The Basis of the Pharmacological Action of Heavy Water in Mammals *

Among all isotopes those which would be expected to show the greatest differences from the original element are the isotopes of hydrogen. The addition of one or two protons to a relatively large atom is an insignificant change in comparison with that undergone by the transformation of hydrogen to its nearest isotope, deuterium, for here the extra proton practically doubles the mass. In fact deuterium is frequently described as "like a new element." The following are some of the changes imparted by deuterium to water when it replaces hydrogen to form deuterium oxide, or heavy water. The melting point and boiling point are increased respectively by 3.82° C. and 1.420 C. The solubility of sodium chloride is decreased by 15 per cent, viscosity is increased by about 25 per cent, the vapor pressure is lowered, and specific gravity is increased to 1.1074. A number of these properties would certainly be expected to lead to changes in living organisms where considerable proportions of the body water are replaced by heavy water. In fact, when Urey41 first made deuterium oxide available, they predicted that it would prove of considerable biological interest. When appreciable amounts of deuterium oxide enter the animal organism problems immediately arise which concern the capacity of the tissues to exchange hydrogen for deuterium. Chemists have already given us information as to the stability of hydrogen variously placed in organic compounds. The following table by Bonhoeffer9 indicates how readily exchange of hydrogen occurs between organic compounds and H20. It was found by tracing the exchange of deuterium atoms: