PRINCIPLES OF TOXICOLOGY - Biology East Borneo

2.2 TRANSFER ACROSS MEMBRANE BARRIERS 39Figure 2.2 Dependence of the ratio of permeability coefficient to partition coefficient on the cylindrical radiusof the diffusing molecule for diffusion of a series of lipophilic amides across the human red cell membrane. Theordinate scale is in relative, not absolute, values. (P—propionamide; B—butyramide; IB—isobutyramide; V—valeramide; IV—isovaleramide). The partition coefficients are P—0.01; B,IB—0.05; and V,IV—0.175. [Data fromSha’afi et al. (1971).]The pK ais the negative logarithm of the acid dissociation constant. If, for example, the acid dissociationconstant K ais 10 –3 , then the pK ais 3.The degree of ionization in body fluids depends on the pH of the medium as well as on the pK aofthe acid or base. This relationship can be expressed by the Henderson–Hasselbalch equations:For acids:For bases:pK a − pH = log(nonionized form)(ionized form)= log (HA)(A − )pK a − pH = log(ionized form)(nonionized form) = log (HB+ )(B)When the pH is equal to the pK a, half of the acid or base is present in the ionized form and half in thenonionized form. At pH values less than the pK a, acids are less completely ionized. At pH greater thanthe pK a, bases are less completely ionized. Another way of stating these relationships is that at a given