Albumin Catabolic Rate Measured by a Stable Isotope

Overview

The goal of this physiological study is to compare albumin catabolic rate measured by a stable isotope labeled amino acid in healthy volunteers and in patients with liver disease. At steady state synthesis and catabolism or degradation are equal. The primary questions it aims to answer are: * Is albumin catabolic rate lower in patients with liver disease? * Is albumin catabolic rate measured by stable isotopes in volunteers like historical controls measured by radio-iodinated albumin at the investigator's laboratory or elsewhere? Subjects will be given an oral dose of the deuterium labeled amino acid phenylalanine that will be incorporated by the liver in newly synthetized albumin molecules, and blood samples will be taken over 12 weeks to determine the catabolic rate of albumin.

This study is part of a greater research program investigating the clinical use of albumin infusions in surgery, liver disease and states of critical illness. Indications are still under debate after more than 70 years of use. Good methodology for assessment of synthesis or degradation can be of immense value in these situations. The gold standard to assess albumin turn-over is to use radio-iodine labeled albumin and take blood samples over several weeks. This method is however associated with many regulatory difficulties and possibly also methodological flaws. Intravenous injection of deuterium labeled D5-phenylalanine has been used for snap-shot assessment of albumin synthesis, but in this study the stable isotope tracer will be given orally. The idea is that a large dose of labeled phenylalanine will over-flow all body compartments and thus in the liver be incorporated in the newly synthetized albumin molecules. After a short time period there is no more tracer in the amino acid pool and the disappearance of labeled albumin molecules can then be used to measure the albumin disappearance rate, that is equal to the catabolic rate and in steady state also equal to the synthesis rate. The investigaors believe that by this new method it is possible to overcome some of the issues of radio-iodinated albumin: 1) no radiation, 2) no risk that the tracer leaves from the albumin molecule ahead of the catabolism of the albumin molecule, 3) no risk that the half-life of the albumin molecule is damaged by the labeling process, 4) a potential to measure turn-over of other long-lasting proteins. Quantitative measurement of enrichment of isotopically labeled phenylalanine is done using gas chromatography-mass spectrometry at the Karolinska Stable Isotope Core where Olav Rooyackers is the laboratory director. Quantification of total phenylalanine (the precursor) is performed by mass spectrometry against an internal standard. The albumin molecule binds to free acid radicals (scavenger function) with its sulfhydryl group in the Cys-34 position (human mercaptoalbumin). During oxidative stress, the sulfhydryl group is oxidized to sulfinic acid (human non-mercaptoalbumin 1, HNA 1). This is a reversible process. Upon continued oxidation, sulfonic acid is formed, which appears to be irreversible (human non-mercaptoalbumin 2, HNA2). The result is a damaged albumin molecule that has lost its scavenger function. Oxidative stress with high levels of HNA2 may have a decisive role in the pathogenesis of severe liver failure. There is no data in the literature regarding changes in oxidized albumin over time. The investigators will try to model albumin turnover by looking at how these fractions and the proportions between them change over time. Previous results, suggesting that the amount of oxidized albumin is higher in patients with liver cirrhosis \[Oettl 2013\], will be reproduced. Finally, the association between albumin oxidation and albumin catabolic rate will be described. The statistical analysis plan including the determination of the number of study subjects is included in the attached study plan.