A radioactive tracer, or radioactive label, is a chemical compound in which one or more atoms have been replaced by a radioisotope so by virtue of its radioactive decay it can be used to explore the mechanism of chemical reactions by tracing the path that the radioisotope follows from reactants to products. Radiolabeling is thus the radioactive form of isotopic labeling.
The early history of (32) P as a radioactive tracer in biochemical research
The concept of using radioactive isotopes as "tracers" of chemical conversions was conceived and developed by inorganic chemist Georg de Hevesy (Nobel Laureate in Chemistry 1943). In 1935, he began to apply the technique to various biological processes using (32) P, and his experiments revealed the dynamic character of physiology and metabolism. Following de Hevesy's lead, Samuel Ruben (University of California, Berkeley) exploited (32) P in 1937-38 for investigation of phospholipid metabolism. Between 1937 and 1940, Ruben and colleague Martin Kamen spearheaded tracer studies in various biological systems using (32) P, short-lived (11) C, and other radioactive isotopes. During this period, Kamen was responsible for cyclotron-produced radioactive tracers and was able to sustain de Hevesy's research by supplying him with (32) P. In 1940, Ruben and Kamen discovered long-lived (14) C, which later proved to be a very powerful tool for analysis of complex biochemical processes, such as the path of carbon in photosynthesis. Between 1946 and 1950, (32) P was used in studies of bacteriophage replication and photosynthetic metabolism. This memoir surveys the history of these early investigations.