Effects of excess intake of leucine and valine deficiency on tryptophan and niacin metabolites in humans.

Nineteen healthy humans (2 men and 17 women) served as experimental subjects in 4 experiments using diets having different levels of leucine and also a valine-deficient diet. The effect of an excess intake of leucine, with and without addition of vitamin B-6, and the effect of a deficiency of valine on urinary excretions of N1-methylnicotinamide, N1-methyl-2-pyridone-5-carboxamide, nicotinic acid, quinolinic acid, and 5-hydroxyindole acetic acid, and on the level of plasma amino acids were investigated. There was no effect of leucine on the excretion of these metabolites, but a marked decrease in the plasma (or serum) valine level was observed. The same decrease was seen when a valine-deficient diet was fed.     

Effect of dietary excess leucine on nicotinamide nucleotide level in rat liver.

1. Six groups of rats were freely fed diets containing casein at 5, 10 and 20% levels with and without nicotinic acid. After 2 weeks on these diets, hepatic nicotinamide nucleotide and free nicotinic acid concentrations were studied. 2. Hepatic nicotinamide nucleotide level was kept in the normal range in rats fed the 10 and 20% casein diets with and without nicotinic acid. 3. L-leucine supplemented at the 5% level to the 10 and 20% casein diets caused significant decrease in hepatic nicotinamide nucleotide level only in rats fed nicotinic acid devoid diet. 4. Hepatic nicotinamide nucleotide in rats fed the diet in which casein was replaced by zein increased significantly by adding nicotinic acid. This increase in the hepatic nicotinamide nucleotide caused by dietary supplemented nicotinic acid was not reduced by the addition of L-leucine. 5. The hepatic free nicotinic acid level did not change even in rats of which hepatic nicotinamide nucleotide was significantly reduced. 6. Urinary excretion of nicotinic acid and N-methylnicotinamide was increased significantly by adding nicotinic acid to the diet but was not affected by adding L-leucine at the 5% level. 7. From the above results, a possible mechanism of L-leucine action was discussed.     

Tryptophan metabolism by the isolated rat liver cells--effects of leucine and its metabolites

1. The effects of leucine and its metabolites, such as alpha-ketoisocaproate and ketone bodies, on the metabolic fluxes of tryptophan were investigated in isolated rat liver cells using [benzene ring-U-14C]- or [methylene-14C]-tryptophan. 2. Tryptophan metabolized through the kynureninase flux decreased while that metabolized to acetyl CoA remained unchanged in the presence of leucine or its metabolites. Accordingly an amount of tryptophan metabolized through the quinolinate-NAD pathway, which was estimated by subtracting an amount of tryptophan metabolized via the acetyl CoA flux from that via the kynureninase flux, was decreased in the presence of leucine or its metabolites. 3. Less quinolinate accumulated during incubation with metabolites of leucine, however, the amount was still sufficient to saturate quinolinate phosphoribosyltransferase (EC 2.4.2.19). 4. Leucine and its metabolites added in vitro at 1 mM level did not inhibit quinolinate phosphoribosyltransferase activity in rat liver homogenate. 5. The results indicate that a decrease in NAD biosynthesis from tryptophan caused by metabolites of leucine in the isolated rat liver cells was neither due to insufficient supply of quinolinate nor to direct inhibitory effect on quinolinate phosphoribosyltransferase.     

Effects of supplemental dietary energy on leucine metabolism in sheep

1. Mixed-breed wethers (40-50 kg), 9 months old, were maintained on high-energy (HED) and low-energy (LED) diets for 2 weeks. 2. After a 15 h fast, a primed-dose constant infusion of L-[U-14C]leucine and alpha-[4,5-3H]ketoisocaproate (KIC) was given. 3. After 2 h, plasma samples were taken and plasma-specific radioactivities of 14C- and 3H-labelled leucine and KIC were measured and analysed by using an open two-pool model. 4. Less than 20% of the total leucine-C entering the circulation was converted to the KIC pool, and 42% of the KIC was converted back to the leucine pool; transamination of the leucine to KIC and reamination of KIC to leucine was much less than in other species. 5. Additional dietary energy resulted in a decrease in tissue protein synthesis, leucine oxidation and interconversion of leucine and KIC. Total leucine-C entry was also lower in sheep given HED, which was most likely due to a suppression of endogenous proteolysis. 6. Plasma glucagon concentration was significantly higher (P less than 0.05) in sheep given LED compared with those given HED. The concentration of glucagon was closely correlated in all treatments with the leucine-C entry (proteolysis + absorbed leucine) and also with KIC-C exit (oxidation).     

