Hypothesis: control of hepatic utilization of alanine by membrane transport or by cellular metabolism? Review

Plasma metabolic fingerprinting of childhood obesity by GC/MS in conjunction with multivariate statistical analysis

Metabolic fingerprinting is a powerful tool for exploring systemic metabolic perturbations and potential biomarkers, thus may shed light on the pathophysiological mechanism of diseases. In this work, a new strategy of metabolic fingerprinting was proposed to exploit the disturbances of metabolic patterns and biomarker candidates of childhood obesity. Plasma samples from children with normal weight, overweight and obesity were first profiled by GC/MS. ULDA (uncorrelated linear discriminant analysis) then revealed that the metabolic patterns of the three groups were different. Furthermore, several metabolites, say isoleucine, glyceric acid, serine, 2,3,4-trihydroxybutyric acid and phenylalanine were screened as potential biomarkers of childhood obesity by both ULDA and CCA (canonical correlation analysis). CCA also shows satisfactory correlation between the metabolic patterns and clinical parameters, and the results further suggest that WHR (waist-hip ratio) together with TG (total triglycerides), TC (total cholesterol), HDL (high density lipoprotein) and LDL (low density lipoprotein) were the most important parameters which are associated closely with the metabolic perturbations of childhood obesity, so as to be paid more attention for dealing with metabolic disturbances of childhood obesity in clinical practice rather than regularly monitored BMI (body-mass index). The results have demonstrated that the proposed metabolic fingerprinting approach may be a useful tool for discovering metabolic abnormalities and possible biomarkers for childhood obesity.

Selective up-regulation of system a transporter mRNA in diabetic liver

The transport of alanine by system A is an important source of carbons for the synthesis of glucose in the liver. Here, we show that the mRNA encoding the ubiquitously expressed isoform of the rat system A transporter (SAT2) is dramatically increased in liver following streptozotocin-induced diabetes. This increase in SAT2 mRNA is intensified in the gluconeogenic periportal hepatocytes and also in hepatocytes surrounding the central vein. SAT3, the more abundant system A mRNA isoform present in liver, is restricted to perivenous hepatocytes and is also increased following this treatment but to a much lesser extent than SAT2 mRNA. SN1, an abundant system N mRNA isoform expressed in both perivenous and periportal hepatocytes, is not affected by streptozotocin treatment. A pharmacological dose of glucagon also increased both SAT2 and SAT3 mRNA levels in liver while SN1 mRNA levels remained unaffected. These results indicate that the increase in system A activity observed in liver following experimentally induced diabetes or glucagon treatment is due to the selective increase in mRNAs encoding system A transporters.

Effects of VIP and forskolin on alanine metabolism in isolated hepatocytes

The effects of vasoactive intestinal polypeptide (VIP) and of forskolin on alanine metabolism in hepatocytes isolated from fed and fasted rats were examined. VIP and 17 microM forskolin stimulated glucose production, gluconeogenesis from alanine, and ureagenesis, and inhibited glyconeogenesis to comparable degrees. However, combination of 17 microM forskolin with a maximal dose of VIP significantly augmented only the inhibition of glyconeogenesis. At 100 microM, forskolin induced metabolic responses comparable to those induced by glucagon, but similarly, in combination with maximal doses of VIP or glucagon, augmented only inhibition of glycogen synthesis. In addition to demonstrating modulation of alanine metabolism by VIP and forskolin, these results raise questions about the nature of the coupling between VIP receptor occupancy and metabolic response.

Stimulation of rat hepatic amino acid transport by burn injury

Burn injury accelerates hepatic amino acid metabolism, but the role of transmembrane substrate delivery in this response has not been investigated. We therefore studied the effects of cutaneous scald injury on the Na+-dependent transport of glutamine and alanine in isolated rat liver plasma membrane vesicles. Scald injury resulted in liver damage and a 1.4- to 2.3-fold and 1.5- to 2.8-fold stimulation of hepatic transport rates for glutamine and alanine, respectively, proportional to the total burned surface area (TBSA) after 24 hours. Enhanced uptake of glutamine and alanine was attributable to increases in the maximum velocity (Vmax) of system N and system A activities, respectively. Hepatic amino acid transport activity remained elevated in vesicles from burned animals after 72 hours, but the degree of stimulation (1.3- to 1.7-fold for glutamine and 1.3- to 1.6-fold for alanine) was less than that observed 24 hours after thermal injury. Liver function tests returned to control values after 72 hours as well, indicating rectification of hepatic damage. In contrast to the induction of hepatic system A and system N activity in catabolic states such as cancer and endotoxemia, further studies showed that tumor necrosis factor (TNF) failed to play a significant role in burn-stimulated amino acid transport rates. When combined with plasma liver enzyme profiles, early transient hepatic amino acid transporter stimulation may support amino acid-dependent pathways involved in the repair of burn-dependent hepatic damage.

