Synthesis of essential amino acids from their alpha-keto analogues by perfused rat liver and muscle

Ketoanalogues Supplemental Low Protein Diet Safely Decreases Short-Term Risk of Dialysis among CKD Stage 4 Patients

Background: Rigid dietary controls and pill burden make a very-low protein (0.3−0.4 g/kg body weight per day), vegetarian diet supplemented with ketoanalogues of amino acids (sVLPD) hard to follow in the long-term. This study aimed to evaluate whether a ketoanalogue supplemental low-protein diet (sLPD) (0.6 g/kg body weight per day) could also reduce the risks of dialysis among CKD stage 4 patients. Methods: Patients aged >20 years with a diagnosis of stage 4 CKD who subsequently received ketosteril treatment, which is the most commonly used ketoanalogue of essential amino acids, between 2003 and 2018 were identified from the Chang Gung Research Database (CGRD). Then, these individuals were divided into two groups according to the continuation of ketosteril for more than three months or not. The primary outcome was ESKD requiring maintenance dialysis. Results: With one-year follow-up, the continuation group (n = 303) exhibited a significantly lower incidence of new-onset end-stage kidney disease (ESKD) requiring maintenance dialysis (6.8% vs. 10.4%, hazard ratio [HR]: 0.62, 95% confidence interval [CI]: 0.41−0.94) in comparison to the discontinuation group (n = 238). Conclusions: This study demonstrated that initiating sLPDs since CKD stage 4 may additionally reduce the short-term risks of commencing dialysis without increasing CV events, infections, or mortality.

Supplemented Low-Protein Diet May Delay the Need for Preemptive Kidney Transplantation: A Nationwide Population-Based Cohort Study

BACKGROUND

Although several studies suggest the benefit of a low-protein diet supplemented with amino acids and keto acids (sLPD) in delaying the initiation of hemodialysis, evidence on whether these nutritional approaches could delay the timing of preemptive transplantation is lacking.

METHODS

Retrospective nationwide cohort study, from Taiwan's National Health Insurance Research Database. Patients having undergone a first preemptive kidney transplantation between 2001 and 2017 were identified and divided into two groups according to the presence of sLPD treatment or not. The primary outcome was the time between the diagnosis of advanced CKD and transplantation. Secondary outcomes were post-transplantation adverse events.

RESULTS

A total of 245 patients who received their first preemptive kidney transplantation were identified from the nationwide database; 63 of them had been on an sLPD prior to transplantation (sLPD group). The duration between the day of advanced CKD diagnosis and the day of transplantation was significantly longer in the sLPD group compared with the non-sLPD group (median duration: 345 vs. 220 days, p = 0.001). The risk of post-transplantation adverse events did not differ between the two groups.

CONCLUSIONS

Within the limits of its observational, retrospective design, this is the first study to suggest that nutritional management with sLPDs can safely delay the timing of preemptive kidney transplantation.

Medical Nutritional Therapy for Patients with Chronic Kidney Disease not on Dialysis: The Low Protein Diet as a Medication

The 2020 Kidney Disease Outcome Quality Initiative (KDOQI) Clinical Practice Guideline for Nutrition in chronic kidney disease (CKD) recommends protein restriction to patients affected by CKD in stages 3 to 5 (not on dialysis), provided that they are metabolically stable, with the goal to delay kidney failure (graded as evidence level 1A) and improve quality of life (graded as evidence level 2C). Despite these strong statements, low protein diets (LPDs) are not prescribed by many nephrologists worldwide. In this review, we challenge the view of protein restriction as an "option" in the management of patients with CKD, and defend it as a core element of care. We argue that LPDs need to be tailored and patient-centered to ensure adherence, efficacy, and safety. Nephrologists, aligned with renal dietitians, may approach the implementation of LPDs similarly to a drug prescription, considering its indications, contra-indications, mechanism of action, dosages, unwanted side effects, and special warnings. Following this framework, we discuss herein the benefits and potential harms of LPDs as a cornerstone in CKD management.

Branched-Chain Amino Acids and Branched-Chain Keto Acids in Hyperammonemic States: Metabolism and as Supplements

In hyperammonemic states, such as liver cirrhosis, urea cycle disorders, and strenuous exercise, the catabolism of branched-chain amino acids (BCAAs; leucine, isoleucine, and valine) is activated and BCAA concentrations decrease. In these conditions, BCAAs are recommended to improve mental functions, protein balance, and muscle performance. However, clinical trials have not demonstrated significant benefits of BCAA-containing supplements. It is hypothesized that, under hyperammonemic conditions, enhanced glutamine availability and decreased BCAA levels facilitate the amination of branched-chain keto acids (BCKAs; α-ketoisocaproate, α-keto-β-methylvalerate, and α-ketoisovalerate) to the corresponding BCAAs, and that BCKA supplementation may offer advantages over BCAAs. Studies examining the effects of ketoanalogues of amino acids have provided proof that subjects with hyperammonemia can effectively synthesize BCAAs from BCKAs. Unfortunately, the benefits of BCKA administration have not been clearly confirmed. The shortcoming of most reports is the use of mixtures intended for patients with renal insufficiency, which might be detrimental for patients with liver injury. It is concluded that (i) BCKA administration may decrease ammonia production, attenuate cataplerosis, correct amino acid imbalance, and improve protein balance and (ii) studies specifically investigating the effects of BCKA, without the interference of other ketoanalogues, are needed to complete the information essential for decisions regarding their suitability in hyperammonemic conditions.

Does a Supplemental Low-Protein Diet Decrease Mortality and Adverse Events After Commencing Dialysis? A Nationwide Cohort Study

Background: A beneficial effect of a ketoanalogue-supplemented low-protein diet (sLPD) in postponing dialysis has been demonstrated in numerous previous studies. However, evidence regarding its effect on long-term survival is limited. Our study assessed the long-term outcomes of patients on an sLPD after commencing dialysis. Methods: This retrospective study examined patients with new-onset end-stage renal disease with permanent dialysis between 2001 and 2013, extracted from Taiwan’s National Health Insurance Research Database. Patients who received more than 3 months of sLPD treatment in the year preceding the start of dialysis were extracted. The outcomes studied were all-cause mortality, infection rate, and major cardiac and cerebrovascular events (MACCEs). Results: After propensity score matching, the sLPD group (n = 2607) showed a lower risk of all-cause mortality (23.1% vs. 27.6%, hazard ratio (HR) 0.77, 95% confidence interval (CI) 0.70–0.84), MACCEs (19.2% vs. 21.5%, HR 0.86, 95% CI 0.78–0.94), and infection-related death (9.9% vs. 12.5%, HR 0.76, 95% CI 0.67–0.87) than the non-sLPD group did. Conclusion: We found that sLPD treatment might be safe without long-term negative consequences after dialysis treatment.

Double-blind placebo-controlled multicenter phase II trial to evaluate D-methionine in preventing/reducing oral mucositis induced by radiation and chemotherapy for head and neck cancer

BACKGROUND

The purpose of this study was to test if oral D-methionine (D-met) reduced mucositis during chemoradiotherapy.

