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Deuterium as an Indicator in the Study of Intermediary
Metabolism. I.
(Schoenheimer, R., and Rittenberg, D. (1935) J. Biol.
Chem. 111, 163-168)
Deuterium as an Indicator in the Study of Intermediary
Metabolism. XI. Further Studies on the Biological Uptake of Deuterium into Organic Substances, with Special Reference to Fat and
Cholesterol Formation
(Rittenberg, D., and Schoenheimer, R. (1937) J. Biol.
Chem. 121, 235-253)
Rudolf Schoenheimer (1898-1941) pioneered the use of isotopes
for the study of metabolism and transformed biochemistry. He was born
and educated in Berlin, receiving his M.D. degree in 1922. He worked
with Karl Thomas in Leipzig for three years to extend his education and
experiences with synthetic chemistry of biological molecules. In 1926, he moved to the University of Freiburg to work with Ludwig Aschoff to
study the role of dietary cholesterol in the development of
atherosclerosis in rabbits. This experience shaped his research
interests for the rest of his career. By 1933, the racial policies of
the Nazi government in Germany forced Schoenheimer, and many other
European Jews, to leave Europe. Hans T. Clarke, Chairman of the
Department of Biological Chemistry at Columbia University, provided
Schoenheimer a faculty position with salary and research support from
the Josiah Macy Foundation. It is remarkable that, in addition to
Schoenheimer and Karl Meyer, the author of a previous Journal of
Biological Chemistry (JBC) Classic (1), many other European
biochemists including Erwin Chargaff, Zacharias Dische, Heinrich
Waelsch, and Erwin Brand also benefited from positions and research
support provided by Clarke. He not only offered sanctuary to these
talented refugees but also created one of the premier biochemistry
departments in the country (2, 3). As Eugene Kennedy rightly reminded us (3) in describing Clarke's account of the recruitment of these
scientists to Columbia, "Clarke modestly omitted to mention that his
own vision and humane instincts in welcoming these gifted refugees were
by no means to be found in every American academic institution."
In 1932, the year before Schoenheimer's arrival at Columbia, Harold
Urey, who was working in the chemistry department at Columbia, discovered deuterium, a heavy isotope of hydrogen. Urey was awarded the
1934 Nobel Prize in Chemistry for this notable achievement. The
successful preparation of D2O, "heavy water," began
immediately in Urey's laboratory as well as in many others. With
Urey's interest and assistance, David Rittenberg, the recent recipient
of a Ph.D. in Physical Chemistry in Urey's department, was recruited
to work with Schoenheimer to explore the biological applications of the deuterium isotope (2, 3). According to Clarke, Rittenberg's arrival in
Schoenheimer's laboratory led to the "idea of employing a stable
isotope as a label in organic compounds, destined for experiments in
intermediary metabolism, which should be biochemically indistinguishable from their natural analogs" (2). The concept of
"tagged" molecules had been applied earlier when metabolically inert phenyl groups were attached to long chain fatty acids to permit
identification of the products of fatty acid metabolism in animals. One
example of this approach is described by H. D. Dakin in an earlier
JBC Classic (4). Schoenheimer points out in the first paper selected as
a Classic that the physical properties of phenyl derivatives are so
different for the natural molecules that the results are suspect.
It was difficult to select the JBC Classic papers to be representative
of Schoenheimer's work because so many qualify. Of the two that were
chosen, the first introduces the rationale and methodology for the use
of deuterium to study metabolism. The second applies that approach to
the study of cholesterol metabolism, a problem Schoenheimer had first
considered while in Freiburg in 1926. In the introduction of the first
Classic, Schoenheimer and Rittenberg summarize nicely the technical
obstacles that had limited the study and understanding of metabolism.
"If substances such as natural fatty acids, amino acids etc. are
administered to animals, we lose track of them the moment they enter
the body, since they are mixed with the same substances already
present. Furthermore, if a substance A is given to an animal and a
substance B is afterwards discovered in the body or in the excretions,
we can never be sure that the substance A has been converted into B ... " Clearly, the administration of specific deuterium-tagged molecules overcame these obstacles. For studies reported in the second
JBC Classic reprinted here, rather than administer a specific deuterium-tagged molecule and follow its fate in the body, Rittenberg and Schoenheimer used a more sophisticated design. They administered D2O to animals and measured the rate of incorporation of
deuterium into fatty acids and cholesterol. These experiments are
meticulous in their reasoning, technique, and rationale. Their
description is preceded by a careful consideration of the kinds of
chemical reactions that would introduce a stable, non-exchangeable
deuterium from D2O into organic molecules by the body. Both
paradigms, administration of specifically tagged molecules to determine
their fate and administration of precursors such as D2O to
determine rates of synthesis, continue to be the design for metabolic
studies. Many studies followed from these initial efforts. Urey
succeeded in accomplishing the enrichment of nitrogen for
15N, which provided a "tag" for amino acids and the
study of protein synthesis. Schoenheimer, Ratner, and Rittenberg
reported that after the administration of 15N-labeled
tyrosine to rats, only about half of the 15N was excreted,
and the rest was retained in the body proteins (5). This finding was a
blow to the prevalent notion that ingested foods were metabolized and
the products were excreted. The conclusions of this and subsequent
experiments made clear that body constituents were not stable or static
but in a dynamic state of turnover. Schoenheimer was invited to present
his important studies in the prestigious Edward K. Dunham Lecture at
Harvard University entitled "The Dynamic State of Body
Constituents." His lecture notes were edited by Hans Clarke, David
Rittenberg, and Sarah Ratner and published posthumously in 1942 (6).
They had an immeasurable influence on a generation of biochemists.
In the course of establishing his research program, Schoenheimer
attracted an interdisciplinary group of notable scientists including
physicists, chemists, and biologists. He provided outstanding leadership during a short life troubled by bouts of depression, leading
to suicide at the age of 43, at the height of his career (3). His young
colleagues were encouraged by Hans Clarke to continue their work (3).
David Rittenberg continued to study protein synthesis and, with Konrad
Bloch, fat metabolism, particularly cholesterol. David Shemin continued
to study amino acid metabolism and later described the pathway for heme
biosynthesis (7). Sarah Ratner continued to work on the metabolism of
nitrogen-containing compounds. DeWitt Stetten Jr. studied fatty acid
metabolism. All would have distinguished careers in biochemistry. Two
of Schoenheimer's young colleagues later received Nobel Prizes, Konrad
Bloch for the elucidation of the biosynthetic pathway for cholesterol
and Rosalyn Yalow for development of the radioimmunoassay with Solomon Berson. Yalow had been interested in science from an early age but the
barriers to careers for women in science had caused her to take a
position as part-time secretary to Schoenheimer, which provided "back
door" access to graduate courses as long as she also took stenography
(8).1
Although radioactive isotopes were beginning to replace stable isotopes
by the early 1940s, the paradigm established by Schoenheimer for the
use of isotopes for metabolic studies is as important today as it was
in 1935.
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