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Dicoumarol Fe + Steroid hormones

(transferrin) (transcortin)

Vitamin B^ Sialic acid Levothyroxine

Figure 10.20 Interactions of drugs and chemicals with plasma proteins; plasma proteins are depicted according to their relative amounts.

Modified from F. W. Putnam, The Proteins, 2nd edn, vol. 3 (ed. H. Neurath), Academic Press, New York, 1965.

necessarily 'preformed'. Fatty acids and warfarin are both capable of inducing conforma-tional changes which result in the formation of hydrophobic 'pools' in the protein.

Plasma proteins other than albumin may also be involved in binding; examples of such interactions are shown in Fig. 10.20. Blood plasma normally contains on average about 6.72 g of protein per 100 cm3, the protein comprising 4.0 g of albumin, 2.3 g of globulins and 0.24 g of fibrinogen. Although albumin is the main binding protein, dicoumarol is also bound to fi- and y-globulins, and certain steroid hormones are specifically and preferentially bound to particular globulin fractions.

An expression allowing the determination of the fraction of drug bound to a protein can be derived as shown in Box 10.2.

10.9.1 Thermodynamics of protein binding

Estimation of the thermodynamic parameters of binding allows interpretation of the mechanisms of interaction. Table 10.9 gives the thermodynamic parameters of binding of a number of agents to bovine serum albumin. The negative value of A G implies that binding is spontaneous. AH is negative, signifying an exothermic process and a reduction in the strength of the association as temperature increases. AS is positive, most likely signifying loss of structured water on binding. A diagrammatic representation of this last process is shown in Fig. 10.21. This diagram shows the change in the ordered water in the hydro-phobic cavity of the albumin and around the nonpolar portion of the drug. The loss of the

Box 10.2 Estimation of the degree of protein binding

Protein binding can be considered to be an adsorption process obeying the law of mass action. If D represents drug and P the protein we can write

D + P e (DP) (protein-drug complex) At equilibrium,

where Df is the molar concentration of unbound drug, Pt is the total molar concentration of protein, and Db is the molar concentration of bound drug (= molar concentration of complex). If one assumes one binding site per molecule, the equilibrium constant, K, is given by

^ k1 rate constant for association (10 9)

k-1 rate constant for dissociation From equation (10.8),

It is obvious that k1 p k_j. The rate constant for dissociation, k_i, is the rate-limiting step in the exchange of drug between free and bound forms. From equation (10.10) we obtain

That is,

Therefore,

Pt 1 + KDf where r is the number of moles of drug bound to total protein in the system. If there are not one, but n, binding sites per protein molecule, then nKDf l + KDf l _ l +

nKDf

Protein binding results are often quoted as the fraction of drug bound, fi. This fraction varies generally with the concentration of both drug and protein, as shown in equation (10.17), which relates fi with n, K and concentration:

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