METHODS

We conducted a placebo-controlled double-blind randomized phase II trial of D-met (100 mg/kg p.o. b.i.d.) testing the rate of severe (grades 3-4) mucositis.

RESULTS

Sixty patients were randomized. Grade 2 + oral pain was higher with placebo (79% vs 45%; P = .0165), whereas grade 2 + body odor was greater with D-met (3% vs 41%; P = .0015). Mucositis was decreased with D-met by the physician (World Health Organization [WHO], P = .007; Radiation Therapy Oncology Group [RTOG], P = .009) and patient functional scales (RTOG, P = .0023). The primary end point of grades 3 to 4 mucositis on the composite scale demonstrated a decrease with D-met (48% vs 24%; P = .058), which was borderline in significance. A planned secondary analysis of a semiquantitative scoring system noted decreased oral ulceration (2.2 vs 1.5; P = .023) and erythema (1.6 vs 1.1; P = .048) with D-met.

CONCLUSION

Although not meeting the primary end point, results of multiple assessments suggest that D-met decreased mucositis.

Retarding Chronic Kidney Disease (CKD) Progression: A Practical Nutritional Approach for Non-Dialysis CKD

This is a case report on a patient with non-dialysis chronic kidney disease (CKD) in whom several nutritional issues are briefly discussed from a practical point of view. The article is accompanied by an editorial published in this Journal in relation to the 2nd International Conference of the European Renal Nutrition working group at ERA-EDTA—“Retarding CKD progression: readily available through comprehensive nutritional management?”— and focuses on several practical topics associated with the nutritional approach for the conservative treatment of non-dialysis CKD. The article is divided into 3 sections—basic nutritional assessment, nutritional targets, and nutritional follow-up in non-dialysis CKD—linked to 3 consecutive steps of the clinical follow-up of the patient and the related nutritional concerns and intervention.

First visit: Baseline nutritional assessment and basic nutritional considerations in non-dialysis chronic kidney disease (CKD) • What nutritional assessment/monitoring for protein-energy wasting (PEW) should be employed? • Is a body mass index (BMI) of 21 kg/m2 adequate? • What phosphate target should be pursued? • What are the nutritional habits in patients with incident CKD? • What protein needs and amount of dietary protein should be pursued? • Does the quality of protein matter? • What amount of dietary salt should be employed? How should this be obtained? • How should normal serum phosphate be achieved? • What diet should be recommended? Is a vegetarian diet an option?

Second visit: Major nutritional targets in non-dialysis CKD • Consequences of unintentional weight loss • What is the role of the renal dietitian in helping the patient adhere to a renal diet?

Intermediate visits: Nutritional follow-up in non-dialysis CKD • What treatment for calcium/parathyroid hormone (PTH) will affect CKD progression? Final visits: • Would a dietary recall/intensive dietary education improve adherence with the diet? • Would a very-low-protein diet (VLPD)/ketodiet be indicated for this patient?

Low-protein diets for chronic kidney disease patients: the Italian experience

BACKGROUND

Nutritional treatment has always represented a major feature of CKD management. Over the decades, the use of nutritional treatment in CKD patients has been marked by several goals. The first of these include the attainment of metabolic and fluid control together with the prevention and correction of signs, symptoms and complications of advanced CKD. The aim of this first stage is the prevention of malnutrition and a delay in the commencement of dialysis. Subsequently, nutritional manipulations have also been applied in association with other therapeutic interventions in an attempt to control several cardiovascular risk factors associated with CKD and to improve the patient's overall outcome. Over time and in reference to multiple aims, the modalities of nutritional treatment have been focused not only on protein intake but also on other nutrients.

DISCUSSION

This paper describes the pathophysiological basis and rationale of nutritional treatment in CKD and also provides a report on extensive experience in the field of renal diets in Italy, with special attention given to approaches in clinical practice and management. Italian nephrologists have a longstanding tradition in implementing low protein diets in the treatment of CKD patients, with the principle objective of alleviating uremic symptoms, improving nutritional status and also a possibility of slowing down the progression of CKD or delaying the start of dialysis. A renewed interest in this field is based on the aim of implementing a wider nutritional therapy other than only reducing the protein intake, paying careful attention to factors such as energy intake, the quality of proteins and phosphate and sodium intakes, making today's low-protein diet program much more ambitious than previous. The motivation was the reduction in progression of renal insufficiency through reduction of proteinuria, a better control of blood pressure values and also through correction of metabolic acidosis. One major goal of the flexible and innovative Italian approach to the low-protein diet in CKD patients is the improvement of patient adherence, a crucial factor in the successful implementation of a low-protein diet program.

Is there a role for ketoacid supplements in the management of CKD?

Ketoacid (KA) analogues of essential amino acids (EAAs) provide several potential advantages for people with advanced chronic kidney disease (CKD). Because KAs lack the amino group bound to the α carbon of an amino acid, they can be converted to their respective amino acids without providing additional nitrogen. It has been well established that a diet with 0.3 to 0.4 g of protein per kilogram per day that is supplemented with KAs and EAAs reduces the generation of potentially toxic metabolic products, as well as the burden of potassium, phosphorus, and possibly sodium, while still providing calcium. These KA/EAA-supplemented very-low-protein diets (VLPDs) can maintain good nutrition, but the appropriate dose of the KA/EAA supplement has not been established. Thus, a KA/EAA dose-response study for good nutrition clearly is needed. Similarly, the composition of the KA/EAA supplement needs to be reexamined; for example, some KA/EAA preparations contain neither the EAA phenylalanine nor its analogue. Indications concerning when to inaugurate a KA/EAA-supplemented VLPD therapy also are unclear. Evidence strongly suggests that these diets can delay the need for maintenance dialysis therapy, but whether they slow the loss of glomerular filtration rate in patients with CKD is less clear, particularly in this era of more vigorous blood pressure control and use of angiotensin/aldosterone blockade. Some clinicians prescribe KA/EAA supplements for patients with CKD or treated with maintenance dialysis, but with diets that have much higher protein levels than the VLPDs in which these supplements have been studied. More research is needed to examine the effectiveness of KA/EAA supplements with higher protein intakes.

α-Ketoglutaramate: an overlooked metabolite of glutamine and a biomarker for hepatic encephalopathy and inborn errors of the urea cycle

Glutamine metabolism is generally regarded as proceeding via glutaminase-catalyzed hydrolysis to glutamate and ammonia, followed by conversion of glutamate to α-ketoglutarate catalyzed by glutamate dehydrogenase or by a glutamate-linked aminotransferase (transaminase). However, another pathway exists for the conversion of glutamine to α-ketoglutarate that is often overlooked, but is widely distributed in nature. This pathway, referred to as the glutaminase II pathway, consists of a glutamine transaminase coupled to ω-amidase. Transamination of glutamine results in formation of the corresponding α-keto acid, namely, α-ketoglutaramate (KGM). KGM is hydrolyzed by ω-amidase to α-ketoglutarate and ammonia. The net glutaminase II reaction is: L - Glutamine + α - keto acid + H2O → α - ketoglutarate + L - amino acid + ammonia. In this mini-review the biochemical importance of the glutaminase II pathway is summarized, with emphasis on the key component KGM. Forty years ago it was noted that the concentration of KGM is increased in the cerebrospinal fluid (CSF) of patients with hepatic encephalopathy (HE) and that the level of KGM in the CSF correlates well with the degree of encephalopathy. In more recent work, we have shown that KGM is markedly elevated in the urine of patients with inborn errors of the urea cycle. It is suggested that KGM may be a useful biomarker for many hyperammonemic diseases including hepatic encephalopathy, inborn errors of the urea cycle, citrin deficiency and lysinuric protein intolerance.

