Depletion of plasma glycine and effect of glycine by mouth on salicylate metabolism during aspirin overdose

Insulin resistance and glycine metabolism in humans

Plasma glycine level is low in patients with obesity or diabetes and the improvement of insulin resistance increases plasma glycine concentration. In prospective studies, hypoglycinemia at baseline predicts the risk of developing type 2 diabetes and higher serum glycine level is associated with decreased risk of incident type 2 diabetes. Consistently, plasma glycine concentration is lower in the lean offspring of parents with type 2 diabetes compared to healthy subjects. Among patients with type 2 diabetes, hypoglycinemia occurs before clinical manifestations of the disease, but the pathophysiological mechanisms underlying glycine deficit and its potential clinical repercussions are unclear. Glycine participates in several metabolic pathways, being required for relevant human physiological processes. Humans synthesize glycine from glyoxylate, glucose (via serine), betaine and likely from threonine and during the endogenous synthesis of L-carnitine. Glycine conjugates bile acids and other acyl moieties producing acyl-glycine derivatives. The glycine cleavage system catalyzes glycine degradation to carbon dioxide and ammonium while tetrahydrofolate is converted into 5,10-methylene-tetrahydrofolate. Glycine is utilized to synthesize serine, sarcosine, purines, creatine, heme group, glutathione, and collagen. Glycine is a major quantitative component of collagen. In addition, the role of glycine maintaining collagen structure is critical, as glycine residues are required to stabilize the triple helix of the collagen molecule. This quality of glycine likely contributes to explain the occurrence of medial arterial calcification and the elevated cardiovascular risk associated with diabetes and chronic kidney disease, as emerging evidence links normal collagen content with the initiation and progression of vascular calcification in humans.

Quantitative determination of five metabolites of aspirin by UHPLC-MS/MS coupled with enzymatic reaction and its application to evaluate the effects of aspirin dosage on the metabolic profile

Acetylsalicylic acid (Aspirin, ASA) is a famous drug for cardiovascular diseases in recent years. Effects of ASA dosage on the metabolic profile have not been fully understood. The purpose of our study is to establish a rapid and reliable method to quantify ASA metabolites in biological matrices, especially for glucuronide metabolites whose standards are not commercially available. Then we applied this method to evaluate the effects of ASA dosage on the metabolic and excretion profile of ASA metabolites in rat urine. Salicylic acid (SA), gentisic acid (GA) and salicyluric acid (SUA) were determined directly by UHPLC-MS/MS, while salicyl phenolic glucuronide (SAPG) and salicyluric acid phenolic glucuronide (SUAPG) were quantified indirectly by measuring the released SA and SUA from SAPG and SUAPG after β-glucuronidase digestion. SUA and SUAPG were the major metabolites of ASA in rat urine 24h after ASA administration, which accounted for 50% (SUA) and 26% (SUAPG). When ASA dosage was increased, the contributions dropped to 32% and 18%, respectively. The excretion of other three metabolites (GA, SA and SAPG) however showed remarkable increases by 16%, 6% and 4%, respectively. In addition, SUA and SUAPG were mainly excreted in the time period of 12-24h, while GA was excreted in the earlier time periods (0-4h and 4-8h). SA was mainly excreted in the time period of 0-4h and 12-24h. And the excretion of SAPG was equally distributed in the four time periods. We went further to show that the excretion of five metabolites in rat urine was delayed when ASA dosage was increased. In conclusion, we have developed a rapid and sensitive method to determine the five ASA metabolites (SA, GA, SUA, SAPG and SUAPG) in rat urine. We showed that ASA dosage could significantly influence the metabolic and excretion profile of ASA metabolites in rat urine.

Metabolomic Characterizations of Liver Injury Caused by Acute Arsenic Toxicity in Zebrafish

Arsenic is one of the most common metalloid contaminants in groundwater and it has both acute and chronic toxicity affecting multiple organs. Details of the mechanism of arsenic toxicity are still lacking and profile studies at metabolic level are very limited. Using gas chromatography coupled with mass spectroscopy (GC/MS), we first generated metabolomic profiles from the livers of arsenic-treated zebrafish and identified 34 significantly altered metabolite peaks as potential markers, including four prominent ones: cholic acid, glycylglycine, glycine and hypotaurine. Combined results from GC/MS, histological examination and pathway analyses suggested a series of alterations, including apoptosis, glycogenolysis, changes in amino acid metabolism and fatty acid composition, accumulation of bile acids and fats, and disturbance in glycolysis related energy metabolism. The alterations in glycolysis partially resemble Warburg effect commonly observed in many cancer cells. However, cellular damages were not reflected in two conventional liver function tests performed, Bilirubin assay and alanine aminotransferase (ALT) assay, probably because the short arsenate exposure was insufficient to induce detectable damage. This study demonstrated that metabolic changes could reflect mild liver impairments induced by arsenic exposure, which underscored their potential in reporting early liver injury.

