Potential methodological errors in the application of equilibrium dialysis to the determination of "free" and "bound" plasma tryptophan

Quantification of free and protein bound uremic toxins in human serum by LC-MS/MS: Comparison of rapid equilibrium dialysis and ultrafiltration

The objectives of this study were (1) to develop a method for the determination of 10 uremic toxins (3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid (CMPF), hippuric acid, indole-3-acetic acid, indoxyl sulfate, kynurenic acid, kynurenine, p-cresyl glucuronide, p-cresyl sulfate, phenylacetylglutamine and trimethylamine N-oxide (TMAO)), and 3 precursors (tyrosine, phenylalanine, tryptophan) in serum and (2) to compare two separation methods to determine the free serum fraction: rapid equilibrium dialysis (RED) and ultrafiltration (UF). The method was developed on a liquid chromatography system coupled to a tandem mass spectrometer. Fifty µL of serum sample were precipitated with methanol after addition of internal standard. The two separation methods were compared using serum samples from patients suffering from renal impairment (n = 30). The method has been validated according to the European Medicines Agency (EMA) guidelines. Calibration curves were linear from 1 to 50 ng/mL up to 10,000-50,000 ng/mL according to the compounds. The comparison between the two separation methods produced similar results for all compounds except kynurenine, tryptophan (around 30% more with UF) and indole-3-acetic acid (around 30% more with RED). This study has allowed the development and validation of a sensitive and robust assay for the quantification of free and total concentrations of 10 uremic toxins and 3 precursors in human serum.

Evaluation of ultrafiltration for the free-fraction determination of single photon emission computed tomography (SPECT) radiotracers: beta-CIT, IBF, and iomazenil

An ultrafiltration system was evaluated for the free-fraction measurement of SPECT radiotracers (beta-CIT, IBF, and iomazenil) used in functional brain imaging. The effect of temperature, storage, centrifugal force, tracer concentration, and percentage filtered demonstrated a relative error of < 9%. As a result of the minimal temperature effect, 25 degrees C was employed for all measurements. A comparison of the ultrafiltration system with equilibrium dialysis revealed < 5% difference for beta-CIT and iomazenil, but 16% for IBF. Additionally, the time and ease of operation considerably favored the ultrafiltration system. The precision quantitated by repetition was < 6% for between-run and within-run variability. In conclusion, ultrafiltration provided rapid results, demonstrated minor analytical errors, revealed generally good correlation with equilibrium dialysis, and allowed excellent precision.

Free and protein-bound tryptophan in serum of untreated patients with chronic renal failure

In fasting sera from 46 untreated patients with chronic renal failure and in 22 normal subjects, non-protein-bound tryptophan, F, was separated by pH-controlled equilibrium dialysis. Total tryptophan, T, and F were measured by HPLC. Results in patients were related to gender, severity of chronic renal failure (as measured by radioisotopic glomerular filtration rate), protein intake (as measured by 24-hr urinary urea N excretion), and protein nutrition (as measured by serum concentrations of albumin and transferrin). T was subnormal in 59% of the patients. In seven hypoalbuminemic patients, F/T was markedly increased (approaching unity) but F was normal. In 39 non-hypoalbuminemic patients, F was again normal but F/T was often increased at glomerular filtration rates below 30 ml/min/3 m2 of height2, especially in females. T was significantly correlated with estimated protein intake (r = 0.54, P = 0.0004), even though neither F nor serum protein levels were correlated with it. We conclude that the serum level of free tryptophan is well-maintained in chronic renal failure, being uninfluenced by severity of renal insufficiency, voluntary protein intake, or serum protein concentrations. On the other hand, protein-bound tryptophan varies with protein intake, decreases markedly in hypoalbuminemic patients, and also decreases in many nonhypoalbuminemic patients (especially females) when the glomerular filtration rate falls below approximately 30 ml/min/3 m2 of height2.