Sites of superoxide and hydrogen peroxide production by muscle mitochondria assessed ex vivo under conditions mimicking rest and exercise

The sites and rates of mitochondrial production of superoxide and H2O2 in vivo are not yet defined. At least 10 different mitochondrial sites can generate these species. Each site has a different maximum capacity (e.g. the outer quinol site in complex III (site IIIQo) has a very high capacity in rat skeletal muscle mitochondria, whereas the flavin site in complex I (site IF) has a very low capacity). The maximum capacities can greatly exceed the actual rates observed in the absence of electron transport chain inhibitors, so maximum capacities are a poor guide to actual rates. Here, we use new approaches to measure the rates at which different mitochondrial sites produce superoxide/H2O2 using isolated muscle mitochondria incubated in media mimicking the cytoplasmic substrate and effector mix of skeletal muscle during rest and exercise. We find that four or five sites dominate during rest in this ex vivo system. Remarkably, the quinol site in complex I (site IQ) and the flavin site in complex II (site IIF) each account for about a quarter of the total measured rate of H2O2 production. Site IF, site IIIQo, and perhaps site EF in the β-oxidation pathway account for most of the remainder. Under conditions mimicking mild and intense aerobic exercise, total production is much less, and the low capacity site IF dominates. These results give novel insights into which mitochondrial sites may produce superoxide/H2O2 in vivo.

Acute hyperammonemia activates branched-chain amino acid catabolism and decreases their extracellular concentrations: different sensitivity of red and white muscle

Hyperammonemia is considered to be the main cause of decreased levels of the branched-chain amino acids (BCAA), valine, leucine, and isoleucine, in liver cirrhosis. In this study we investigated whether the decrease in BCAA is caused by the direct effect of ammonia on BCAA metabolism and the effect of ammonia on BCAA and protein metabolism in different types of skeletal muscle. M. soleus (SOL, slow-twitch, red muscle) and m. extensor digitorum longus (EDL, fast-twitch, white muscle) of white rat were isolated and incubated in a medium with or without 500 μM ammonia. We measured the exchange of amino acids between the muscle and the medium, amino acid concentrations in the muscle, release of branched-chain keto acids (BCKA), leucine oxidation, total and myofibrillar proteolysis, and protein synthesis. Hyperammonemia inhibited the BCAA release (81% in SOL and 60% in EDL vs. controls), increased the release of BCKA (133% in SOL and 161% in EDL vs. controls) and glutamine (138% in SOL and 145% in EDL vs. controls), and increased the leucine oxidation in EDL (174% of controls). Ammonia also induced a significant increase in glutamine concentration in skeletal muscle. The effect of ammonia on intracellular BCAA concentration, protein synthesis and on total and myofibrillar proteolysis was insignificant. The data indicates that hyperammonemia directly affects the BCAA metabolism in skeletal muscle which results in decreased levels of BCAA in the extracellular fluid. The effect is associated with activated synthesis of glutamine, increased BCAA oxidation, decreased release of BCAA, and enhanced release of BCKA. These metabolic changes are not directly associated with marked changes in protein turnover. The effect of ammonia is more pronounced in muscles with high content of white fibres.

Inhibitors of Pyruvate Carboxylase

This review aims to discuss the varied types of inhibitors of biotin-dependent carboxylases, with an emphasis on the inhibitors of pyruvate carboxylase. Some of these inhibitors are physiologically relevant, in that they provide ways of regulating the cellular activities of the enzymes e.g. aspartate and prohibitin inhibition of pyruvate carboxylase. Most of the inhibitors that will be discussed have been used to probe various aspects of the structure and function of these enzymes. They target particular parts of the structure e.g. avidin - biotin, FTP - ATP binding site, oxamate - pyruvate binding site, phosphonoacetate - binding site of the putative carboxyphosphate intermediate.

Evaluation of D-methionine as a novel oral radiation protector for prevention of mucositis

PURPOSE

Oral mucositis is a common acute morbidity associated with radiation and/or chemotherapy treatment for cancer. D-Methionine (D-Met), the dextro-isomer of the common amino acid l-methionine, has been documented to protect normal tissues from a diverse array of oxidative insults.

EXPERIMENTAL DESIGN

We evaluated if D-Met could selectively prevent radiation-induced oral mucositis using in vitro cell culture models as well as an in vivo model of radiation injury to the oral mucosa in C3H mice.

RESULTS

Unlike free-radical scavengers, which protected both normal and transformed tumor cells in vitro from radiation-induced cell death, treatment with d-Met in culture protected nontransformed primary human cells from radiation-induced cell death (protective factor between 1.2 and 1.6; P<0.05) whereas it did not confer a similar protection on transformed tumor cells. D-Met treatment also provided significant protection to normal human fibroblasts, but not to tumor cell lines, from radiation-induced loss of clonogenicity (protection factor, 1.6+/-0.15). D-Met treatment did not alter DNA damage (as measured by histone phosphorylation) following irradiation but seemed to selectively mitigate the loss of mitochondrial membrane potential in nontransformed cells, whereas it did not provide a similar protection to tumor cells. Tumor control of implanted xenografts treated with radiation or concurrent cisplatin and radiation was not altered by D-Met treatment. Pharmacokinetics following administration of a liquid suspension of D-Met in rats showed 68% bioavailability relative to i.v. administration. Finally, in a murine model of mucositis, a dose-dependent increase in protection was observed with the protective factor increasing from 1.6 to 2.6 over a range of oral D-Met doses between 200 and 500 mg/kg (P<0.0003).

CONCLUSIONS

D-Met protected normal tissues, but not tumor cells, in culture from radiation-induced cell death; it also protected normal cells from radiation-induced mucosal injury in a murine model but did not alter tumor response to therapy. Further studies on the use of D-Met to protect from oral mucositis are warranted.

