(A father of Biological pH)
(b. Torsaker, Sweden, 30 June 1873; d. Lund, Sweden, 4 December 1952)
The son of Per Erik Thunberg, a merchant, and of Wendela Maria Elisabeth Hård, Thunberg studied medicine at the University of Uppsala from 1891 to 1900 and obtained the M.D. degree with a dissertation on epidermal sensory perception. He was demonstrator for Olof Hammarsten at the Institute of Physiological Chemistry in 1893–1894 and for Frithiof Holmgren at the Institute of Physiology at Uppsala in 1894–1896. He was reader in physiology at Uppsala in 1897–1904. Immediately after the death of Magnus Blix in 1904, Thunberg went to Lund as temporary occupant of the chair of physiology and embryology; the following year he was appointed to the chair of physiology, from which he retired in 1938.
Thunbergs first work, on sensory physiology, was published in 1893 and his last, in 1953. Thunberg extensively studied the physiology of epidermal sensations, showing, among other things, that a pinprick gives rise to two sensations of pain, the second occurring some seconds after the first. This phenomenon was interpreted much later by Zotter-man, who demonstrated the existence of two groups of pain fibers, one of which transmits impulses more slowly than the other. In 1905 Thunberg wrote the chapter “Physiologie der Druck-, Temperatur- und Schmerzempfindungen” for Nagel’s Handhuch der Physiologie des Menschen. With this work he left a field into which he had been led more by chance than by ability.
In 1903, when Thunberg began to study the elementary processes of metabolism–studies that constitute his major contributions to science–nothing was known of the oxidation processes in the tissue cells. Energy is derived in the cells by combustion (cellular respiration), whereby oxygen is consumed and carbon dioxide produced. Lavoisier had proved that respiration is chemically a combustion process in the 1770’s. From then until 1875, the consumption of oxygen and the production of carbon dioxide were thought to be confined to the lungs and the blood. In that year Pflüger proclaimed: “Cells are constantly burning, although we do not see their light with our weak vision.” Everyday experience shows that organic material does not burn in oxygen at body temperature, so it was assumed that oxygen was activated in some way. From about 1840 a series of oxygen activation theories appeared: but valid knowledge had to await the work of Thunberg, Otto Warburg, and Heinrich Wieland. Warburg introduced the term Atmungsferment and argued that this enzyme that catalyzed oxidation was an iron-pyrrole complex. His idea was based on the orthodox concept that the degradation and combustion of food to carbon dioxide and water took place through the direct attack of oxygen on the carbon atoms of the food. Thunberg and Wieland developed an entirely new conception, rivaling that of Warburg.
In 1908 Thunberg began to study the capacity of cells to burn various organic acids, including acetic, propionic, oxalic, malic, succinic, and citric. These acids were not then recognized as normal constituents of the body that played a role in intracellular metabolism; rather, they were known as products of putrefaction or fermentation. From among all the organic acids Thunberg chose precisely those that proved useful to his aims, thus revealing his unerring sense for the right path to follow.
Wieland turned to this area of investigation just when his cooperation was needed to elucidate the nature of the processes of biological oxidation. In 1912 he found that organic compounds can be oxidized through removal of hydrogen in the presence of a catalyst and that the hydrogen can be taken up by suitable acceptors, such as methylene blue. From 1910 Thunberg integrated Wieland’s discoveries with his own, especially those on the biological oxidation of succinate, and initiated the concept of the specific, hydrogen-activating, chain-forming enzyme systems. When Thunberg began this work, all that was known of intracellular enzymes was merely that postmortem autolysis was catalyzed by proteolytic enzymes. Oxidases also had been found.
Thunberg saw that the oxidation of succinate was initiated by an agent in the cells that endowed a hydrogen group in the succinate with a reactivity that it had not previously possessed. The reactivity could easily be demonstrated with methylene blue, which was decolorized by hydrogen uptake. Under the influence of this agent in the cells, the succinate emerged as “hydrogen donator” and released a hydrogen group to the “hydrogen acceptor” methylene blue. The terms “donator” and “acceptor” were introduced by Thunberg and are now in general use. He called the enzymatic agent a dehydrogenase, in this case succinate dehydrogenase. The introduction of the methylene blue method in 1916 opened up a worldwide search for dehydrogenases. The chainlike degradation of the various organic molecules in the organism could now be charted.
Thunberg finally formulated the following generalization concerning the oxidative degradation of food: the degradation is accomplished by a chain of consecutive splittings of hydrogen atoms carried out by a series of dehydrogenases, each with a specific purpose.
With the discovery of a hydrogen-carrying flavo-protein, das gelbe Atmungsferment, Warburg in 1932 contributed substantially to Thunberg’s conception of hydrogen transport from one system to another as a central mechanism in oxidative metabolism. Thunberg rightfully considered himself responsible for a revolution in the concept of the mechanism of biological oxidation.
In 1905, the year of his appointment to the chair of physiology at Lund, Thunberg recorded that nerve tissue respires, taking up oxygen and giving up carbon dioxide. It had previously been thought that nerve fibers conducted impulses like an electric cable, without measurable energy consumption. Thunberg made this discovery with his micro-respirometer, an ingenious device with which he could measure oxygen consumption and carbon dioxide production in small units of tissue. Using this apparatus, he also could demonstrate that traces of certain metals, such as manganese salts, strongly catalyze tissue respiration. This field held his interest throughout his retirement.
Although a scientist first, Thunberg was not unfamiliar with the nonscientific world. As a student at Uppsala he was associated with the radical-liberal group Verdandis, headed by the physiologist Hjalmar Öhrvall. During five decades– through books, popular journals, pamphlets, and the daily press, and as an adviser to the government–he disseminated information on hygiene and medicine that in scope and quality was unparalleled in Sweden.
Thunberg’s enormous capacity for work was combined with good health, and he reached the age of seventy before becoming seriously ill, with pneumonia, from which he recovered. He was fully active for more than nine years, until October 1952, when he stumbled over the doorstep of his house and broke his femur. Despite the best care, he died early in December. He had been elected a member of the Royal Swedish Academy of Sciences in 1928, and later of many foreign learned societies.
Obituaries include Georg Kahlson, “Thorsten Thunberg.” in Acta physiologica scandinavica, 30 (1953), supp. 1ll: and F. G. Young, “Prof. T. Thunberg,” in Nature (12 Dec. 1953).