Enzymic acylation of glutamine by phenylacetic acid

Safety, tolerability, and pharmacokinetics of l-ornithine phenylacetate in patients with acute liver injury/failure and hyperammonemia

Cerebral edema remains a significant cause of morbidity and mortality in patients with acute liver failure (ALF) and has been linked to elevated blood ammonia levels. l-ornithine phenylacetate (OPA) may decrease ammonia by promoting its renal excretion as phenylacetylglutamine (PAGN), decreasing the risk of cerebral edema. We evaluated the safety, tolerability, and pharmacokinetics of OPA in patients with ALF and acute liver injury (ALI), including those with renal failure. Forty-seven patients with ALI/ALF and ammonia ≥60 μM were enrolled. Patients received OPA in a dose escalation scheme from 3.3 g every 24 hours to 10 g every 24 hours; 15 patients received 20 g every 24 hours throughout the infusion for up to 120 hours. Plasma phenylacetate (PA) concentrations were uniformly below target (<75 μg/mL) in those receiving 3.3 g every 24 hours (median [interquartile range] 5.0 [5.0] μg/mL), and increased to target levels in all but one who received 20 g every 24 hours (150 [100] μg/mL). Plasma [PAGN] increased, and conversion of PA to PAGN became saturated, with increasing OPA dose. Urinary PAGN clearance and creatinine clearance were linearly related (r = 0.831, P < 0.0001). Mean ammonia concentrations based on the area under the curve decreased to a greater extent in patients who received 20 g of OPA every 24 hours compared with those who received the maximal dose of 3.3 or 6.7 g every 24 hours (P = 0.046 and 0.022, respectively). Of the reported serious adverse events (AEs), which included 11 deaths, none was attributable to study medication. The only nonserious AEs possibly related to study drug were headache and nausea/vomiting.

CONCLUSION

OPA was well-tolerated in patients with ALI/ALF, and no safety signals were identified. Target [PA] was achieved at infusion rates of 20 g every 24 hours, leading to ammonia excretion in urine as PAGN in proportion to renal function. Randomized, controlled studies of high-dose OPA are needed to determine its use as an ammonia-scavenging agent in patients with ALF. (Hepatology 2018;67:1003-1013).

Taurine in development and nutrition

Taurine is an amino acid that is widely distributed in the fluids and tissues of man. In mammals, taurine is a major end-product of methionine metabolism. Taurine is found in most mammalian tissues but is only present in trace amounts in many plants. During fetal development of the brain in man and other mammals taurine is present in high concentrations and declines to lower, adult concentrations during neonatal life. However, during this time there is a net accumulation of taurine when the amount per brain rather than per gram of tissue is calculated. In man, taurine is apparently an essential nutrient, unlike in other animals which have a much greater capacity to synthesize this compound. The human infant, is particular, needs a dietary supply of taurine to synthesize the bile salt taurocholate. Thus, taurine appears to be an important component of the developing brain and must be supplied to man in the diet.

The FEMA GRAS assessment of phenethyl alcohol, aldehyde, acid, and related acetals and esters used as flavor ingredients

This publication is the ninth in a series of safety evaluations performed by the Expert Panel of the Flavor and Extract Manufacturers Association (FEMA). In 1993, the Panel initiated a comprehensive program to re-evaluate the safety of more than 1700 GRAS flavoring substances under conditions of intended use. Elements that are fundamental to the safety evaluation of flavor ingredients include exposure, structural analogy, metabolism, pharmacokinetics and toxicology. Flavor ingredients are evaluated individually and in the context of the available scientific information on the group of structurally related substances. Scientific data relevant to the safety evaluation of the use of phenethyl alcohol, aldehyde, acid, and related acetals and esters as flavoring ingredients is evaluated. The group of phenethylalcohol, aldehyde, acid, and related acetals and esters was reaffirmed as GRAS (GRASr) based, in part, on their self-limiting properties as flavoring substances in food, their rapid absorption, metabolic detoxication, and excretion in humans and other animals, their low level of flavor use, the wide margins of safety between the conservative estimates of intake and the no-observed-adverse effect levels determined from subchronic and chronic studies and the lack of significant genotoxic and mutagenic potential. This evidence of safety is supported by the fact that the intake of phenethyl alcohol, aldehyde, acid, and related acetals and esters as natural components of traditional foods is greater than their intake as intentionally added flavoring substances.