Very low protein diet supplemented with ketoanalogs improves blood pressure control in chronic kidney disease

Blood pressure (BP) is hardly controlled in chronic kidney disease (CKD). We compared the effect of very low protein diet (VLPD) supplemented with ketoanalogs of essential amino acids (0.35 g/kg/day), low protein diet (LPD, 0.60 g/kg/day), and free diet (FD) on BP in patients with CKD stages 4 and 5. Vegetable proteins were higher in VLPD (66%) than in LPD (48%). LPD was prescribed to 110 consecutive patients; after run-in, they were invited to start VLPD. Thirty subjects accepted; 57 decided to continue LPD; 23 refused either diet (FD group). At baseline, protein intake (g/kg/day) was 0.79+/-0.09 in VLPD, 0.78+/-0.11 in LPD, and 1.11+/-0.18 in FD (P<0.0001). After 6 months, protein intake was lower in VLPD than LPD and FD (0.54+/-0.11, 0.78+/-0.10, and 1.04+/-0.21 g/kg/day, respectively; P<0.0001). BP diminished only in VLPD, from 143+/-19/84+/-10 to 128+/-16/78+/-7 mm Hg (P<0.0001), despite reduction of antihypertensive drugs (from 2.6+/-1.1 to 1.8+/-1.2; P<0.001). Urinary urea excretion directly correlated with urinary sodium excretion, which diminished in VLPD (from 181+/-32 to 131+/-36 mEq/day; P<0.001). At multiple regression analysis (R2=0.270, P<0.0001), BP results independently related to urinary sodium excretion (P=0.023) and VLPD prescription (P=0.003), but not to the level of protein intake. Thus, in moderate to advanced CKD, VLPD has an antihypertensive effect likely due to reduction of salt intake, type of proteins, and ketoanalogs supplementation, independent of actual protein intake.

Candidate's thesis: enhancing intrinsic cochlear stress defenses to reduce noise-induced hearing loss

OBJECTIVES/HYPOTHESIS

Oxidative stress plays a substantial role in the genesis of noise-induced cochlear injury that causes permanent hearing loss. We present the results of three different approaches to enhance intrinsic cochlear defense mechanisms against oxidative stress. This article explores, through the following set of hypotheses, some of the postulated causes of noise-induced cochlear oxidative stress (NICOS) and how noise-induced cochlear damage may be reduced pharmacologically. 1) NICOS is in part related to defects in mitochondrial bioenergetics and biogenesis. Therefore, NICOS can be reduced by acetyl-L carnitine (ALCAR), an endogenous mitochondrial membrane compound that helps maintain mitochondrial bioenergetics and biogenesis in the face of oxidative stress. 2) A contributing factor in NICOS injury is glutamate excitotoxicity, which can be reduced by antagonizing the action of cochlear -methyl-D-aspartate (NMDA) receptors using carbamathione, which acts as a glutamate antagonist. 3) Noise-induced hearing loss (NIHL) may be characterized as a cochlear-reduced glutathione (GSH) deficiency state; therefore, strategies to enhance cochlear GSH levels may reduce noise-induced cochlear injury. The objective of this study was to document the reduction in noise-induced hearing and hair cell loss, following application of ALCAR, carbamathione, and a GSH repletion drug D-methionine (MET), to a model of noise-induced hearing loss.

STUDY DESIGN

This was a prospective, blinded observer study using the above-listed agents as modulators of the noise-induced cochlear injury response in the species chinchilla langier.

METHODS

Adult chinchilla langier had baseline-hearing thresholds determined by auditory brainstem response (ABR) recording. The animals then received injections of saline or saline plus active experimental compound starting before and continuing after a 6-hour 105 dB SPL continuous 4-kHz octave band noise exposure. ABRs were obtained immediately after noise exposure and weekly for 3 weeks. After euthanization, cochlear hair cell counts were obtained and analyzed. RESULTS ALCAR administration reduced noise-induced threshold shifts. Three weeks after noise exposure, no threshold shift at 2 to 4 kHz and <10 dB threshold shifts were seen at 6 to 8 kHz in ALCAR-treated animals compared with 30 to 35 dB in control animals. ALCAR treatment reduced both inner and outer hair cell loss. OHC loss averaged <10% for the 4- to 10-kHz region in ALCAR-treated animals and 60% in saline-injected-noise-exposed control animals. Noise-induced threshold shifts were also reduced in carbamathione-treated animals. At 3 weeks, threshold shifts averaged 15 dB or less at all frequencies in treated animals and 30 to 35 dB in control animals. Averaged OHC losses were 30% to 40% in carbamathione-treated animals and 60% in control animals. IHC losses were 5% in the 4- to 10-kHz region in treated animals and 10% to 20% in control animals. MET administration reduced noise-induced threshold shifts. ANOVA revealed a significant difference (P <.001). Mean OHC and IHC losses were also significantly reduced (P <.001).

CONCLUSIONS

These data lend further support to the growing body of evidence that oxidative stress, generated in part by glutamate excitotoxicity, impaired mitochondrial function and GSH depletion causes cochlear injury induced by noise. Enhancing the cellular oxidative stress defense pathways in the cochlea eliminates noise-induced cochlear injury. The data also suggest strategies for therapeutic intervention to reduce NIHL clinically.

Relation between glutamine, branched-chain amino acids, and protein metabolism

The branched-chain amino acids (BCAAs; valine, isoleucine, and leucine) are the major nitrogen source for glutamine and alanine synthesis in muscle. Synthesis of glutamine, alanine, and BCAA use is activated in critical illnesses such as in sepsis, cancer, and trauma. The use of glutamine often exceeds its synthesis, resulting in the lack of glutamine in plasma and tissues. In critical illness, resynthesis of BCAA from branched-chain keto acids is activated, particularly in hepatic tissue. The BCAA released to circulation may be used for protein synthesis or synthesis of alanine and glutamine. Glutamine and/or alanine infusion has an inhibitory effect on the breakdown of body proteins and decreases BCAA catabolism in postabsorptive control, endotoxemic, and irradiated rats. Decreased protein breakdown also was observed when glutamine synthesis was activated by ammonia infusion. In conclusion some favorable effects of BCAA supply can be explained by its role in the synthesis of glutamine and some positive effects of glutamine exogenous supply can be explained by its effect on metabolism of BCAA.

Is alpha-ketoisocaproyl-glutamine a suitable glutamine precursor to sustain fibroblast growth?

BACKGROUND

Glutamine is considered an essential nutrient for cellular growth.

AIM

To test the suitability of alpha-ketoisocaproyl-Gln (Kic-Gln) as a new glutamine (Gln) precursor to sustain human fibroblast growth.

METHODS

[3H] thymidine uptake into cellular DNA of human fibroblasts. Extracellular and intracellular amino acid patterns were determined with peptides and acylated compounds.

RESULTS

L-alanyl-L-glutamine (used here as a recognized Gln precursor) promoted DNA synthesis, while N-acetyl-L-glutamine (used here as a negative control since it is known to be a poor Gln precursor) and alpha-ketoisocaproyl-glutamine had no effect. Alanyl-glutamine progressively gave rise to free glutamine in the growth medium. In contrast, glutamine supplied in acylated form was poorly available and did not appear in free form in the medium. In addition, only alanyl-glutamine increased intracellular glutamine and glutamate levels. In contrast, Kic-Gln was able to sustain net protein synthesis as judged by total protein content and reduced intracellular levels of most essential amino acids.

CONCLUSION

Kic-Gln appears to be a poor extra-cellular precursor of Gln to sustain cell growth.

