not come about at once. Until about 1930 many biochemists denied the protein nature of enzymes, and relatively little importance was assigned to Michaelis’ mathematical treatment of enzyme kinetics. The climate of opinion began to change only after the crystallization of pepsin by John Howard Northrop and the appearance of the valuable book by J. B. S. Haldane4.

In addition to studies on the dependence of initial rates on substrate concentration for enzymes other than invertase, Michaelis also extended Henri’s treatment of the inhibition of enzyme action and made a clear distinction between inhibitors that act by competition with the substrate for the free enzyme and those that affect the rate at which the enzymesubstrate complex decomposes to form reaction products and to regenerate the free enzyme. After World War II the mathematical treatment of this aspect of enzymology was greatly refined because of its importance in the study of enzyme mechanisms and in the development of new chemotherapeutic agents.

To the above achievements of the period between 1905 and 1921 may be added studies on the adsorption of ions and small molecules by such surface-active materials as charcoal and cellulose. This work, largely conducted in association with Rona, reflected the then-current preoccupation of biologists with the colloidal state of living matter. In his typical fashion Michaelis wrote a book on the subject, entitled Dynamik der Oberflächen, published in 1909; an English translation (by W. H. Perkins) appeared in 1914. He also wrote an introductory textbook in mathematics for biologists and chemists, published in 1912, with later editions in 1922 and 1927, and Praktikum der physikalischen Chemie, insbesondere der Kolloidchemie für Mediziner und Biologen. The latter appeared in 1921 (subsequent editions were published in 1922, 1926, and 1930), and an English translation (by T. R. Parsons) of the second edition was published in 1925. French and Spanish translations soon followed. Michaelis’ collaboration with Rona included a series of studies (1908) on the estimation of blood sugar. As an outgrowth of their interest in adsorption phenomena, methods were devised for the removal of serum proteins by means of kaolin or ferric hydroxide. In addition, Michaelis published experimental papers on bacterial agglutination and pH indicators, as well as on various clinical topics.

In 1921 the post-Wilhelmine government gave Michaelis the title of Professor extraordinarius for physical chemistry applied to medicine and biology at the University of Berlin, but without salary or facilities for research. By that time he had acquired worldwide recognition, and his modest laboratory at the “am Urban” hospital had attracted many young German physicians and postdoctoral guests from abroad. The city administration, however, had not changed its policy of discouraging basic research in the Berlin, municipal hospitals. Michaelis therefore accepted an appointment with the Berlin firm Vereinigte Fabriken für Laboratoriumsbedarf, where he was given a research laboratory and served as a consultant in the manufacture of scientific apparatus. This association was brief, for in 1922 he was invited to become a visiting full professor at the Aichi Prefectural Medical School (in Nagoya, Japan), whose status had just been raised to that of a university; it later became part of the University of Nagoya.

Michaelis remained in Japan until 1925. He later wrote enthusiastically about his experience there, for he was given a good laboratory and had many research students, with whom he published numerous papers. He continued work on many of the problems studied in his Berlin laboratory, in particular adsorption phenomena and electrochemistry, and also embarked on a sustained investigation of semipermeable membranes. Michaelis also learned to speak Japanese and studied Chinese.

Among Michaelis’ American admirers was Jacques Loeb, whose remarkable career as an experimental biologist was nearing its close with his studies on the physical chemistry of proteins at the Rockefeller Institute for Medical Research.5 In 1923 Loeb wrote to Michaelis in Japan, inviting him to come to the United States for a lecture tour, which Michaelis undertook in May and June 1924. The tour was carefully arranged by Loeb and proceeded according to schedule, but Michaelis’ pleasure at this recognition was dimmed by his distress at Loeb’s sudden death that February. The lectures were collected in his book The Effects of lons in Colloidal Systems, published in 1925, and dealt with Michaelis’ studies on the adsorption of ions, electrical double layers, and other aspects of biophysical chemistry. One of the stops on the lecture tour was the Johns Hopkins School of Medicine, and on that occasion Michaelis was invited to come there as a resident lecturer for three years, after the expiration of his appointment in Japan. He accepted and began work in Baltimore in the spring of 1926.

