ARA

The sodium salt of arachidonic acid is a soap, the same as might be prepared for any other long-chain fatty acid. It can be dissolved fairly readily in aqueous solution. This is in complete contrast to arachidonic free acid, which is an insoluble oil. Interconversion of the salt (ionic) and nonionic forms of arachidonic acid occurs in the range of normal physiological pH. The high pKa of arachidonic acid is crucial, as it sets the solubility properties and the possible distribution of the unesterified fatty acid in cells. As most studies of enzymatic transformations and biological activities of arachidonic acid are dependent on addition of exogenous arachidonic acid to cells and tissues.

Cells have hydrophobic (membrane and protein) sites and aqueous/polar sites, so “choices” are available for the different ionic forms of the fatty acid. Like other fatty acids (and certain other membrane components, including acylated proteins and phospholipids), arachidonic acid is amphipathic, and its hydrophobic tail can remain in a lipid bilayer while its polar carboxyl group (charged or uncharged) can emerge into the aqueous environment outside the membrane.

The physical chemistry of solutions of polyunsaturated fatty acids such as linoleic and arachidonic acids is not completely defined. If arachidonate sodium salt is dissolved in a weakly alkaline solution and then titrated with HCl, the clear solution starts to become cloudy. The observed pKa in the titration, the point of 50% ionization, is noted around pH 8 At this stage, a 1 millimolar (0.3 mg/ml) solution of arachidonate Na salt would be almost opaque, as half the molecules are converted to the insoluble free acid. Strangely, when the same experiment is carried out with more dilute solutions of polyunsaturated fatty acid, the apparent pKa falls towards pH 7 . This trend in decreasing pKa implies that at a constant pH (e.g., pH 7.4) the lower the concentration of the fatty acid, the better its solubility. The explanation for this change in apparent pKa may relate to a tendency for the long carbon chains of different molecules to bunch together in an aqueous system, an effect that would be less prevalent at dilute concentrations. This may change the accessibility or reactivity of the carboxyl group to acid and alkali. These properties have practical significance in that they determine the aqueous solubility of arachidonic acid in the concentrations ranges likely to be used in biological experiments. The ionic environment also influences solubility: for example, the calcium salts of long-chain fatty acids are water insoluble, as clearly evidenced by the appearance of a scum when hard-water (containing CaCO3) is mixed with soap. Similarly, solutions of arachidonic acid salts will tend to precipitate in the presence of millimolar solutions of calcium ions.

Protein binding can increase the overall concentration of arachidonic acid that can be present in an aqueous environment by effectively decreasing the concentration of free molecules in solution. Albumin, in particular, binds specifically to fatty acids . Because of its high concentration in human plasma (35 mg/ml, 0.6 mM), this protein greatly reduces the effective concentration of free fatty acid molecules and permits millimolar concentrations to be stabilized in an aqueous environment. Similarly, the extracellular fluid has an albumin concentration of 0.1 mM, and physiologically, this is also critical to the presentation of low concentrations of the free fatty acid (unbound) to cells. In the presence of albumin, the concentrations of free fatty acids are likely kept well below 0.1μM .