Metabolism of alpha-ketoisocaproic acid in isolated perfused liver of cirrhotic rats

To determine the hepatic fate of alpha-ketoisocaproate (KIC) in cirrhosis, six groups of isolated rat livers were perfused with 0, 0.5, 1 (with or without alpha-[1-14C]KIC), 2, and 5 mM KIC; control livers from healthy rats were studied in parallel under similar conditions. KIC was rapidly removed by the normal livers, whereas uptake was lower in the cirrhotic livers at all concentrations tested (at 2 mM, 4.04 +/- 0.33 vs. 6.32 +/- 0.58 mumol/min; P < or = 0.05). The transamination pathway, evaluated by leucine exchanges, was more important in the cirrhotic livers (25.4 vs. 6.8% in controls at 2 mM). The incorporation of alpha-[1-14C]KIC in proteins of cirrhotic liver was increased compared with controls (0.25 +/- 0.04% of alpha-[1-14C]KIC was incorporated in proteins excreted in perfusate vs. 0.20 +/- 0.04 in controls; P < or = 0.05). In addition, a line of evidence suggests that glutamine rather than glutamate is the N donor for leucine synthesis from KIC. The decarboxylation pathway evaluated by beta-hydroxybutyrate production and by 14CO2 release from alpha-[1-14C]KIC was reduced, respectively, by 40-85% (according to KIC dose) and by 24% at 90 min in cirrhotic livers compared with healthy livers. These results indicate a dramatic modification of KIC metabolism in the cirrhotic liver; its uptake by the liver is decreased and its incorporation into proteins is increased via an enhancement of transamination to leucine, probably as a consequence of an inhibition of branched-chain keto acid dehydrogenase.

Glutamine: a major gluconeogenic precursor and vehicle for interorgan carbon transport in man

To compare glutamine and alanine as gluconeogenic precursors, we simultaneously measured their systemic turnovers, clearances, and incorporation into plasma glucose, their skeletal muscle uptake and release, and the proportion of their appearance in plasma directly due to their release from protein in postabsorptive normal volunteers. We infused the volunteers with [U-14C] glutamine, [3-13C] alanine, [2H5] phenylalanine, and [6-3H] glucose to isotopic steady state and used the forearm balance technique. We found that glutamine appearance in plasma exceeded that of alanine (5.76 +/- 0.26 vs. 4.40 +/- 0.33 mumol.kg-1.min-1, P < 0.001), while alanine clearance exceeded glutamine clearance (14.7 +/- 1.3 vs. 9.3 +/- 0.8 ml.kg-1.min-1, P < 0.001). Glutamine appearance in plasma directly due to its release from protein was more than double that of alanine (2.45 +/- 0.25 vs. 1.16 +/- 0.12 mumol.kg-1.min-1, P < 0.001). Although overall carbon transfer to glucose from glutamine and alanine was comparable (3.53 +/- 0.24 vs 3.47 +/- 0.32 atoms.kg-1.min-1), nearly twice as much glucose carbon came from protein derived glutamine than alanine (1.48 +/- 0.15 vs 0.88 +/- 0.09 atoms.kg-1.min-1, P < 0.01). Finally, forearm muscle released more glutamine than alanine (0.88 +/- 0.05 vs 0.48 +/- 0.05 mumol.100 ml-1.min-1, P < 0.01). We conclude that in postabsorptive humans glutamine is quantitatively more important than alanine for transporting protein-derived carbon through plasma and adding these carbons to the glucose pool.

Branched-chain amino acid metabolism in isolated perfused liver of cirrhotic rats

We examined the possible contribution of the liver to the alterations in branched-chain amino acid (BCAA) metabolism in cirrhosis. The livers of male Sprague-Dawley rats with CCl4-induced cirrhosis were removed and placed in a recirculating perfusion system. Net amino acid uptake and release were determined over 55 min. Results were compared with those obtained with control animals, which were either pair-fed or fed ad libitum. Intrahepatic amino acid concentrations were determined at the end of the perfusion. The release of isoleucine and leucine was significantly lower in the cirrhotic livers than in the controls fed ad libitum. There was no difference between the cirrhotic and pair-fed groups with regard to the fluxes of the three BCAA. Intrahepatic concentrations of BCAA were reduced only in pair-fed controls. These results suggest that both cirrhosis and a low protein/calorie diet alter hepatic BCAA flux, but via different mechanisms. In cirrhosis, alterations could be due both to low food intake and to BCAA metabolism in non-parenchymal cells.

On the mechanism of L-alloisoleucine formation: studies on a healthy subject and in fibroblasts from normals and patients with maple syrup urine disease

L-Alloisoleucine formation from L-isoleucine was studied in vitro and in vivo. When a healthy subject was loaded with L-isoleucine, plasma levels of L-isoleucine and 3-methyl-2-oxopentanoate (KMV), as well as L-alloisoleucine, increased. Peak values were reached successively and were in the order L-isoleucine much greater than KMV much greater than L-alloisoleucine. Metabolic clearance of L-isoleucine and KMV was rapid; clearance of L-alloisoleucine was considerably delayed. When human skin fibroblast cultures were challenged with L-isoleucine, KMV accumulated at a gradually decreased rate, whereas L-alloisoleucine accumulated at a gradually accelerated rate. KMV and L-alloisoleucine formation were related and depended on the L-isoleucine concentration applied. In cell lines derived from MSUD patients (classical form), metabolite formation was only about 2-fold higher than in control strains. The relatively small difference between normal and MSUD fibroblasts in vitro as opposed to the striking differences between healthy subjects and MSUD patients in vivo are discussed with respect to the significance of physiological mechanisms participating in the formation and degradation of L-alloisoleucine in man.

Nutritional therapy for selected inborn errors of metabolism

Nutritional approaches are available for the management of several different classes of inborn metabolism errors. In phenylketonuria (PKU), phenylalanine is not properly metabolized; and its accumulation leads to neurologic dysfunction and metal retardation. Altering the diet to limit phenylalanine intake led to remarkable improvement in children with PKU. It was later found that instituting dietary therapy immediately after identification of the disorder in newborns prevented mental retardation. Throughout the 1960s nutritional therapies were found for other inborn disorders, including galactosemia, maple syrup urine disease, and homocystinuria. For the group of disorders associated with defects in the urea cycle, leading to profound hyperammonemia, therapy based on the concept of waste nitrogen excretion (i.e., by increasing excretion of urea cycle intermediates in the urine, nitrogen that would otherwise recycle as ammonia can be eliminated) dramatically produced better control of hyperammonemia and its consequences. Some inborn errors of metabolism respond to vitamin therapy. Biotin-related multiple carboxylase synthetase deficiency can be produced by either of two enzyme defects--holocarboxylase synthetase deficiency or biotinidase deficiency. Both are treatable with biotin supplementation. The symptoms of multiple carboxylase deficiency can also occur after intestinal resection or ingestion of raw eggs. Multiple carboxylase deficiency has been treated successfully in utero by giving the mother biotin supplements. Peroxisomal disorders may respond to dietary management. Liver disease in hereditary tyrosinemia may be accentuated by hypermethioninemia and treated by controlling the blood methionine level. Glycogen storage disease Type I, which causes hypoglycemia, can be controlled by oral administration of cornstarch.

