Dose-dependent quantitative effects of acute fructose administration on hepatic de novo lipogenesis in healthy humans

Fructose feeding increases hepatic de novo lipogenesis (DNL) and is associated with nonalcoholic fatty liver disease. Little is known, however, about individual variation in susceptibility to fructose stimulation of DNL. In this three-period crossover study, 17 healthy male subjects were enrolled to evaluate the within- and between-subject variability of acute fructose feeding on hepatic fractional DNL. During each assessment, [1-13C1]acetate was infused to measure DNL in the fasting state and during fructose feeding. Subjects randomly received a high dose of fructose (10 mg·kg fat-free mass−1·min−1) on two occasions and a low dose (5 mg·kg fat-free mass−1·min−1) on another. Fructose solutions were administered orally every 30 min for 9.5 h. Ten subjects completed all three study periods. DNL was assessed as the fractional contribution of newly synthesized palmitate into very-low-density lipoprotein triglycerides using mass isotopomer distribution analysis. Mean fasting DNL was 5.3 ± 2.8%, with significant within- and between-subject variability. DNL increased dose dependently during fructose feeding to 15 ± 2% for low- and 29 ± 2% for high-dose fructose. The DNL response to high-dose fructose was very reproducible within an individual ( r = 0.93, P < 0.001) and independent of fasting DNL. However, it was variable between individuals and significantly correlated to influx of unlabeled acetyl-CoA ( r = 0.7, P < 0.001). Unlike fasting DNL, fructose-stimulated DNL is a robust and reproducible measure of hepatic lipogenic activity for a given individual and may be a useful indicator of metabolic disease susceptibility and treatment response.

Utility of high resolution accurate mass spectrometry (HRMS) in the mass isotopomer distribution analysis (MIDA) of CSF proteins modified by stable isotope labeling in mammals (SILAM) methodology applied to neurodegenerative diseases

Stable isotope labeling of proteins affords indicators at the molecular level, specifically biomarkers, which may providein vivodata on disease diagnosis, progression, and treatment.

Assessment of cardiac proteome dynamics with heavy water: slower protein synthesis rates in interfibrillar than subsarcolemmal mitochondria

Traditional proteomics provides static assessment of protein content, but not synthetic rates. Recently, proteome dynamics with heavy water (2H2O) was introduced, where 2H labels amino acids that are incorporated into proteins, and the synthesis rate of individual proteins is calculated using mass isotopomer distribution analysis. We refine this approach with a novel algorithm and rigorous selection criteria that improve the accuracy and precision of the calculation of synthesis rates and use it to measure protein kinetics in spatially distinct cardiac mitochondrial subpopulations. Subsarcolemmal mitochondria (SSM) and interfibrillar mitochondria (IFM) were isolated from adult rats, which were given 2H2O in the drinking water for up to 60 days. Plasma 2H2O and myocardial 2H-enrichment of amino acids were stable throughout the experimental protocol. Multiple tryptic peptides were identified from 28 proteins in both SSM and IFM and showed a time-dependent increase in heavy mass isotopomers that was consistent within a given protein. Mitochondrial protein synthesis was relatively slow (average half-life of 30 days, 2.4% per day). Although the synthesis rates for individual proteins were correlated between IFM and SSM ( R2 = 0.84; P < 0.0001), values in IFM were 15% less than SSM ( P < 0.001). In conclusion, administration of 2H2O results in stable enrichment of the cardiac precursor amino acid pool, with the use of refined analytical and computational methods coupled with cell fractionation one can measure synthesis rates for cardiac proteins in subcellular compartments in vivo, and protein synthesis is slower in mitochondria located among the myofibrils than in the subsarcolemmal region.

Metabolic pathways promoting intrahepatic fatty acid accumulation in methionine and choline deficiency: implications for the pathogenesis of steatohepatitis

The pathological mechanisms that distinguish simple steatosis from steatohepatitis (or NASH, with consequent risk of cirrhosis and hepatocellular cancer) remain incompletely defined. Whereas both a methionine- and choline-deficient diet (MCDD) and a choline-deficient diet (CDD) lead to hepatic triglyceride accumulation, MCDD alone is associated with hepatic insulin resistance and inflammation (steatohepatitis). We used metabolic tracer techniques, including stable isotope ([13C4]palmitate) dilution and mass isotopomer distribution analysis (MIDA) of [13C2]acetate, to define differences in intrahepatic fatty acid metabolism that could explain the contrasting effect of MCDD and CDD on NASH in C57Bl6 mice. Compared with control-supplemented (CS) diet, liver triglyceride pool sizes were similarly elevated in CDD and MCDD groups (24.37 ± 2.4, 45.94 ± 3.9, and 43.30 ± 3.5 μmol/liver for CS, CDD, and MCDD, respectively), but intrahepatic neutrophil infiltration and plasma alanine aminotransferase (31 ± 3, 48 ± 4, 231 ± 79 U/l, P < 0.05) were elevated only in MCDD mice. However, despite loss of peripheral fat in MCDD mice, neither the rate of appearance of palmitate (27.2 ± 3.5, 26.3 ± 2.3, and 28.3 ± 3.5 μmol·kg−1·min−1) nor the contribution of circulating fatty acids to the liver triglyceride pool differed between groups. Unlike CDD, MCDD had a defect in hepatic triglyceride export that was confirmed using intravenous tyloxapol (142 ± 21, 122 ± 15, and 80 ± 7 mg·kg−1·h−1, P < 0.05). Moreover, hepatic de novo lipogenesis was significantly elevated in the MCDD group only (1.4 ± 0.3, 2.3 ± 0.4, and 3.4 ± 0.4 μmol/day, P < 0.01). These findings suggest that important alterations in hepatic fatty acid metabolism may promote the development of steatohepatitis. Similar mechanisms may predispose to hepatocyte damage in human NASH.

Determination of protein synthesis in vivo using labeling from deuterated water and analysis of MALDI-TOF spectrum

This paper describes a method of determining protein synthesis and turnover using in vivo labeling of protein with deuterated water and analysis of matrix-assisted laser desorption time-of-flight mass spectrometer (MALDI-TOF) spectrum. Protein synthesis is calculated using mass isotopomer distribution analysis instead of precursor to product amino acid enrichment ratio. During protein synthesis, the incorporation of deuterium from water changes the mass isotopomer distribution (isotope envelop) according to the number of deuterium atoms (0, 1, 2, 3, etc.) incorporated, and the distribution of the protein with 0, 1, 2, 3,… atoms of deuterium follows a binomial distribution. A mathematical algorithm by which the distribution of deuterium isotopomers can be extracted from the observed MALDI-TOF spectrum is presented. Since deuterium isotopomers are unique to newly synthesized proteins, the quantitation of their distribution provides a method for the quantitation of newly synthesized proteins. The combined use of postsource decay sequence identification and mass isotopomer distribution analysis makes the use of in vivo labeling with deuterated water a precise method to determine specific protein synthesis.