The glycine deportation system and its pharmacological consequences

The glycine deportation system is an essential component of glycine catabolism in man whereby 400 to 800mg glycine per day are deported into urine as hippuric acid. The molecular escort for this deportation is benzoic acid, which derives from the diet and from gut microbiota metabolism of dietary precursors. Three components of this system, involving hepatic and renal metabolism, and renal active tubular secretion help regulate systemic and central nervous system levels of glycine. When glycine levels are pathologically high, as in congenital nonketotic hyperglycinemia, the glycine deportation system can be upregulated with pharmacological doses of benzoic acid to assist in normalization of glycine homeostasis. In congenital urea cycle enzymopathies, similar activation of the glycine deportation system with benzoic acid is useful for the excretion of excess nitrogen in the form of glycine. Drugs which can substitute for benzoic acid as substrates for the glycine deportation system have adverse reactions that may involve perturbations of glycine homeostasis. The cancer chemotherapeutic agent ifosfamide has an unacceptably high incidence of encephalopathy. This would appear to arise as a result of the production of toxic aldehyde metabolites which deplete ATP production and sequester NADH in the mitochondrial matrix, thereby inhibiting the glycine deportation system and causing de novo glycine synthesis by the glycine cleavage system. We hypothesize that this would result in hyperglycinemia and encephalopathy. This understanding may lead to novel prophylactic strategies for ifosfamide encephalopathy. Thus, the glycine deportation system plays multiple key roles in physiological and neurotoxicological processes involving glycine.

UGT1A6 polymorphism and salicylic acid glucuronidation following aspirin

OBJECTIVES

In vivo, aspirin (acetylsalicylic acid) is rapidly deacetylated to form salicylic acid, which then undergoes primary or secondary glucuronidation catalyzed by UDP-glucuronosyltransferases (UGTs). The variant UGT1A6*2 (T181A, R184S) is associated with altered enzyme function. Our objective was to compare salicylic acid glucuronidation in individuals with different UGT1A6 genotypes.

METHODS

Following orally dosing with 650 mg aspirin, saliva and urine samples were collected over a period of 24 h from healthy individuals with homozygous wild-type UGT1A6 *1/*1 (n=19) and homozygous variant UGT1A6 *2/*2 (T181A, R184S) (n=9) genotypes.

RESULTS

No statistically significant differences were observed in salivary pharmacokinetic parameters. Urinary excretion of the sum of aspirin and its metabolites (salicyluric acid, salicyluric acid phenolic glucuronide, salicyl phenolic glucuronide, salicyl acyl glucuronide, salicylic acid) during the early period of 2-4 h of collection was significantly lower in UGT1A6 *1/*1 than in UGT1A6 *2/*2 individuals. Further, UGT1A6 *1/*1 individuals excreted a lower percentage of aspirin and its metabolites in the first 12 h and a greater percentage after 12 h than UGT1A6 *2/*2 individuals.

CONCLUSIONS

The variant UGT1A6*2 or polymorphisms in other UGTs that are in linkage disequilibrium with UGT1A6*2 may confer more rapid glucuronidation of salicylic acid than the wild-type UGT1A6 *1/*1.

Glycine--an important neurotransmitter and cytoprotective agent

BACKGROUND

Glycine, the simplest of the amino acids, is an essential component of important biological molecules, a key substance in many metabolic reactions, the major inhibitory neurotransmitter in the spinal cord and brain stem, and an anti-inflammatory, cytoprotective, and immune modulating substance.

MATERIAL AND METHODS

Based on available literature, we discuss some of the important biological properties of glycine. In addition, we describe some clinical disorders where glycine plays a central role, either as an essential structural element, or through its metabolism or receptors.

RESULTS

The past few years have witnessed a broadening of glycine research. The traditional prime interest in aspects related to its role as an inhibitory neurotransmitter in the central nervous system has been expanded to equally emphasize other organs and tissues. With the demonstration of glycine-gated chloride channels on neurons in the central nervous system, on most leukocytes, and subsequently on other cells as well, a unifying mechanism of action accounting for many of the widespread effects of glycine has been found.

CONCLUSIONS

Glycine is a simple, easily available, and inexpensive substance with few and innocuous side-effects. The diversity of biological activities is well documented in the literature. Despite this, glycine has only gained a modest place in clinical medicine.

Does Urine Alkalinization Increase Salicylate Elimination? If so, Why?

