THE COMBINATION OF ORGANIC ANIONS WITH SERUM ALBUMIN

Albumin as fatty acid transporter

SUMMARY

Fatty acids play critical roles in mammalian energy metabolism. Moreover, they are important substrates for the synthesis of membrane phospholipids and biologically active compounds like eicosanoids and leukotrienes. Because of their low solubility in aqueous solutions such as blood plasma and interstitial fluid, fatty acids are in need of binding proteins to increase their concentration in vascular and interstitial compartments. Albumin acts as main fatty acid binding protein in extracellular fluids. Plasma albumin possesses about 7 binding sites for fatty acids with moderate to high affinity, enhancing the concentration of fatty acids by a several orders of magnitude. Despite the high affinity of albumin for fatty acids, uptake of fatty acids by parenchymal cells such as skeletal and cardiac myocytes seems not to be hampered by albumin. In contrast, experimental findings suggest that albumin may facilitate the uptake of fatty acids by organs in need of these substrates. In the present overview the following issues will be briefly discussed: (i) transport and storage of fatty acids in the mammalian body, (ii) biosynthesis of albumin in the liver, (iii) localization and concentration of albumin in body fluids, (iv) interactions between albumin and fatty acids, (v) albumin structure and fatty acid binding sites, (vi) uptake of fatty acids by organs and roles for plasma albumin and (vii) lessons from patients and experimental animals lacking plasma albumin.

Measurement of phenylacetic acid in cerebrospinal fluid and plasma using combined gas chromatography/electron capture chemical ionization mass spectrometry

Details are presented of an ultra-sensitive gas chromatographic/mass spectrometric assay for phenylacetic acid in plasma and cerebrospinal fluid based on measurements of the relative intensities of the carboxylate anions, derived from the penta- and tetrafluorobenzyl esters under electron capture chemical ionization conditions, of unlabeled and a (13C2)-labeled internal standard. The limits of detection for the penta- and tetrafluorobenzyl esters are 0.85 and 4.0 pg respectively, and the assay is capable of measuring phenylacetic acid concentrations in samples as small as 20 microliter of CSF and plasma. The penta- and tetrafluorobenzyl esters are chromatographically separated on the gas chromatograph column, which allows for their co-injection and independent measurement from the same chromatogram.

A general method for the assay of binders. Application to the potentiometric determination of albumin in human serum, plasma and whole blood

From the generalized theory of binding (stepwise equilibrium model, Scatchard model) it can be derived that in a solution containing the binder and a specific ligand, the bound ligand concentration is linearly related to the total binder concentration only if the free ligand concentration is constant. We designed a general method which is based on that principle and we have shown its validity for the albumin assay in serum, plasma and whole blood. We have tested the ligands bromcresol purple and picrate and used ligand-ion selective electrodes to monitor free ligand concentration in a homogeneous solution. The method has excellent linearity. We chose picrate (PIC) as ligand, because of better specificity. Comparison with the bromcresol purple (BCP) and bromcresol green (BCG) spectrophotometric procedures for 93 samples gave the regression equations: y(PIC) = 0.98x(BCP) + 0.76 (g/L, r = 0.979) and y(PIC) = 0.95x(BCG) - 1.3 (r = 0.947), respectively. Within-day and day-to-day precision was up to 1.5% and 2.3%, respectively. Results for albumin in plasma, when transformed to expected whole blood values, were in close agreement with those obtained by direct analysis of whole blood (in the presence of erythrocytes).

THE BINDING OF ANIONS BY LYSOZYME, CALF THYMUS HISTONE SULPHATE, AND PROTAMINE SULPHATE

The adsorption of a large series of anions by positively charged lysozyme, calf thymus histone sulphate, and protamine sulphate has been studied by dialysis equilibrium. One group of anions was not adsorbed by lysozyme while the adsorption isotherms for Orange I, Orange II, and 2, 4-dinitro-1-naphthol-7-sulphonic acid were S-shaped. These anomalous isotherms, which were also obtained with histone sulphate and protamine sulphate, have an appreciable temperature coefficient at intermediate dye concentrations. It is emphasized that this type of adsorption is inconsistent with the exclusively electrostatic view of protein binding of anions. It is also shown that −ΔF, −ΔH, and ΔS per mole of anion bound must pass through a maximum for intermediate free anion concentrations in such systems. The lack of binding of some anions and the anomalous isotherms for others is interpreted in terms of a solvation sheath about the charged protein. Their significance for the general process of binding of ions by proteins is discussed.