In vitro metabolism of creatinine, methylamine and amino acids by intestinal contents of normal and uraemic subjects

Challenges of Serum Creatinine Level in GFR Assessment and Drug Dosing Decisions in Kidney Injury

Serum creatinine (SCr) is widely regarded as a standard biomarker for assessing glomerular filtration rate (GFR) and is commonly used to guide dose adjustments for renally eliminated drugs. However, the application of SCr as a marker for evaluating GFR and drug dosing in kidney injury has significant limitations that are often overlooked in clinical practice. This oversight can result in subtherapeutic drug concentrations or adverse drug reactions due to inappropriate dosing adjustments based on SCr levels alone. This review aimed to highlight the factors affecting serum creatinine (SCr) and the challenges associated with using SCr as a biomarker for assessing GFR and adjusting drug doses with regard to its limitations and variability. The findings of this review underscore the complexity of SCr regulation, which is affected by its synthesis, metabolism, and excretion processes (glomerular filtration, tubular secretion, tubular reabsorption and extra-renal elimination), and disease states (such as trauma-induced hyperfiltration and HIV) and the use of medications (drug-creatinine interactions) lead to altered renal excretion of creatinine, either increasing or decreasing its levels. Additionally, the renal excretion pathways for drugs and creatinine are not entirely the same, making it difficult to use creatinine to evaluate drug renal excretion. In conclusion, SCr is an imperfect index of GFR and adjusting drug dosing, and the development of multi-biomarker panels, incorporating biomarkers from different excretory pathways-particularly those involving tubular transport-holds promise for improving the evaluation of renal excretory function and ensuring safer and more effective drug dosing.

Methylamine in human urine

BACKGROUND

Methylamine is the simplest aliphatic amine found in human urine. In the body it is thought to play a significant part in central nervous system disturbances observed during renal and hepatic disease and also has a role in general toxicity caused by oxidative stress. The present study provides data on the daily urinary excretion of methylamine in a population of unrelated healthy volunteers and investigates a variety of food substances as potential dietary sources.

METHODS

Complete 0-24 h urine samples were collected from 203 volunteers (102 male, age 22.2 +/- 4.5 years, mean +/- S.D.; 101 female, age 21.6 +/- 5.0 years) maintained on their normal diets. Six male subjects also consumed, on different occasions separated by at least 1 week, 41 foods and collected the subsequent 0-8 h urine. In addition, these subjects also ingested various dietary precursors (betaine, carnitine, choline, creatinine, lecithin) and collected the following 0-3 day urine. All urine samples were analysed for their methylamine content.

RESULTS

The average daily output of methylamine was 11.00 +/- 8.17 mg (12.73 +/- 9.35 male; 9.27 +/- 6.35 female) with a range of values spreading from 1.68 to 62.30 mg. Dietary studies suggested that certain fish and seafoods (clam, crab, haddock, halibut, octopus, tuna) and fruit and vegetables (pear, peas, tomato) may add to this urinary output. Ingestion of creatinine also increased urinary methylamine levels.

CONCLUSIONS

For the first time, standard daily excretion values for methylamine have been established for a large population. Chemical and dietary precursor studies indicated that there was no major exogenous source of this amine and suggested that the origin of the majority of human urinary methylamine is endogenous with only subtle contributions from the diet.

Creatine and creatinine metabolism

The goal of this review is to present a comprehensive survey of the many intriguing facets of creatine (Cr) and creatinine metabolism, encompassing the pathways and regulation of Cr biosynthesis and degradation, species and tissue distribution of the enzymes and metabolites involved, and of the inherent implications for physiology and human pathology. Very recently, a series of new discoveries have been made that are bound to have distinguished implications for bioenergetics, physiology, human pathology, and clinical diagnosis and that suggest that deregulation of the creatine kinase (CK) system is associated with a variety of diseases. Disturbances of the CK system have been observed in muscle, brain, cardiac, and renal diseases as well as in cancer. On the other hand, Cr and Cr analogs such as cyclocreatine were found to have antitumor, antiviral, and antidiabetic effects and to protect tissues from hypoxic, ischemic, neurodegenerative, or muscle damage. Oral Cr ingestion is used in sports as an ergogenic aid, and some data suggest that Cr and creatinine may be precursors of food mutagens and uremic toxins. These findings are discussed in depth, the interrelationships are outlined, and all is put into a broader context to provide a more detailed understanding of the biological functions of Cr and of the CK system.

Dietary precursors of trimethylamine in man: a pilot study

An increased urinary excretion of trimethylamine and its N-oxide were observed in man following the oral intake (15 mmol) of choline (63% dose as trimethylamine and its N-oxide), D,L-carnitine (31% dose) and trimethylamine N-oxide (78% dose). Similar ingestion of betaine, creatinine or lecithin failed to elicit any significant increases. Of 46 different foods investigated, only fish and other sea-products gave rise to significant increases in urinary trimethylamine and N-oxide. Ingestion of fruits, vegetables, cereal and dairy produce, and meats had no measurable effects. Reasons for the apparent lack of trimethylamine provision by foods previously thought to be precursors are given and the role of gut microflora highlighted.

Dimethylamine formation in the rat from various related amine precursors

Dimethylamine is the immediate precursor of dimethylnitrosamine, a known potent carcinogen in a wide variety of animal species. Although small amounts of dimethylamine are ingested directly, the major dietary source is believed to be via choline and related materials. Owing to quantitative recoveries following oral administration, urinary dimethylamine levels provide good overall measures of body exposure. The oral administration of equimolar amounts (1 mmol/kg body weight) of potential amine precursors to male Wistar rats produced only small increases in urinary dimethylamine after choline (+ 11%; 0.60 +/- 0.36% dose), dimethylaminopropanol (+ 32%; 1.49 +/- 0.30% dose), dimethylaminoethyl chloride (+ 110% 5.38 +/- 1.72% dose) and trimethylamine (+ 51%; 1.6 +/- 0.80% dose) input, whereas significantly larger increases were found following trimethylamine N-oxide ingestion (+ 355%; 12.93 +/- 1.13% dose; t-test, P < 0.001). These data suggest that trimethylamine N-oxide is a major dietary source of dimethylamine, by direct conversion and not by sequential reduction (to trimethylamine) and demethylation, and that in this respect it is of greater importance, on a molar basis, than choline.

Fecal methylamine and dimethylamine in chronic renal failure

Determinations of methylamine and dimethylamine in the fecal samples from normal subjects (n = 22), nonhemodialysis patients (n = 10), and hemodialysis patients (n = 14) with chronic renal failure have been made by high-performance liquid chromatography of their 2,4-dinitrophenyl derivatives. Fecal methylamine level was significantly lower in the normal group than in the nonhemodialysis group (P less than 0.05) and in the hemodialysis group (P less than 0.05). The mean dimethylamine value of the hemodialysis group was significantly higher than that of the nonhemodialysis group (P less than 0.01) and that of the normal group (P less than 0.005). The method has also been applied to the determination of the two amines in the fecal samples from two patients with leukemia who had been isolated and sterilized in the laminar air flow rooms. Preliminary in vitro experiments were given of the possible pathway for the production of these amines by the incubation of normal fecal samples with added creatinine.