Amino acid metabolism in uremia

The uremic syndrome is multifactorial, and affects most tissues and organs. Disturbances in protein and amino acid metabolism may play important roles, especially in chronic uremia, either directly or by production of toxic metabolites, with resultant negative nitrogen (N) balance, muscle wasting, reduced protein synthesis, and characteristically abnormal intracellular free amino acid concentrations. There are also grossly abnormal amino acid levels in the plasma of uremic patients, e.g., increases in conjugated amino acids, high levels of several nonessential and low levels of essential amino acids. The ratios of tyrosine/phenylalanine and of valine/glycine are decreased. The low tryptophan levels may contribute to encephalopathy as a result of an imbalance in neurotransmitter synthesis. Citrulline is found in excess; the explanation is unresolved. There are elevated concentrations of the sulfur-containing amino acids: cystine, taurine, cystathionine, and homocysteine. Excess of the latter is implicated in the atherogenesis of renal failure. Disturbed metabolism and interorgan exchange of amino acids in the uremic state explains some of the abnormalities in tissue and plasma concentrations of individual amino acids. Enzymatic defects are involved in the disturbed metabolism of branched chain amino acids (BCAA), with possible antagonism among them, which impairs growth and amino acid utilization. Carbohydrate intolerance, associated with insensitivity of peripheral tissues to insulin and hyperinsulinemia, elicits decreased plasma BCAA. Protein synthesis rates in normal and pathological conditions are more closely related to the intracellular amino acid pool than to plasma amino acid levels. Concentrations of individual amino acids in the plasma pool are poor indicators of their intracellular concentrations. Muscle contains the largest pool of protein and free amino acids in the body. In chronic renal failure patients, the intracellular concentrations of valine, threonine, lysine, and carnosine are low. With low protein diets and in hemodialysis, serine, tyrosine, and taurine often are also low. The low taurine may be related to fatigue and to uremic cardiomyopathies. The commonly used amino acid supplements generally fail to correct the intracellular amino acid deficits. A "New Formula" has been developed to correct these intracellular amino acid abnormalities, and to supplement a low protein diet. It provides more valine than leucine, increased tyrosine and threonine, and less histidine, leucine, isoleucine, lysine, methionine, and phenylalanine than in formulas customarily used for patients with chronic renal failure. It is uncertain whether other ap

Transamination and oxidative decarboxylation of L-isoleucine, L-alloisoleucine and related 2-oxo acids in perfused rat hind limb muscle

Metabolism of L-isoleucine, L-alloisoleucine and corresponding 2-oxo acids in rat hind limb muscle was comparatively studied under steady-state perfusion conditions. At 0.5 mM L-[1-14C]isoleucine, apparent transamination and 2-oxo acid decarboxylation rates amounted to about 17 and 4 nmol/min per g of muscle, respectively. With L-allo[1-14C]isoleucine, the corresponding rates were about 5- and 10-fold lower, respectively. After addition of dichloroacetate (1-5 mM), the portion of (S)- and (R)-methyl-2-oxopentanoate undergoing further oxidative decarboxylation within the tissue was similarly increased by over 40%. In perfusions with 0.5 mM (R,S)-3-methyl-2-oxopentanoate and tracer doses of 1-14C-labeled (S)- or (R)-enantiomer, the 14CO2 production was comparable (about 0.5 nmol/min per g of muscle). Dichloroacetate caused a several-fold increase in 14CO2 release from either enantiomer, apparent 2-oxo acid transamination rates remaining unaffected. Indications for a racemization of 2-oxo acid were not obtained in the experiments. The results are discussed with respect to the appearance/disappearance of L-alloisoleucine in vivo and to the fact that (R)-3-methyl-2-oxopentanoate, but not L-alloisoleucine, can support growth of rats on a diet deficient in L-isoleucine.

Effects of branched-chain amino acids on protein turnover

Amino acid availability rapidly regulates protein synthesis and degradation. Increasing amino acid concentrations above the levels found in post-absorptive plasma stimulates protein synthesis in a dose-dependent manner at the level of mRNA translation-initiation and inhibits protein degradation by inhibiting lysosomal autophagy. The anabolic effects of insulin on protein synthesis and protein degradation are exerted at the same sites (i.e., peptide chain initiation and lysosomal stabilization) allowing for a rapid synergistic response when both amino acids and insulin increase after a protein-containing meal. In perfused liver preparations, protein anabolic effects are exerted by a group of amino acids acting in concert. The BCAA are among the amino acids required for stimulation of hepatic protein synthesis, but there is no evidence that BCAA or leucine alone are effective. Leucine alone is an important inhibitor of hepatic protein degradation, but maximal inhibition requires in addition several other regulatory amino acids. In heart and skeletal muscle in vitro, increasing the concentration of the three BCAA or of leucine alone reproduces the effects of increasing the supply of all amino acids in stimulating protein synthesis and inhibiting protein degradation. Skeletal muscle is the largest repository of metabolically active protein and a major contributor to total body nitrogen balance. Supplying energy alone (i.e., carbohydrate and lipids) cannot prevent negative nitrogen balance (net protein catabolism) in animals or humans; only provision of amino acids allows the attainment of nitrogen balance. In rats and in humans nourished parenterally, provision of balanced amino acid solutions or of only the three BCAA cause similar improvements in nitrogen balance for several days. There is some evidence that infusions of leucine alone can stimulate muscle protein synthesis in vivo; the effect may be transitory and was not observed by all investigators; provisions of excess leucine alone does not seem to affect total body or muscle protein degradation in vivo. In postabsorptive rats, in vivo, infusion of the three BCAA together stimulates muscle protein synthesis as much as the infusion of a complete amino acid mixture or of a mixture of essential amino acids; the in vivo effect requires coinfusion of glucose or of small (physiological) doses of insulin, suggesting synergism between insulin and amino acids.(ABSTRACT TRUNCATED AT 400 WORDS)

Measurement of branched-chain alpha-keto acid dehydrogenase flux rates in perfused heart and liver

The methods described here represent a flexible set of procedures for investigating the metabolism of the branched-chain alpha-keto acids and other substances in perfused organs, notably the rat heart and liver. These procedures have been used to investigate many aspects of the metabolism of the branched-chain alpha-keto acids not discussed here, such as the effects on branched-chain alpha-keto acid metabolism by exposure to alpha-adrenergic agents, by inhibition of the monocarboxylate translocator, and by the coinfusion of other metabolites.

Utilization of alpha-ketoisocaproate for protein synthesis in uremic rats

We have recently shown that the nutritional efficiency, R, of alpha-ketoisocaproate (KIC) as a substitute for leucine, defined as the ratio of the dose of leucine to the dose of KIC (on a leucine-free diet) for equal growth, can be evaluated isotopically: 14C-KIC and 3H-leucine are administered p.o.; six hours later, 14C/3H in the leucine of whole body protein, divided by 14C/3H in the injectate, gives a value distinguishable from R assessed in the same animals by growth experiments. To see how chronic uremia affects R, 11/12 nephrectomized rats and sham-operated controls were fed a regular diet for 15 days and then given these isotopes p.o. Six hours later, R, measured in whole body protein, and in the protein of brain, heart, muscle, salivary gland, liver, and the kidney remnant was significantly greater than in sham-operated controls. The greatest difference (39%) was seen in liver protein and the smallest difference (19%) in muscle. Thus chronic uremia increases the efficiency, relative to leucine, with which KIC is utilized for protein synthesis in all of these organs and in the body as a whole. Possible explanations are discussed.

