Leucine metabolism in man: lessons from modeling

Recycling, channeling and heterogeneous protein turnover estimation using a model of whole-body protein turnover based on leucine kinetics in rodents

In the companion paper, a whole-body, mechanistic model of protein turnover in a rodent was described and evaluated with independent data sets that used the flooding dose method. On the basis of fitted fluxes, the model was able to predict specific radioactivity changes in the protein and free leucine pools and whole-body protein fractional synthesis rate (FSR). In this paper, results of model simulations of specific radioactivity changes in the flooding dose, pulse dose and continuous infusion methods were compared and the influence of recycling, channeling and multiple protein pools on model behavior were analyzed. For all methods, the percentage of channeling must be estimated to determine whether the extracellular or intracellular pool specific radioactivities better approximate the aminoacyl tRNA pool specific radioactivity. Recycling also affects the specific radioactivity of the aminoacyl-tRNA pool and therefore must be estimated. An analysis of fits of the flooding dose data indicated that 100% channeling was occurring, but the percentage of recycling could not be determined. Multiple protein pools turning over at different rates overestimated FSR by 2-3% at early time points (5 min) and underestimated FSR by 3-6% at 60 min in the flooding dose method. For the pulse dose method, FSR was underestimated by 40-50% at 5 min and underestimated by 9-10% at 60 min. An increase in time to measure FSR caused a decrease in the estimate of FSR (18% over 3 h) for the flooding dose method and an increase in the estimate of FSR (144% over 3 h) for the pulse dose method.

Ketoisocaproate contamination errors in protein synthesis determinations using L[1-14C]leucine

Protein synthesis rate determinations in vivo using L-[1-14C]-leucine may be underestimated because of contamination by radioactive ketoisocaproate (KIC) resulting from leucine metabolism. The aim of this work was to set up a reliable method to determine the KIC/leucine radioactivity ratio in protein-free homogenates, and to apply it to study the extent of the protein synthesis ratio error due to KIC contamination. Cation-exchange chromatography using Dowex AG 50W-X8 resin was used to separate KIC from leucine, eluting KIC with water and leucine with 4 M ammonia. The errors found in the protein synthesis ratio were 6.20% in liver and 2.34% in jejunum.

Protein and amino acid metabolism in cancer cachexia: investigative techniques and therapeutic interventions

Cancer cachexia is a complex syndrome characterized primarily by diminished nutrient intake and progressive tissue depletion that is manifest clinically as anorexia and host weight loss. The gradual loss of host protein stores is central to this process. This review outlines the techniques that have been used to evaluate human amino acid metabolism, their application in patients with cancer cachexia, and possible therapeutic interventions designed to overcome alterations in host protein and amino acid metabolism associated with malignant cachexia. The techniques of nitrogen balance and 3-methylhistidine excretion provide indirect estimates of overall nitrogen metabolism and skeletal muscle myofibrillar protein breakdown. Measurement of circulating amino acid concentrations, particularly when combined with assessment of arterial-venous differences and regional amino acid balance allows for investigation of interorgan amino acid metabolism. One of the most significant advances in in vivo amino acid metabolic research has been the development of labeled amino acid tracer studies to evaluate whole body and regional amino acid kinetics. The use of stable and unstable amino acid isotopes in these techniques is reviewed in detail. Virtually all of these techniques have now been employed in the evaluation of human cancer cachexia. The results of studies evaluating amino acid concentrations, regional amino acid balance, and 3-methylhistidine excretion are summarized. The use of regional and whole body kinetic studies in cancer cachexia are reviewed extensively. Most investigators have observed increased rates of whole body protein turnover, synthesis, and catabolism in both weight-stable and weight-losing cancer patients. Some studies have suggested a relationship between the extent of disease and the degree of aberration in amino acid kinetic parameters. Investigators have attempted to reverse some of these alterations by provision of substrate (nutritional support) or administration of specific pharmacologic or anabolic agents such as hydrazine sulfate, insulin, growth hormone, and beta-2 agonists. The role of total parenteral nutrition (TPN) in cancer and its effects on protein and amino acid kinetics and tumor growth are addressed. The possible benefits of specific amino acid nutritional formulations with increased branched chain amino acids, arginine, and glutamine are reviewed. Although many of these approaches appear promising, significant impact on clinically definable parameters remains to be demonstrated. A better understanding of the underlying protein catabolic mechanisms of cancer cachexia will likely lead to more effective therapies to reverse the protein calorie malnutrition associated with cancer cachexia.

Domain of validity of classical models of leucine metabolism assessed by compartmental modeling

Whole-body modeling of in vivo leucine (an essential amino acid) metabolism is fundamentally difficult due to the complexity of the system. This has favored the use of two simple kinetic models, the so-called primary and reciprocal pool models, to interpret tracer data, but their domain of validity is uncertain. We define here the error of these two approaches by using comprehensive compartmental models of leucine metabolism as true representations of the leucine system. Of particular interest is the comparison of the two simple models with an 11-compartment model characterized by a rich intracellular compartmentation that has recently been proposed as a sound physiological description of the system. Formulas are derived that define in structural terms the error of the primary and reciprocal pool models.

Accessible pool and system parameters: assumptions and models

Quantitative assessment of substrate metabolism from in vivo tracer kinetic data requires a model of the system, i.e., a hypothesis on the structure and functioning of the system. Some fundamentals of modeling important for studying intermediary metabolism in the steady state will be discussed. Accessible pool and system parameters are defined. Although the calculation of accessible pool parameters is structure-free, that of system parameters requires the use of non-compartmental or compartmental structures. Assumptions, bases for choice, and relative merits of these two modeling strategies are discussed. Glucose and leucine metabolism serve as prototypes to illustrate the theoretical points.