Effects of dietary picolinate on mice deprived of tryptophan, niacin and vitamin B-6

Tryptophan (Trp) is usually catabolized to CO2, H2O and urea in mammals. Quantitatively, minor branches from this pathway lead to the production of picolinate (PA) and NAD+. Although niacin is a required nutrient when NAD+ synthesis from Trp is insufficient, it is not known if dietary PA is beneficial in such conditions. Two kinds of experiment were used to investigate the effects of dietary PA. First, mice were fed on a diet deficient in Trp for six weeks to diminish endogenous PA synthesis and stores. After this time the body mass of the mice was reduced by approximately 30%. Tryptophan-deficient mice subsequently fed a nutritionally complete diet gained weight more rapidly during the first but not subsequent week(s) of recovery when PA was added to their food. Second, mice were fed on a diet deficient in Trp, niacin and vitamin B-6 during the preimplantation period of pregnancy in an attempt to acutely lower PA synthesis and perhaps alter the embryos thus produced. The rate at which preimplantation blastocysts formed trophoblastic outgrowths when cultured in vitro was stimulated by dietary PA only if these blastocysts came from mice fed on a nutritionally deficient diet. The results suggest that PA serves as a beneficial nutrient under some conditions where its endogenous synthesis may be diminished.     

Effect of elevated dietary tryptophan on protein synthesis in rat liver

This study investigated whether rats force-fed for three days an elevated (1%) tryptophan diet compared to a control (0.2% tryptophan) diet had changes in hepatic protein metabolism. Earlier, we showed that a single administration of L-tryptophan to fasted rats caused a rapid increase in hepatic protein synthesis. In the present study rats force-fed a high tryptophan diet for 3 days and killed the fourth morning had increased rats of hapatic protein synthesis, cytochrome P-450 and b5 activities, in vitro nuclear RNA release (cell sap and nuclear effects) and nuclear envelope nucleoside triphosphatases activity compared to animals force-fed the control diet. We noted little or no change in hapatic total polyribosomal aggregation patterns or plasma and hepatic free amino acid levels.     

The effect of a high level of dietary leucine on the niacin status of dogs.

Two feeding trials were designed to precipitate niacin deficiency in puppies receiving low levels of niacin by adding 15 g/kg supplementary l-leucine to a diet containing 180 g/kg casein. We failed to produce such an effect and, as niacin levels were gradually reduced, the times at which control dogs became deficient (and then responded to injections of the vitamin) were not significantly different from those for dogs receiving the leucine supplement. Differences between the conditions of our experiments and of the experiment in which this effect was found are discussed. Two pairs of littermates in trial 2 died suddenly while apparently in fairly good condition, but revealing fatty livers and/or changes in heart muscle on autopsy. Similar observations have been reported by others using purified diets with dogs over long periods; there is no certain explanation.     

Effects of norepinephrine on hepatic amino acid metabolism in isolated perfused rat liver

The effects of norepinephrine (NE) on hepatic amino acid exchanges were studied in the isolated perfused rat liver using a recirculating system with a medium containing amino acids at twice physiologic concentrations. Norepinephrine induced a significant decrease (25%) in portal blood flow at a concentration of 2 ng/ml (10(-8) M). The hormone also increased total hepatic amino acid uptake, essentially through a switch from glutamine release to net uptake. There was no modification in free intracellular amino acids, but glycogen was slightly decreased, and glucose production was increased. Taken as a whole, these results suggest that NE modulates hepatic protein balance.     

Nutritional and metabolic effects of dietary leucine excess in preruminant lamb

The effects of excess leucine intake on food intake, branched-chain amino acid and branched-chain alpha-keto acid concentrations in plasma and nitrogen retention were investigated in the preruminant lamb. Lambs were fed leucine in excess in either an adequate protein diet [24% of dry matter (DM)] or a low protein diet (15% DM) for 2 d. Increasing the dietary leucine content of 2.3 to 10.6 or 12.6% DM led to a significant decrease in food intake. This depressing effect was not influenced by dietary protein content. Increasing the dietary leucine content from 2.3 to 6.4% DM in an adequate protein diet for a week did not significantly improve nitrogen retention in the preruminant lamb. Plasma leucine and its alpha-keto acid concentrations increased with leucine intake. Plasma valine and isoleucine concentrations were significantly decreased only in lambs fed the highest excess leucine diet. Surprisingly, a maximal 50% decrease of their plasma alpha-keto acid concentrations occurred even in the group fed the lowest excess leucine diet. Our results might be explained by an inhibition of the rate of transamination of valine and isoleucine by high leucine concentration.     

Effect of dietary cystine on methionine metabolism in rat liver

Cystine supplementation of adequate diets resulted in significantly higher hepatic levels of betaine-homocysteine methyltransferase. Other changes occurred but were a function of the basal diet. When the latter contained 0.25% methionine + 0.5% cystine, the additional cystine caused a markedly lower hepatic cystathionine synthase activity and lower levels of both adenosylmethionine and serine. The metabolic effect of these changes may be enhanced methionine retention and diminished transsulfuration.