Transfer, biological membranes distribution

After absorption, a chemical compound enters the circulation, which transfers it to all parts of the body. After this phase, the most important factor affecting the distribution is the passage of the compound through biological membranes. From the point of view of the distribution of a chemical compound, the organism can be divided into three different compartments (1) the plasma compartment (2) the intercellular compartment and (3) the intracellular compartment. In all these compartments, a chemical compound can be bound to biological macromolecules. The proportion of bound and unbound (free) chemical compound depends on the characteristics of both the chemical... 

The distribution of a chemical in tissue depends on the binding/par-titioning between circulatory blood and tissues, the transfer across biological membranes and the tissue-blood perfusion. After incorporation, contaminants are distributed from blood to high perfusion tissues (e.g. liver, kidney), then to low perfusion ones (skin, muscle) and finally to lipoidal tissues [2], establishing kinetically different compartments (internal organs > liver > head skin) and different times to equilibrium [18]. Surfactants (LAS, AS, AES, AEO and APEO) have been... 

More recent penetration experiments were carried out in biological systems, that is, large intact nuclei [35], giant liposomes [36], and mammalian cells [37]. Such experiments can provide information about the distribution of electroactive species inside the cell, potentials, and ion transfers across biological membranes (see Section V.F). 

It is thus evident that in order to be absorbed, distributed and eliminated drug molecules have to pass through a number of biological membranes of different kinds. In the vast majority of cases the transfer of drug molecules occurs by simple diffusion which is a passive process not requiring a supply of external energy. In a limited number of cases, facilitated diffusion, active transport or even pinocytosis (engulfing of substances by the membrane) are involved. 

Knowledge of the size of the exchangeable pool of lipid in a substrate is necessary to determine the rate constants of the exchange process and establish the specificity of a transfer protein for different classes of phospholipids. The size of the exchangeable pool of each class of phospholipid must be determined individually because phospholipids are often asymmetrically distributed across biological and artificial membranes (Op den Kamp, 1979). 

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