Alanine transport in rat liver plasma membrane vesicles during the acute-phase response in young and old rats

The Na(+)-dependent transport of L-alanine into liver plasma membrane vesicles isolated from young, 2-month-old, and aged, 24-month-old, normal and turpentine-treated rats, to induce an aseptic inflammatory condition and the acute-phase response, was studied. In old rats, alanine transport maintained the same features observed in young controls, being strictly Na(+)-and electrical potential-dependent. However, old rats showed a twofold increase in the Vmax value for alanine uptake compared with young controls, the affinity constant (Km) remaining unmodified. Four hours after turpentine treatment, the Vmax value of alanine transport in young rats was 2.5-fold over that observed in untreated controls. In contrast, old turpentine-treated rats demonstrated only slight increases in this value, compared to untreated old controls. In both young and old turpentine-treated rats, the Km values of alanine transport remained almost unmodified. Since dysregulation of interleukin 6 (IL-6) gene expression, a multifunctional cytokine that is proving to be a major contributor to the acute phase response, occurs with advancing age in different animal species, the increased alanine transport across liver plasma membrane vesicles observed in old "normal" rats might be related to an increased IL-6 production, inducing an acute phase protein synthetic activity, in these animals.

Ontogeny and plasma-membrane domain localization of amino acid transport system L in rat liver

Na(+)-independent hepatic transport of branched-chain amino acids occurs via at least two distinct transport processes. System L1, characterized by micromolar Km values, predominates in hepatoma and fetal hepatocytes, whereas System L2, distinguished by Km values in the millimolar range and sensitivity to inhibition by N-ethylmaleimide (NEM), predominates in adult hepatocytes. To determine the plasma-membrane domain localization and ontogeny of System L activity in the rat, we prepared membrane vesicles from the livers of suckling (10 days old) and adult rats enriched for either basolateral (BLMV) or canalicular (CMV) domains. The initial rate of [3H]leucine uptake into BLMV and CMV derived from adult liver was significantly inhibited by the addition of 5 mM NEM; transport into BLMV and CMV derived from 10-day-old rat liver was not affected. Michaelis-Menten kinetic parameters estimated in BLMV derived from adult liver were consistent with System L2 (Km = 2.16 +/- 0.62 mM, Vmax. = 781 +/- 109 pmol/5 s per mg of protein), as were those estimated in adult CMV (Km = 0.83 +/- 0.21 mM, Vmax. = 385 +/- 38 pmol/5 s per mg of protein). Conversely, kinetic parameters estimated in BLMV derived from livers of suckling rats were consistent with System L1 (Km = 0.041 +/- 0.024 mM, Vmax. = 8.8 +/- 1.5 pmol/5 s per mg of protein), as were those from CMV of suckling rats (Km = 0.023 +/- 0.09 mM, Vmax. = 28.1 +/- 2.1 pmol/5 s per mg of protein). We conclude that NEM-inhibitable Na(+)-independent leucine transport activity consistent with System L2 is present in both BLMV and CMV derived from adult rat liver, whereas System L1 predominates in 10-day-old rat liver tissue.

Differences in L-alanine uptake by livers of Wistar and lean Zucker rats

The L-alanine uptake by livers of Wistar and lean Zucker rats has been studied. The hepatic uptake and fractional extraction rates of alanine were estimated in 50-55 day old rats. No significant differences in amino acid concentrations and blood flows in afferent and efferent liver vessels were seen in lean Zucker rats when compared with Wistar rats. However, the hepatic uptake (1.6 +/- 0.1 and 0.7 +/- 0.1 mumol/min/100 g bw, p less than 0.01) and the fractional extraction (26.8 +/- 2.1 and 15.2 +/- 3.1%, p less than 0.05) were much lower in Zucker than in Wistar rats. The hepatic active transport of L-alanine was determined in vitro using isolated plasma membrane vesicles. Vesicles isolated from livers of lean Zucker rats showed similar values of Km (2.5 +/- 0.7 vs. 2.0 +/- 0.5 mM for Wistar and Zucker respectively, N.S.), but lower values of Vmax when compared with Wistar rats (1.1 +/- 0.1 vs 0.6 +/- 0.005 nmol/mg prot 5 s, p less than 0.01, for Wistar and lean Zucker rats respectively). These results indicate that, the liver of lean Zucker rats concentrates alanine less efficiently than the liver of Wistar rats. This fact correlates well with a lower capacity of the Na(+)-dependent L-alanine transport in liver plasma membrane vesicles from lean Zucker rats.