Changes in the excretion of endogenous glycine conjugate as a possible artifact in toxicological experiments

Changes in the urinary excretion of hippuric acid (HIA) and phenaceturic acid (PUA) as well as their metabolic precursors, i.e. benzoic (BA) and phenylacetic acid (PAA), in rats housed in glass metabolic cages for 4 days were monitored using gas-liquid chromatography. The amount of HIA excreted was 128 +/- 63 mumol/kg for female and 79 +/- 43 mumol/kg for male rats in the first 24 h and decreased to 11 +/- 7 mumol/kg (p less than 0.01) for female and 3.2 +/- 2.4 mumol/kg (p less than 0.001) for male rats on the 2nd day. These values remained nearly at the same level until the end of the experiment. The amount of PUA decreased from 48 +/- 12 mumol/kg on the 1st day to 22 +/- 9 mumol/kg (p less than 0.05) on the 2nd day by male rats, whereas by the females the decrease from 30 +/- 9 mumol/kg to 21 +/- 8 mumol/kg was not significant. The decrease in the excretion of glycine conjugates was compensated by a parallel increase in the level of unconjugated BA and PAA.

Some observations on the urinary excretion of glycine conjugates by laboratory animals

1. An isotope dilution assay has been developed for measuring the amount of conjugated glycine present in urine. The average daily excretion of conjugated glycine by male and female mouse, rat, guinea-pig and rabbit has been determined. 2. The glycine conjugates excreted by the four species have been identified. All species excreted only benzoylglycine and phenylacetylglycine. 3. The metabolism of [carboxy-14C]benzoic acid and [1-14C]phenylacetic acid has been investigated in the Sprague-Dawley rat. When administered over a range of concn. from 10 microgram/kg to 1g/kg, benzoic acid is converted to hippuric acid while phenylacetic acid is converted to phenylacetylglycine and phenylacetylglutamine. 4. Neither the rate of excretion nor the composition of the urinary metabolites arising from each acid is changed when low doses of one acid are co-administered with a high dose of the other. 5. The origin of the conjugated benzoic and phenylacetic acids is discussed.

The kinetics of intramolecular distribution of 15N in uric acid after administration of (15N) glycine. A reappraisal of the significance of preferential labeling of N-(3+9) of uric acid in primary gout

THE CONCEPT OF AN ABNORMALITY OF GLUTAMINE METABOLISM IN PRIMARY GOUT WAS FIRST PROPOSED ON THE BASIS OF ISOTOPE DATA: when [(15)N]glycine was administered to gouty subjects, there was disproportionately great enrichment of N-(3 + 9) of uric acid, which derive from the amide-N of glutamine. An unduly high concentration of (15)N in glutamine was postulated, and attributed to a hypothetical defect in catabolism of glutamine. Excess glutamine was proposed as the driving force of uric acid overproduction. WE HAVE REEXAMINED THIS PROPOSITION IN FOUR GOUTY SUBJECTS: one mild overproducer of uric acid with "idiopathic gout," one marked overproducer with high-grade but "partial" hypoxanthine-guanine phosphoribosyl-transferase deficiency, and two extraordinary overproducers with superactive phosphoribosylpyrophosphate synthetases. In the last three, the driving force of excessive purine biosynthesis is a known surplus of alpha-5-phosphoribosyl-1-pyrophosphate. Disproportionately high labeling of N-(3 + 9) was present in all four gouty subjects, most marked in the most flamboyant overproducers. The precursor glucine pool was sampled by periodic administration of benzoic acid and isolation of urinary hippuric acid. Similarly, the precursor glutamine pool was sampled by periodic administration of phenylacetic acid and isolation of the amide-N of urinary phenylacetylglutamine. The time course of (15)N enrichment of hippurate differed from that of the amide-N of glutamine. Whereas initial enrichment values of hippurate were very high, those of glutamine-amide-N were low, increasing to a maximum at about 3 h, and then declining less rapidly than those of hippurate. However, enrichment values of hippurate and of phenacetyl glutamine were normal in all of the gouty subjects studied. Thus, preferential enrichment of N-(3 + 9) in gouty overproducers given [(15)N]glycine does not necessarily reflect a specific abnormality of glutamine metabolism, but rather appears to be a kinetic phenomenon associated with accelerated purine biosynthesis per se.In addition, greater enrichment of N-9 than of N-3 on days 1 and 2 provided suggestive evidence for a second pathway for synthesis of the initial precursor of purine biosynthesis, phosphoribosylamine, perhaps utilizing ammonia rather than the amide-N of glutamine as nitrogen donor. In this limited study, the activity of this potential second pathway did not appear to be selectively increased in gout.

