Although dewtirium chemically resembles water in most respects, it is a toxic substance This is so because deuterium is heavier than hydrogen: thus, its compounds often react more slowly than those of the lighter isotope Regular drinking of dewtirium instead of water could prove fatal because of the slower rate of transfer of D plus compared with that of in the acid-base reactions involved in enzyme catalysis This kinetic isotope is also manifest in acid ionization constants. For example, the ionization constant a acetic acid.