Branched-chain amino acid interactions in skeletal muscle: isoleucine and L-alloisoleucine

Parenteral administration of a mixture of branched-chain amino acid (BCAA) solutions is known to alter plasma levels of the BCAA (ILE, LEU, VAL), their corresponding alpha-ketoacids (KMV, KIC, KIV) and L-alloisoleucine (ALLO), a stereoisomer of ILE. Although variously formulated mixtures of BCAA are administered, the metabolic implications of individual BCAA interactions have been only partially elucidated. Using the incubated, isolated, and intact rat epitrochlearis muscle, we measured the effect of graded increases (0.05, 0.10, 0.5, and 1.0 mM) in media concentrations of a single BCAA or ALLO on (1) the rate of decarboxylation of the other BCAA, and (2) the release of branched chain ketoacids from muscle. A graded increase of media ILE concentration to 1.0 mM changed the decarboxylation of LEU by -28%, and the release of KIC by +23%, but VAL decarboxylation increased by +25%, and the release of KIV declined by -56%. A graded increase of media LEU to 1.0 mM increased ILE decarboxylation by +146%, and its corresponding ketoacid (KMV) by +61%. However, VAL decarboxylation changed by only +25% and KIV release declined by -65%. A graded increase in media VAL to 1.0 mM accelerated ILE decarboxylation by +37%, but KMV release was unchanged. Similarly, LEU decarboxylation fell by -26%, and the release of KIC by only +6%. ALLO 0.025 mM increased ILE release by +55% but had an inconsistent effect on ILE decarboxylation and did not alter protein synthesis or degradation (estimated by phenylalanine incorporation and tyrosine release, respectively). Increasing ILE did not affect ALLO release.(ABSTRACT TRUNCATED AT 250 WORDS)

Utilization of alpha-ketoisocaproate for synthesis of hepatic export proteins and peripheral proteins in normal and cirrhotic subjects

The ratio R, defined as (percent of dose of 14C)/(percent of dose of 3H) in the leucine of plasma fibrinogen, albumin, immunoglobulin G (IgG), red cell globin, and salivary mucin, was measured in 7 normal adults and in 5 cirrhotic patients during continuous intragastric infusion of 1-14C-labeled alpha-ketoisocaproate (KIC) and 3H-labeled leucine. The ratio R measured in whole body protein has been shown in rat experiments to be a measure of the nutritional efficiency of KIC relative to leucine. In normal subjects, R in albumin and fibrinogen became constant (0.63 +/- 0.05) after the third hour and were indistinguishable from one another. The ratio R in IgG was similar and constant. The ratio R in plasma leucine (0.62 +/- 0.06) was significantly lower than R in mucin (0.86 +/- 0.04) or globin (0.73 +/- 0.04), indicating that these latter proteins derive a significant fraction of their leucine from KIC transaminated locally, rather than from circulating leucine. Results in 5 cirrhotic patients were the same, except that R in IgG and R in globin were significantly increased. Thus, cirrhosis does not alter the efficiency, relative to leucine, with which oral KIC is used for synthesis of export proteins by the liver, but increases the efficiency with which it is used for the synthesis of some proteins peripherally.

Regulation of oxidative decarboxylation of branched-chain 2-oxo acids in rat liver mitochondria

At 0.1 mM 2-oxo[1-14C]isocaproate or 2-oxo[1-14C]isovalerate plots of the reciprocal of the rate of 14CO2 formation by branched-chain 2-oxo acid dehydrogenase complex in mitochondria vs alpha-cyanocinamate concentration were linear up to high inhibitor concentrations, indicating that the monocarboxylate carrier-mediated transport was the rate-limiting step. At low (0.025 mM) concentration of 2-oxo[1-14C]isocaproate or 2-oxo[1-14C]isovalerate the 1/v vs I plots became nonlinear indicating that the branched-chain 2-oxo acid dehydrogenase activity determined the rate of 14CO2 formation. Inhibition of branched-chain 2-oxo acid dehydrogenase complex by clofibric acid or arsenite showed that at 0.1 mM 2-oxoisovalerate the activity of the complex became the rate-limiting step of the pathway. The availability of the 2-oxoisocaproate or 2-oxoisovalerate seems to affect the phosphorylation and the activity of the branched-chain 2-oxo acid dehydrogenase complex only at low, physiological concentrations of these substrates (less than 0.025 mM).

Glutamine metabolism in isolated perfused rat liver. The transamination pathway

In isolated perfused rat liver, added 4-methyl-thio-2-oxobutyrate and phenylpyruvate are rapidly transaminated to the corresponding amino acids with glutamine, the latter being supplied via the portal vein or by endogenous synthesis. With portal glutamine concentrations below 5mM and in the presence of a oxo-acid acceptor, the flux through glutamine transaminases exceeded the ammonium ion-stimulated glutaminase flux. 4-Methylthio-2-oxobutyrate-induced extra glutamine uptake was not dependent on the perfusate pH in the range of pH 7 to 8. During glutamine/4-methylthio-2-oxobutyrate transamination, the amide nitrogen of glutamine is fully recovered as glutamate, ammonia, urea and alanine. Oxoglutarate formed by omega-amidase activity is released as glutamate or oxidized by oxoglutarate dehydrogenase. alpha-Cyanocinnamate, the inhibitor of the monocarboxylate translocator in the mitochondrial membrane inhibited 4-methylthio-2-oxobutyrate-induced glutamine uptake and methionine release by about 30%. This might indicate that about 2/3 of glutamine transaminase flux is cytosolic. alpha-Cyanocinnamate inhibited 4-methylthio-2-oxobutyrate-induced glutamate efflux by about 90%. Stimulation of flux through glutamine transaminases is accompanied by a 70-80% inhibition of glutaminase flux. This is not explained by a direct inhibition of glutaminase by 4-methylthio-2-oxobutyrate but by a substrate competition between glutaminase and glutamine transaminases. 4-Methylthio-2-oxobutyrate decreases glutamine release by the liver due to withdrawal by transamination. The oxo acid itself is without effect on glutamine synthetase flux. With respect to hepatocyte heterogeneity there is no evidence for a zonal distribution of glutamine transaminase activities, as it has been shown for glutamine synthetase and glutaminase activities.

