Evaluation of the Carbon 13-Labeled Ketoisocaproate Breath Test To Assess Mitochondrial Dysfunction in Patients with High Alcohol Consumption

Mitochondria Matter: Systemic Aspects of Nonalcoholic Fatty Liver Disease (NAFLD) and Diagnostic Assessment of Liver Function by Stable Isotope Dynamic Breath Tests

The liver plays a key role in systemic metabolic processes, which include detoxification, synthesis, storage, and export of carbohydrates, lipids, and proteins. The raising trends of obesity and metabolic disorders worldwide is often associated with the nonalcoholic fatty liver disease (NAFLD), which has become the most frequent type of chronic liver disorder with risk of progression to cirrhosis and hepatocellular carcinoma. Liver mitochondria play a key role in degrading the pathways of carbohydrates, proteins, lipids, and xenobiotics, and to provide energy for the body cells. The morphological and functional integrity of mitochondria guarantee the proper functioning of β-oxidation of free fatty acids and of the tricarboxylic acid cycle. Evaluation of the liver in clinical medicine needs to be accurate in NAFLD patients and includes history, physical exam, imaging, and laboratory assays. Evaluation of mitochondrial function in chronic liver disease and NAFLD is now possible by novel diagnostic tools. "Dynamic" liver function tests include the breath test (BT) based on the use of substrates marked with the non-radioactive, naturally occurring stable isotope 13C. Hepatocellular metabolization of the substrate will generate 13CO2, which is excreted in breath and measured by mass spectrometry or infrared spectroscopy. Breath levels of 13CO2 are biomarkers of specific metabolic processes occurring in the hepatocyte cytosol, microsomes, and mitochondria. 13C-BTs explore distinct chronic liver diseases including simple liver steatosis, non-alcoholic steatohepatitis, liver fibrosis, cirrhosis, hepatocellular carcinoma, drug, and alcohol effects. In NAFLD, 13C-BT use substrates such as α-ketoisocaproic acid, methionine, and octanoic acid to assess mitochondrial oxidation capacity which can be impaired at an early stage of disease. 13C-BTs represent an indirect, cost-effective, and easy method to evaluate dynamic liver function. Further applications are expected in clinical medicine. In this review, we discuss the involvement of liver mitochondria in the progression of NAFLD, together with the role of 13C-BT in assessing mitochondrial function and its potential use in the prevention and management of NAFLD.

Exploring Liver Mitochondrial Function by 13C-Stable Isotope Breath Tests: Implications in Clinical Biochemistry

The liver is at the crossroad of key metabolic processes, which include detoxification, glycolipidic storage and export, and protein synthesis. The gut-liver axis, moreover, provides hepatocytes with a series of bacterial products and metabolites, which contribute to maintain liver function in health and disease. Breath tests (BTs) are developed as diagnostic tools for indirect, rapid, noninvasive assessment of several metabolic processes in the liver. BTs monitor the appearance of CO₂ in breath as a marker of a specific substrate metabolized in the liver, typically within microsomes, cytosol, or mitochondria. The noninvasiveness of BTs originates from the use of the, nonradioactive, naturally occurring stable isotope ¹³C marking a specific substrate which is metabolized in the liver, leading to the appearance of ¹³CO₂ in expired air. Some substrates (ketoisocaproic acid, methionine, and octanoic acid) provide information about dynamic liver mitochondrial function in health and disease. In humans, the application of ¹³C-breath tests ranges from nonalcoholic and alcoholic liver diseases to liver cirrhosis, hepatocarcinoma, preoperative and postoperative assessment of liver function, and drug-induced liver damage. ¹³C-BTs are an indirect, cost-effective, and easy method to evaluate dynamic liver function and gastric kinetics in health and disease, with ongoing studies focusing on further applications in clinical medicine.