After settling in the United States, Michaelis continued his research on semipermeable membranes, but by 1928 he had begun to shift his attention to the electrometric study of oxidation-reduction processes. His first efforts in this field (with Louis B. Flexner and E. S. G. Barron) were not notably successful because they dealt with the cysteinecystine system, which responds sluggishly at metallic electrodes. Also, as was his wont, he wrote a book, Oxidations-Reduktions-Potentiale, mit besondere Berücksichtigung ihrer physiologischen Bedeutung (Berlin, 1929); an English translation (by Louis B. Flexner) appeared a year later, and a second German edition was published in 1933. In connection with this shift in Michaelis’ scientific interests, it is noteworthy that in 1927 William Mansfield Clark was appointed professor of physiological chemistry at the Johns Hopkins School of Medicine. Beginning in 1920, Clark had conducted studies on the oxidation-reduction potentials of organic dyes (for instance, methylene blue), and he continued to make decisive contributions to this field in Baltimore.6

Michaelis finally attained his hope of securing a permanent position in 1929, when he was appointed a member of the Rockefeller Institute for Medical Research. He remained there until his death, and his research was not interrupted by transfer to emeritus status in 1941. Shortly after moving to New York, Michaelis made his most significant contribution to the study of the reversible oxidation-reduction of organic substances. At that time, largely owing to the work of Clark, it was accepted that in the reduction of the oxidized form of an organic dye (for instance, methylene blue) to its reduced form (leucomethylene blue), two electrons are transferred from the reductant, and it was considered that this electron transfer occurs without intermediate steps. In 1931, however, Michaelis (with Ernst Friedheim) reported that in the electrometric oxidation-reduction titration of the natural pigment pyocyanine, the titration curve in alkaline solution was consistent with simultaneous two-electron transfer, but at acidic pH values there was a stepwise one electron transfer, with the intermediate formation of a free radical that Michaelis termed a “semi-quinone.” It should be noted that this discovery was made independently at about the same time by Bene Elema when he was a graduate student at Delft.

At first Michaelis’ interpretation of his data was greeted with skepticism, but he soon produced massive evidence in support of his conclusions. Apart from potentiometric studies showing that the principle of stepwise univalent oxidation-reduction applied to other organic substances, he also used the methods of magnetochemistry to demonstrate that the semiquinones are paramagnetic, owing to the presence of an unpaired electron. In the latter work he enlisted the aid of experimental physicists but also contributed to the methodology by modifying the magnetic balance for the purpose of his studies. Moreover, Michaelis proceeded to acquaint himself thoroughly with the quantum theory underlying the principles of magnetochemistry. His enthusiasm as a teacher was again evident in the series of lectures he gave on quantum mechanics at the Rockefeller Institute during the 1930’s. To this should be added the words of Clark: “As one whose work occasionally overlapped that of Dr. Michaelis, I wish to say that he taught me much and that true to the instincts of a good teacher he generously ignored the fact that I had had an opportunity to find what he later discovered.”7

Other problems were also investigated in Michaelis’ laboratory at the Rockefeller Institute. The stability of semiquinones in acid solutions permitted an extension of the pH scale for aqueous solutions up to 11 molar sulfuric acid; this complemented the “acidity scale” developed by Louis Plack Hammett. Also, the dimerization of organic radicals derived from dyes led Michaelis to study the relation of this process to metachromatic effects in histological staining. In addition, there was a series of studies on cysteine, especially the mechanism of its iron-catalyzed oxidation and its reaction with iodoacetic acid. Related to this work was the finding, in association with David Goddard, that the reduction of the cystine-rich hair keratin by means of thioglycolate gives a soluble form of the protein, digestible by proteolytic enzymes; this observation was later adopted in the cosmetic industry to produce a “cold” permanent wave.

Another area of investigation, largely developed by Michaelis’ associate Sam Granick, was the study of the iron-containing protein ferritin. In addition to Granick, Michaelis’ chief research associates at the Rockefeller Institute were Maxwell Schubert, Carl Smythe, and Edgar Smith Hill. Among the numerous guest investigators, apart from Goddard, were James Baumberger, Jannik Bjerrum, John Runnström, Kurt Salomon, Gerold Schwarzenbach, and Kurt G. Stern.

