Tissue constituents, binding

In aqueous solutions, at a physiological pH, HA is represented by negatively charged hyaluronate macromolecules (pK = 3.21) [15] with extended conformations. In a polyanionic form, hyaluronan functional groups make the biopolymer so hydrophilic that it binds 1000 times more water than is predicted from its molar mass. The heterogeneity and hydrophilicity of HA facilitate its interaction with a variety of tissue constituents inside and outside the cells. In the extracellular space, HA controls the retention of water, ionic and molecular diffusion and provides a 3D-structural meshwork [16]. 

An immediate problem that must be overcome when conducting an RRA is the relatively low density of receptors specific for the ligand vis-a-vis other tissue constituents that are available for nonspecific interactions. Indeed, in many competitive binding assays, the amount of ligand specifically bound when all receptors are occupied (saturated) is on the order of fmoles/mg protein Because m most applications, and particularly for RRAs, non-... 

Materials may be absorbed by a variety of mechanisms. Depending on the nature of the material and the site of absorption, there may be passive diffusion, filtration processes, faciHtated diffusion, active transport and the formation of microvesicles for the cell membrane (pinocytosis) (61). EoUowing absorption, materials are transported in the circulation either free or bound to constituents such as plasma proteins or blood cells. The degree of binding of the absorbed material may influence the availabiHty of the material to tissue, or limit its elimination from the body (excretion). After passing from plasma to tissues, materials may have a variety of effects and fates, including no effect on the tissue, production of injury, biochemical conversion (metaboli2ed or biotransformed), or excretion (eg, from liver and kidney). 

Methyl paraoxon may also be made unavailable by binding to noncritical tissue and plasma constituents (Benke and Murphy 1975), including cholinesterase (Parkinson 1996). In addition, the parent compound is bound to albumin, in serum, as discussed previously in Section 3.4.2.4, but this binding does not appear to limit the availability of methyl parathion to the tissues, indicating that it is reversible. Tissue binding appears to be more important than serum binding (Braeckman et al. 1980, 1983). 

In the last case, this may be a physical problem resulting from incomplete penetration by the extraction solvent into the matrix. Alternatively, incomplete recovery of the analyte may result from chemical binding between the analyte and a constituent of the matrix. This is particularly important in the determination of drugs in body tissues where binding to proteins is known to occur. Problems of this kind are documented in the literature. If a new procedure is being developed, it is necessary to investigate the extraction step, e.g. by using radioactive tracers. 

Immunohistochemistry is used for detecting the constituents of tissue by using specific antibody tagged to a visibie iabei which will bind to the antigen inside the tissue. The tissues are paraffinised, frozen or embedded in resin. 

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