The evaluation of the repeatability of the 13C-ketoisocaproate breath test for assessing hepatic mitochondrial function

The ¹³C-ketoisocaproate (¹³C-KICA) breath test (BT) has been recently proposed as a non-invasive test for assessing hepatic mitochondrial function. Results of the ¹³C-KICA BT can be expressed as different parameters. However, the best parameter for expressing the ¹³C-KICA BT result is uncertain which hinders use of the BT in routine clinical practice. We have investigated the repeatability of different parameters of ¹³C-KICA BT. Thirteen healthy adult subjects (5 men and 8 women) underwent a ¹³C-KICA BT on two occasions separated by a gap of approximately 30 days. There were no significant differences between the repeated measurements for all the test parameters over 30 days. Furthermore, the Bland Altman statistics showed no fixed or proportional bias for any of the test parameters. The cumulative ¹³C-dose enrichment over 60 min had the lowest within-subject variability of 12% compared to all other test parameters. The cumulative ¹³C-dose enrichment over 60 min could be a very useful parameter for the ¹³C-KICA BT to detect impaired hepatic mitochondrial function in patients with chronic liver diseases.

Exploring liver mitochondrial function by ¹³C-stable isotope breath tests: implications in clinical biochemistry

The liver plays a pivotal role in a myriad of metabolic processes, including detoxification, glycolipidic storage and export, and protein synthesis. Breath tests employing (13)C as stable isotope have been introduced to explore such energy-dependent pathways involving mitochondrial function in the liver. Specific substrates are ketoisocaproic acid, methionine, and octanoic acid. In humans, the application of (13)C-breath tests ranges from nonalcoholic and alcoholic liver diseases to liver cirrhosis, hepatocarcinoma, preoperative and postoperative assessment of liver function, and drug-induced liver damage. Studying liver mitochondrial function by (13)C-breath tests represents a complementary tool to monitor complex metabolic processes in health and disease.

Immunoproteomic identification of biotransformed self-proteins from the livers of female Balb/c mice following chronic ethanol administration

Chronic alcohol consumption culminates in alcoholic hepatitis which is characterized by ballooning degeneration of hepatocytes and perivenous inflammation. The aldehydes produced by ethanol oxidation and lipid peroxidation form adducts with the hepatic proteins rendering them immunogenic and initiating an autoimmune response. The present study was designed to identify these immunoreactive hepatic proteins in ethanol-treated Balb/c mice. Liver cytosolic, mitochondrial, and microsomal proteins from the ethanol-treated and control female Balb/c mice were size fractionated on SDS-PAGE and immunoblotted with the sera from the individual animal. The immunoreactive proteins were identified using antimouse IgG antibody and characterized by MALDI-TOF. It is the first report demonstrating that 15 hepatic proteins show immunoreactivity following alcohol administration. The identified autoreactive proteins ranged in function from metabolism to cytoskeletal support. Remarkably, three key enzymes of ethanol metabolism, namely alcohol dehydrogenase, aldehyde dehydrogenase I and III as well as important antioxidant enzyme glutathione S-transferase were found to be autoreactive upon ethanol treatment. We conclude that ethanol treatment induces biotransformation of host proteins from almost every compartment of the cell, especially the enzymes involved in the detoxification of ethanolic insult being the major target for biotransformation. Hence, we propose that these proteins can be the potential candidates for the biomarker studies.

13C-breath tests for clinical investigation of liver mitochondrial function

BACKGROUND

Mitochondria play a major role in cell energetic metabolism; therefore, mitochondrial dysfunction inevitably participates in or even determines the onset and progression of chronic liver diseases. The assessment of mitochondrial function in vivo, by providing more insight into the pathogenesis of liver diseases, would be a helpful tool to study specific hepatic functions and to develop rational diagnostic, prognostic and therapeutic strategies.

DESIGN

This review focuses on the utility of breath tests to assess mitochondrial function in humans and experimental animals.

RESULTS

The introduction in the clinical setting of specific breath tests may allow elegantly and noninvasively overcoming the difficulties caused by previous complex techniques and might provide clinically relevant information, i.e the effects of drugs on mitochondria. Substrates meeting this requirement are alpha-keto-isocaproic acid and methionine that are both decarboxylated by mitochondria. Long-and medium-chain fatty acids that are metabolized through the Krebs cycle, and benzoic acid which undergoes glycine conjugation, may also reflect the function of mitochondria.

CONCLUSIONS

Breath tests to assess in vivo mitochondrial function in humans represent a potentially useful diagnostic and prognostic tool in clinical investigation.