Regulation of hepatic glutamate metabolism. Role of 2-oxoacids in glutamate release from isolated perfused rat liver

In isolated perfused rat liver, addition of the oxoanalogues of leucine, isoleucine, methionine and phenylalanine is followed by a rapid and reversible stimulation of glutamate release. This is not observed with the corresponding amino acids or 2-oxoisovalerate, 2-oxoglutarate or oxaloacetate. The increased glutamate release by the liver is accompanied by a decrease in the tissue contents of 2-oxoglutarate and glutamate by about 25% and 50%, respectively. During the metabolism of glutamine, i.e. conditions with elevated tissue glutamate concentrations, 2-oxoacid-induced glutamate release is stimulated. In the presence of glutamine (5 mM), 2-oxoisocaproate, 2-oxo-4-methylvalerate and 2-oxo-4-methylthiobutyrate were found to be most effective and glutamate release by the liver increased linearly from about 80 nmol g-1 min-1 to 600 nmol g-1 min-1 at increasing 2-oxoacid concentrations up to 1 mM. When glutamate tissue levels were decreased by phenylephrine, stimulation of glutamate release by 2-oxoisocaproate was markedly diminished. 2-Oxoacid-stimulated glutamate release is independent of oxoacid metabolism, indicating that the effect is probably not explained by a 2-oxoacid/glutamate exchange across the liver plasma membrane. 2-Oxoacid-induced glutamate export predominantly occurs in a sodium-independent way. At low concentrations of 2-oxoisocaproate (below 0.2 mM), the increased glutamate release was accompanied by a slight inhibition of 14CO2 production from added [14C]glutamate, indicating a simultaneous glutamate uptake and release also under these conditions. Stimulation of glutamate release by 2-oxoisocaproate is followed by a decreased rate of urea and glutamine synthesis from portal ammonia, as a consequence of an increased glutamate release.

Enzymatic and pharmacokinetic studies on the metabolism of branched chain alpha-keto acids in the rat

Michaelis-constants and enzyme activities for dehydrogenation and transamination of the three branched chain alpha-keto acids in liver, kidney, skeletal muscle, and brain of rats are reported. After oral load only 11-22% of the keto acids pass the liver unchanged. Blood levels in pharmacokinetic and absorption studies are related to the Michaelis-constants. At the low keto-acid concentrations after oral application, dehydrogenation in the non-hepatic tissues is supposed to prevail over transamination. Data on feed efficiency of branched chain alpha-keto acids reported in the literature support this view. The chance for transamination is better after intravenous administration. The transferability of our data to humans, and various factors influencing the efficiency of branched chain alpha-keto acids are discussed in connection with data reported in the literature.

Role of insulin in the regulation of leucine kinetics in the conscious dog

To study the effect of insulin on leucine kinetics, three groups of conscious dogs were studied after an overnight fast (16-18 h). One, saline-infused group (n = 5), served as control. The other two groups were infused with somatostatin and constant replacement amount of glucagon; one group (n = 6) received no insulin replacement, to produce acute insulin deficiency, and the other (n = 6) was constantly replaced with 600 muU/kg per min insulin, to produce twice basal hyperinsulinemia. Hepatic and extrahepatic splanchnic (gut) balance of leucine and alpha-ketoisocaproate (KIC) were calculated using the arteriovenous difference technique. l,4,5,[(3)H]Leucine was used to measure the rates (micromoles per kilogram per minute) of appearance (Ra) and disappearance (Rd), and clearance (Cl) of plasma leucine (milliliters per kilogram per minute). Saline infusion for 7 h resulted in isotopic steady state, where Ra and Rd were equal (3.2+/-0.2 mumol/kg per min). Acute insulin withdrawal of 4-h duration caused the plasma leucine to increase by 40% (P < 0.005). This change was caused by a decrease in the outflow of leucine (Cl) from the plasma, since Ra did not change. The net hepatic release of the amino acid (0.24+/-0.03 mumol/kg per min) did not change significantly; the arterio-deep femoral venous differences of leucine (-10+/-1 mumol/liter) and KIC (-12+/-2 mumol/liter) did not change significantly indicating net release of the amino and ketoacids across the hindlimb. Selective twice basal hyperinsulinemia resulted in a 36% drop in plasma leucine (from control levels of 128+/-8 to 82+/-7 mumol/liter, P < 0.005) within 4 h. This was accompanied by a 15% reduction in Ra and a 56% rise in clearance (P < 0.001, both). Net hepatic leucine production and net release of leucine and KIC across the hindlimb fell markedly. These studies indicate that physiologic changes in circulating insulin levels result in a differential dose-dependent effect on total body leucine metabolism in the intact animal. Acute insulin withdrawal exerts no effect on leucine rate of appearance, while at twice basal levels, insulin inhibited leucine rate of appearance and stimulated its rate of disappearance.

Disposal of alpha-ketoisocaproate: roles of liver, gut, and kidneys

The alpha-keto analogues of the branched-chain amino acids, and particularly that of leucine, alpha-ketoisocaproate (KIC), have been found to reduce urea synthesis and as a result have been proposed for the treatment of uremia and portal systemic encephalopathy. Because little is known about the fate of these keto acids in the intact animal, we examined the disposal of a KIC load in five conscious overnight-fasted dogs with catheters previously implanted in an artery, and in the portal, hepatic, and renal veins. During the absorptive period (54 +/- 9 min; range, 20-75 min), 62 +/- 5% of the administered load (6,358 +/- 662 mumol) of the keto acid was absorbed as KIC and 23 +/- 3% was transaminated across the gut and entered as leucine. The hepatic uptake of KIC was equivalent to 35 +/- 5% (2.316 +/- 419 mumol) of the administered load, and of that, one-third was transaminated to leucine and two-thirds were converted to ketone bodies. The splanchnic output of KIC amounted to 1,732 +/- 256 mumol of 27 +/- 2% of the administered load, half of which was transaminated across the kidneys to leucine. As a result, the amount of KIC reaching the extrahepatic extrarenal tissues as KIC carbon amounted to 15% of the load administered. We conclude that the majority of an intragastrically administered KIC load reaches the (extrarenal) peripheral tissues in the form of leucine or ketone bodies. The study also underscores the importance of the "gut," the kidneys, and the liver in metabolism of the absorbed KIC load.

Long-term effects of a new ketoacid-amino acid supplement in patients with chronic renal failure

Nine patients with severe chronic renal failure (mean glomerular filtration rate 4.8 ml/min; mean serum creatinine 11.3 mg/dl) who were previously on a protein-restricted diet were treated with a diet containing an average of 33 kcal/kg and 22.5 g/day of mixed quality protein, supplemented by a combination of amino acids and mixed salts formed between basic amino acids and keto-analogues of essential amino acids. The supplement was designed to minimize or reverse the amino acid abnormalities of chronic renal failure rather than to meet the normal requirements for the essential amino acids; it contained tyrosine, ornithine, and a high proportion of branched-chain ketoacids, but no phenylalanine or tryptophan and very little methionine. Within one month, serum urea nitrogen fell and serum albumin and transferrin rose significantly; serum creatinine fell slightly. Hyperphosphatemia (present in three patients) was corrected. Nitrogen balance, measured in seven of the nine patients, on the average was neutral, as it was in a preceding control period on a 40 to 50 g/day protein diet. Plasma tyrosine and threonine, which were subnormal before therapy, rose to normal or high normal levels. Branched-chain amino acids did not change. During a total of 63 patient-months of therapy, no side effects or toxicity were observed, and serum albumin and transferrin did not change further. It is concluded that this specially designed supplement added to a 20 to 25 g/d protein diet is an acceptable regimen which can improve or maintain protein nutrition in patients with severe chronic renal failure who would otherwise require dialysis.