Michaelis was a short man of stocky stature and unprepossessing appearance. Although kindly toward younger scientists, he was not always tactful with senior colleagues. In times of relaxation, as at the Woods Hole Marine Biological Laboratory, where he spent many summers, he was a lively conversationalist. A talented pianist, he delighted in entertaining his friends with musical improvisations. He did not seek honors and received few: his election to the National Academy of Sciences came in 1943, and he was awarded an honorary degree by the University of California at Los Angeles in 1945.


1. L. Michaelis, Biographical Memoirs, National Academy of Sciences, 31 (1958), 284.

2. S. P. L. Sørensen, “Enzymstudien II. Über die Messung und die Bedeutung der Wasserstoffionenkonzentration bei enzymatischen Prozessen,” in Biochemische Zeitschrift, 21 (1908), 131–200.

3. W. M. Bayliss, The Nature of Enzyme Action (London, 1908).

4. J. B. S. Haldane, Enzymes (London, 1930); see also T. R. C. Boyde, Foundation Stones of Biochemistry (Hong Kong, 1980).

5. Jacques Loeb, Proteins and the Theory of Colloidal Behavior (New York, 1922).

6. William Mansfield Clark, Oxidation-Reduction Potentials of Organic Systems (Baltimore 1960); “Notes on a Half-Century of Research, Teaching and Administration”, in Annual Review of Biochemistry, 31 (1962), 1-24.

7. William Mansfield Clark, “Leonor Michaelis 1875-1949”, in Science,III (1950), 55.


I. Original Works. A list of Michaeles’ publications, appended to his autobiographical sketch in Biographical Memoirs. National Academy of Science, 31 (1958), 282–321, includes 484 items. Regrettably, the list is riddled with errors. The books he wrote are listed in the text. Among his more important articles in scientific journals are the following: “Die vitale Färbung, eine Darstellungsmthode der Zellgranula”, in Archiv für mikroskopische Anatomie und Entwicklungsgeschichte, 55 (1900), 558-575; “Über die Adsorption der Neutralsälze”, in Zeitschrift für Elektrochemie,17 (1911), 1-5, 917-919, with H. Lachs; “Die Wirkung der Wasserstoffionen aufdas Invertin”, in Biochemische Zeitschrift, 35 (1911), 386-412, with Heinrich Davidsohn; “Die Kinetik der Invertinwirkung”, ibid., 49 (1913), 333-369, with M. L. Menten; “Über die verschiedenartige Natur der Hemmungen der Invertasewirkung”, ibid., 60 (1914), 79-90, with H. Pechstein; “Potentiometric Study of Pyocyanine”, in Journal of Biological Chemistry, 91 (1931), 355-368, with E. Friedheim; “A Study on Keratin”, ibid., 106 (1934), 605-614, with David R. Goddard; “The Paramagnetism of the Semiquinone of Phenanthrenequinone-3-sulfonate”, in Journal of the American Chemical Society,60 (1938), 202-204, with G. F. Boeker and R. K. Reber; “Ferritin and Apoferritin”, in Science, 95 (1942), 439-440, with Sam Granick; and “The Semiquinone Radical of Tocopherol”, ibid., 109 (1949), 313-314, with S. A. Wollman. Michaelis contributed to the history of science in “Zur Erinnerung an Paul Ehrlich; Seine wiedergefundene Doktor-Dissertation”, in Die Naturwissenschaften, 7 (1919), 165-168.

II. Secondary Literature. There is no adequate biography of Michaelis, and most of the obituary notices on him are based on his autobiographical sketch (with brief addendum by D. A. Maclnnes and Sam Granick) in Biographical Memoirs. National Academy of Sciences (see above). Among them are those by E. S. G. Barron in Biological Bulletin, 101 (1951), 13-16; William Mansfield Clark, in Science, 111 (1950), 55; and Sam Granick, in Nature, 165 (1950), 299-300. See also George W. Corner, A History of the Rockefeller Institute, 1901-1953 (New York, 1964), 178-181.

Joseph S. Fruton

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