The stable isotope ketoisocaproic acid breath test as a measure of hepatic decarboxylation capacity: a quantitative analysis in normal subjects after oral and intravenous administration

BACKGROUND AND AIMS

There is no generally accepted kinetic evaluation method for the stable isotope [(13)C]ketoisocaproic acid (KIC) breath test. Differences found in the results between women and men are contradictory.

METHODS

Oral and intravenous breath tests using 1 mg/kg stable isotope-labelled KIC were performed in healthy male and female volunteers. A power exponential function was fitted to the mass spectrometric data of breath (13)CO(2) enrichment, allowing mathematical analysis of time-to-peak-excretion, half-excretion time, percent label recovery and parameters describing the shape of the curve. Body composition was determined using bioelectrical impedance analysis.

RESULTS

After oral administration, total label recovery after 3 h was about 22% and was not different between men (n=7) and women (n=8). The time to maximal label excretion was 0.67 +/- 0.12 h in men and 0.9 +/- 0.32 h in women (P=0.028) and the excretion curve showed an initially slower rise in women compared with men. Adjusting for lean body mass or body water abrogated the sex differences. Total label recovery after intravenous administration was about 9%, suggesting that the substrate was rapidly catabolized in the muscle compartment after intravenous administration.

CONCLUSIONS

The modified power exponential function described allows standardized estimates of the KIC breath test results. When corrected for body composition, there are no differences in breath test results between men and women. The comparison between oral and intravenous results provides robust evidence that the KIC breath test measures predominantly hepatic and not muscle decarboxylation and is thus a highly specific liver function test.

Improvement of mitochondrial function evaluated by ketoisocaproic acid breath test in patients with HCV infection undergoing albumin dialysis

BACKGROUND AND AIM

Oxidative injury occurs as a direct result of hepatitis C virus (HCV) core protein expression both in vitro and in vivo, and may be due to a direct effect on mitochondria. The ketoisocaproic acid (KICA) breath test is a simple, reliable, and noninvasive test to evaluate hepatic mitochondrial function. Albumin dialysis (MARS) is an effective bridge treatment for patients with acute failure superimposed on chronic liver disease. The aim of our study was to evaluate the improvement of mitochondrial function measured by KICA in patients undergoing MARS for acute-on-chronic HCV liver failure.

MATERIALS AND METHODS

Five patients with HCV chronic infection undergoing MARS treatment for acute decompensation were enrolled. Before and after each MARS treatment, patients underwent blood testing for the main hematochemical parameters as well as for mitochondrial function by the KICA breath test and the arterial ketone bodies ratio (AKBR).

RESULTS

MARS treatment effectively decreased the serum level of total bilirubin, bile acids, urea, and ammonium. Moreover, MARS treatment produced an increase in AKBR and in the cumulative percentage of (13)CO(2) recovered in exhaled air 2 hours after KICA ingestion.

CONCLUSION

Liver mitochondrial function appears to be beneficially affected by MARS treatment.

Postprandial insulin response and mitochondrial oxidation in obese men nutritionally treated to lose weight

Obesity, hyperglycemia, and insulin resistance have been associated to an oxidative mitochondrial dysfunction. The aim of this research was to evaluate the relation between carbohydrate metabolism and mitochondrial oxidation, as affected by the weight status and the weight loss induced by a calorie-restricted diet. Lean control men (BMI<25 kg/m2, n = 6) and obese men (BMI>30 kg/m2, n = 14), who were characterized as insulin resistant (n = 6) or insulin sensitive (n = 8) based on HOMA index values, participated in the trial. Plasma insulin levels and mitochondrial oxidation estimated by the 2-keto(1-13C)isocaproate breath test, were measured after ingestion of a test meal during 3 h. Obese subjects repeated the breath test protocol after a 10-week caloric restriction diet to lose weight. Postprandial insulin secretion tended to be marginally higher (P = 0.059) in both obese groups than in controls, while the rate of postprandial mitochondrial oxidation was markedly decreased (P = 0.019) in the obese subjects as compared with lean individuals. The nutritionally induced weight loss produced a rise in the postprandial oxidative process in volunteers initially considered as insulin resistant (P = 0.036), while no statistical differences in the insulin-sensitive obese (P = 0.241) were found. Interestingly, the percentage of oxidized tracer was inversely related to postprandial insulin secretion (r = -0.56; P = 0.001). In conclusion, these results support the hypothetized relation between carbohydrate metabolism and mitochondrial oxidation at a postprandial state in obese subjects, raising interest about mitochondria stimulation as a target in the therapy of obesity.

