As noted above, an enzyme inhibitor is characterised by its two dissociation constants, "K"i and "K"i', to the enzyme and to the enzyme-substrate complex, respectively. The enzyme-inhibitor constant "K"i can be measured directly by various methods; one extremely accurate method is isothermal titration calorimetry, in which the inhibitor is titrated into a solution of enzyme and the heat released or absorbed is measured. However, the other dissociation constant "K"i' is difficult to measure directly, since the enzyme-substrate complex is short-lived and undergoing a chemical reaction to form the product. Hence, "K"i' is usually measured indirectly, by observing the enzyme activity under various substrate and inhibitor concentrations, and fitting the data to a modified Michaelis–Menten equation