Fluxes and membrane transport of amino acids in rat liver under different protein diets

The aim of the present work was to evaluate in vivo the role of the transport step in hepatic amino acid metabolism. To vary hepatic utilization of amino acids, rats were adapted to diets containing various concentrations of casein (5, 15, and 60%). In rats fed 5 or 15% casein diets, Gln and Glu were released by the liver, and there was a significant uptake of Ala. Hepatic fluxes of amino acids increased considerably after adaptation to high-casein diet (up to 1.55 mumol.min-1.g liver-1 for Ala), because of the rise in afferent concentrations as well as enhanced uptake percentage (peaking at 60-75% for most glucogenic amino acids). Adaptation to a high-protein diet led to induction of not only system A but also of most of the other transport systems (Gly, anionic, T, y+, and to a lesser extent system N); only systems ASC and L were unchanged. The study of amino acid repartition between liver and plasma with different diets indicates that transport could modulate utilization of Ala, Ser, Thr, Gly, Gln, and Asp. For Arg and Asn, present in very low concentrations in liver under any condition, the transport step should be the major locus of control of their metabolism. For amino acids chiefly transported by nonconcentrative systems, such as aromatic amino acids, cellular metabolism could also be limited by the transport process. In conclusion, during adaptation to a high-protein diet, there is apparently a coordinated adaptation of amino acid transport and of their intracellular metabolism. For some amino acids, induction of catabolic enzymes seems greater than that of transport, so that the transport step may play an important role in control of metabolic fluxes. For example, concentration of amino acids such as Thr may be markedly depressed in rats adapted to a high-protein diet.

Na(+)-dependent transport of alanine and serine by liver plasma-membrane vesicles from rats fed a low-protein or a high-protein diet

Plasma-membrane vesicles prepared from the liver of rats fed either a low-(LP) or a high-protein (HP) diet exhibited Na(+)-dependent active transport of alanine and serine. The process gave apparent kinetic parameters compatible with a single saturable component for both amino acids. Na,K-ATPase (EC 3.6.1.37), marker of the basolateral domain of the hepatocyte plasma-membrane, was chosen as reference for the expression of amino acid transport in vesicle preparations. The high-protein diet induced a significant increase in liver Na,K-ATPase activity also found in corresponding plasma-membrane preparations, in parallel with an increase in the capacity towards amino acid transport. This suggests that in rats fed the high protein diet, transcellular Na+ exchange, although increased, remains well balanced. N-Methylaminoisobutyric acid (MeAIB), due to its poor velocity, proved unsuitable to distinguish between systems A and ASC in the experimental model. Comparing Na(+)- and Li(+)-driven transport, a family of carriers with strict Na(+)-dependency (A-like) was evidenced in LP vesicles but not in HP vesicles. The sensitivity to the lowering of the pH from 7.5 to 6.5 in the external medium was similar in both type of vesicles when Na+ was the driving ion. In the HP vesicles the Li(+)-tolerant, pH-insensitive component (ASC-like) was increased in parallel with overall Na(+)-dependent transport. These functional properties suggest that the carriers involved in the stimulation of transport in HP vesicles are composite in nature. Increasing concentrations of an amino acid mixture mimicking the changes of portal aminoacidemia inhibited the transport of alanine and of serine. The degree of inhibition was correlated with the relative concentration of substrate and was independent of the nutritional treatment.

Histidine transport in plasma membrane vesicles from rat liver

The transport of L-histidine, a selective substrate for system N, across liver plasma membrane vesicles has been studied. The amino acid is accumulated sevenfold within the intravesicular space in the presence of a sodium gradient. Lithium can replace sodium to some extent in the cotransport mechanism. The amino acid translocation is not influenced by the transmembrane electrical potential difference. Histidine kinetics involves a saturable component plus a linear one both in the presence and in the absence of sodium. Sodium affects mainly the affinity of the translocator and to a lesser extent its mobility. Histidine uptake is competitively inhibited by glutamine and it is affected by alanine in a noncompetitive manner.

Organic ion transport during seven decades. The amino acids

The amino acids are ions of various charge combinations, and one can argue that historically they were the first ions for which the ongoing problem of membrane transport was presented; also that among transported ions these may undergo a highly detailed molecular recognition. Furthermore, the distribution of charge on the amino acid molecule determines by what route or routes it is conducted across the biological membrane, with what directional and structural specificity, and therefore what regulation is imposed, and where. Cases where a presumably charged chemical group behaves as if it were somehow absent from the amino acid have been observed to fall into several categories: Straightforward cases where the pH has been low enough or high enough to remove the charge by protonation or deprotonation, even in free solution. Cases where that protonation or deprotonation is facilitated at the binding site, and perhaps by the total transport process. The cystine molecule can apparently thus be rendered either a tripolar anion or a tripolar cation for transport. Cases where an otherwise co-transported Na+ is omitted to redress charge, or where a Na+ serves as a surrogate for a missing charged group on the amino acid molecule. A case where the protonation occurs reversibly at the receptor site rather than on the amino acid molecule.