Methodological characterization of the 2-keto [1-13C]isocaproate breath test to measure in vivo human mitochondrial function: application in alcoholic liver disease assessment

BACKGROUND

The 2-keto[1-13C]isocaproate oxidation measurement has been shown as a helpful tool in the in vivo assessment of liver mitochondrial function.

METHODS

The aim of this work was to study the variability of the 2-keto[1-13C]isocaproate breath test in 24 healthy controls (8 men and 16 women) and to evaluate its clinical usefulness in 20 patients (14 men and 6 women) with liver disease (7 men with history of alcoholism). Breath test was performed by measuring 13CO2 enrichment in breath before and after the oral administration of the tracer and by using isotope ratio mass spectrometry.

RESULTS

The intrasubject and intersubject variability of the percentage of tracer oxidized were 8 and 14%, respectively. The 2-keto[1-13C]isocaproate oxidation in women was faster (p = 0.004) and tended to be higher (p = 0.050) than in men. The percentage of oxidized tracer was lower in those patients with alcoholic liver disease than in healthy volunteers (p = 0.001) and in nonalcoholic patients (p = 0.003).

CONCLUSIONS

The percentage of tracer oxidized appears as a convenient parameter to detect impairment in liver mitochondrial oxidation related to alcoholism by the 2-keto[1-13C]isocaproate breath test, establishing different cutoff values depending on gender.

In vivo assessment of the mitochondrial response to caloric restriction in obese women by the 2-keto[1-C]isocaproate breath test

The 2-keto[1-(13)C]isocaproate breath test has been proposed as a tool to detect mitochondrial dysfunction in alcoholic liver disease. The aim of this study was to evaluate if the 2-keto[1-(13)C]isocaproate breath test could detect in vivo dynamic changes on mitochondrial activity due to caloric restriction in obese women. Fifteen obese women (body mass index [BMI] > 30 kg/m(2)) participated in the study at baseline. Ten of these women agreed to participate on a diet program to induce body weight loss. Fifteen lean women (BMI < 25 kg/m(2)) were included as a control group. The breath test was performed by the oral administration of the tracer measuring (13)CO(2) enrichment in breath before and after ingestion using isotope ratio mass spectrometry. Body composition, resting energy expenditure, and plasma levels of insulin and leptin were measured. There were no relationships observed between the 2-keto[1-(13)C]isocaproate breath test and the plasma insulin (before diet: P =.863; after diet: P =.879), or leptin (before diet: P =.500; after diet: P =.637). In obese women before treatment, kilograms of fat free mass (P =.108), resting energy expenditure adjusted for body composition (P =.312), and the 2-keto[1-(13)C]isocaproate breath test (P =.205) were similar in comparison to lean women. However, 2-keto[1-(13)C]isocaproate oxidation tended to increase after dieting and was significantly higher than in controls (P =.015). These data suggest that the 2-keto[1-(13)C]isocaproate breath test reflected the adaptive modifications in mitochondrial oxidation in response to caloric restriction in obese women.

Review article: breath testing for human liver function assessment

Carbon-labelled breath tests were proposed as tools for the evaluation of human liver function 30 years ago, but have never become part of clinical routine. One reason for this is the complex role of the liver in metabolic regulation, making it difficult to provide essential information for the management of patients with liver disease with a single test and to satisfy the hepatology community. As a result, a battery of breath tests have been developed. Depending on the test compound administered, different metabolic pathways (microsomal, cytosolic, mitochondrial) can be examined. Most available data come from microsomal function tests, whilst information about cytosolic and mitochondrial liver function is more limited. However, breath tests have shown promise in some studies, in particular to predict the outcome of patients with chronic liver disease or to monitor hepatic function after treatment. Whilst we await new substrates that can be used to measure liver function in a more valid manner, and large prospective studies to assess the usefulness of available test compounds, the aim of this review is to describe how far we have come in this controversial and unresolved issue.