Urinary aromatic acid excretion by fed and fasted sheep in relation to protein metabolism in the rumen

1. Two adult wether sheep were maintained on a diet of hay and two on a diet of dried grass for 3 weeks before starvation for a period of 10 d. Urinary excretion of the following acids was determined when the animals were fed and when they were fasted: total diethyl ethersoluble acids of hydrolysed and unhydrolysed urine, hippuric acid, benzoic acid and phenylacetic acid. By the 5th day of fasting, urinary output of all acids had attained stable levels that did not change during the remaining starvation period. The output of all urine fractions except phenylacetic acid declined rapidly during the first 4 d of fasting: phenylacetic acid output by all sheep increased to a maximum during the first 4 d of fast and then declined to stable values (0·42–0·73 g/24 h) which were greater than those observed when the sheep were fed. It is concluded that prolonged retention of food and microbial residues in the digestive tract is responsible for the large output of phenylacetic acid in the urine of fasted sheep.

2. Solutions of casein which supplied between 6·3. and 26·5 g nitrogen/24 h were infused into the rumens (fifteen experiments) or abomasums (sixteen experiments) of eight adult wether sheep. Ruminal infusions of casein caused increments in the urinary excretion of diethyl ethersoluble acids and phenylacetic acid. Both these increments were described by linear regression equations (P< 0·001), the coefficients of which showed that 284 ± 44 and 220 ± 21 mg benzoic acid equivalent were excreted as diethyl ether-soluble acids and phenylacetic acid respectively per g casein N infused. The phenylacetic acid excreted was equivalent to 95% of the phenylalanine of the infused casein. No increments in urinary benzoic acid were observed. One sheep scoured when it was given an abomasal infusion of casein. This was the only animal to show any increment in urinary aromatic acids when casein was infused into the abomasum.

3. When four sheep were given two rations containing an excess of carbohydrate as sugarbeet pulp or rolled barley, 11 and 16% respectively of their phenylalanine intakes were excreted in the urine as phenylacetic acid. When the same sheep were given two rations containing an excess of N as linseed meal or field beans, 51 and 59% respectively of their phenylalanine intakes appeared in the urine as phenylacetic acid.

4. Methods for the determination of creatinine, and of benzoic, phenylacetic, 3-phenylpropionic, cinnamic, hippuric and phenaceturic acids are described.

5. It is suggested that the amount of phenylacetic acid excreted in the urine is a measure of the equilibrium occurring in the rumen between catabolism of phenylalanine and reutilization of the products of catabolism for phenylalanine synthesis.

The urinary excretion of aromatic acids by starved sheep

1. Four wether sheep were maintained on a diet of hay for 2 weeks and then starved for a period of 4 days.

2. Immediately before and during starvation the urinary excretion in the following fractions was determined: hippuric acid, creatinine, total diethyl ether-soluble acids of hydrolysed and unhydrolysed urine, total aromatic acids in hydrolysed and unhydrolysed urine and the proportion of the former present as benzoic and phenylacetic acids.

3. A method for determining the benzoic acid content of light petroleum extracts of urine has been developed and is described.

4. Starvation had little effect on the urinary excretion of phenylacetic acid or creatinine, but during the first 2 days of starvation there were large decreases in the excretion of all the other urinary fractions studied.

5. Of the fractions examined, 43% of the diethyl ether-soluble acids of hydrolysed urine and 42% of those of unhydrolysed urine were of exogenous origin; 76% of the total urinary aromatic acids were of exogenous origin. Partition of the aromatic acids in the urine of two of the four sheep indicated that the reduction in aromatic acid excretion on starvation was completely accounted for by the decline in benzoic acid output. Almost all the hippuric acid (97%) was of exogenous origin.

6. These results have been compared with the urinary output of aromatic acids by nonruminants when fasted, and possible reasons for the relatively large amounts of phenylacetic acid found in the urine of starved sheep have been discussed.