Urine alkalinization is a treatment regimen that increases poison elimination by the administration of intravenous sodium bicarbonate to produce urine with a pH > or = 7.5. Experimental and clinical studies confirm that urinary alkalinization increases salicylate elimination, although the mechanisms by which this occurs have not been elucidated. The conventional view is that ionisation of a weak acid, such as salicylic acid, is increased in an alkaline environment. Since the ionisation constant (pKa) is a logarithmic function then, theoretically, a small change in urine pH will have a disproportionately larger effect on salicylate clearance. Hence, elimination of salicylic acid by the kidneys is increased substantially in alkaline urine. However, as salicylic acid is almost completely ionised within physiological pH limits, alkalinization of the urine could not, therefore, significantly increase the extent of ionisation further and the conventional view of the mechanism by which alkalinization is effective is patently impossible. Further experimental studies are required to clarify the mechanisms by which urine alkalinization enhances salicylate elimination.

Inhibition of a medium chain acyl-CoA synthetase involved in glycine conjugation by carboxylic acids

Molecular characteristics of carboxylic acids were investigated for the ability to inhibit a purified medium chain acyl-CoA synthetase, using hexanoic acid as a substrate. Salicylic acid, 4-methylsalicylic acid, 2-hydroxynaphtoic acid, and 2-hydroxyoctanoic acid, which do not act as substrates for the medium chain acyl-CoA synthetase, were potent as inhibitors. Valproic acid was not an inhibitor. Salicylic acid, 2-hydroxynaphthoic acid, and 2-hydroxyoctanoic acid inhibited the medium chain acyl-CoA synthetase with Ki values of 37, 5.2, and 500 microM, respectively. 4-Methylsalicylic acid was more potent than salicylic acid. The inhibitory carboxylic acids were competitive with respect to hexanoic acid. The distance of the hydroxyl group from the carboxylic acid group of the benzene ring influenced the inhibitory activity. The hydroxyl group on the carbon adjacent to the carboxylic acid group was required for inhibitory activity. In addition, there was a good correlation between the lipophilicity of the carboxylic acids and the Ki values, suggesting that the lipophilicity of the carboxylic acids is a major determinant for inhibition of the medium chain acyl-CoA synthetase.

Participation of a medium chain acyl-CoA synthetase in glycine conjugation of the benzoic acid derivatives with the electron-donating groups

Glycine conjugation of a series of benzoic acid derivatives was investigated in bovine liver mitochondria. Benzoic acids with chlorine, methyl, methoxy or ethoxy substituents in the para-or meta-positions of the benzene ring showed a high degree of glycine conjugation. In contrast, the acids with cyano, nitro, amino, or acetylamino groups were conjugated to a small extent with glycine. A medium chain acyl-CoA synthetase that activates carboxylic acids was purified from bovine liver mitochondria. The purified medium chain acyl-CoA synthetase accepted not only medium chain fatty acids but also aromatic and arylacetic acids as substrates. There was a good correlation between the activity of the purified medium chain acyl-CoA synthetase and glycine conjugation of ten benzoic acids with electron-donating substituents. These findings indicate that the purified medium chain acyl-CoA synthetase is a major enzyme for glycine conjugation of benzoic acids with electron-donating groups in bovine live mitochondria.

Dependence of glycine conjugation on availability of glycine: role of the glycine cleavage system

1. Glycine conjugation of benzoic acid was investigated in anaesthetized rats by measuring the disappearance of benzoate from blood, and the appearance of benzoylglycine in blood and urine. 2. Administration of glycine (1-10 mmol/kg,i.v.) increased the capacity of benzoylglycine formation in a dose-dependent fashion, with a maximal rate (8.1 mumol/kg per min) occurring after administration of 5 mmol/kg glycine. The normal endogenous glycine supply (1.7 mM in liver) permits glycine conjugation only at an approximate half-maximal rate (4.5 mumol/kg/per min). 3. The increase in benzoylglycine formation in response to exogenous glycine supply is also a function of the benzoate dosage. Decreased responsiveness at high benzoate dosage indicates that the availability of coenzyme A is another factor that also limits the capacity of glycine conjugation. 4. Cysteamine (200 mg/kg, i.p.), a potent inhibitor of the mitochondrial glycine cleavage system, rapidly increased hepatic glycine concentration 2-3-fold without affecting the concentration of the other co-substrates (i.e. coenzyme A and ATP) of glycine conjugation. 5. Administration of cysteamine increased the blood clearance of benzoate by 50%, the appearance of benzoylglycine in blood, and the urinary excretion of benzoylglycine. 6. It is concluded that the activity of glycine cleavage system is an important determinant of glycine supply and, thereby, the capacity of glycine conjugation of xenobiotics.