Free amino acids in biopsy specimens from the human colonic mucosa

Machine learning-assisted label-free colorectal cancer diagnosis using plasmonic needle-endoscopy system

Early and accurate detection of colorectal cancer (CRC) is critical for improving patient outcomes. Existing diagnostic techniques are often invasive and carry risks of complications. Herein, we introduce a plasmonic gold nanopolyhedron (AuNH)-coated needle-based surface-enhanced Raman scattering (SERS) sensor, integrated with endoscopy, for direct mucus sampling and label-free detection of CRC. The thin and flexible stainless-steel needle is coated with polymerized dopamine, which serves as an adhesive layer and simultaneously initiates the nucleation of gold nanoparticle (AuNP) seeds on the needle surface. The AuNP seeds are further grown through a surface-directed reduction using Au ions-hydroxylamine hydrochloride solution, resulting in the formation of dense AuNHs. The formation mechanism of AuNHs and the layered structure of the plasmonic needle-based SERS (PNS) sensor are thoroughly analyzed. Furthermore, a strong field enhancement of the PNS sensor is observed, amplified around the edges of the polyhedral shapes and at nanogap sites between AuNHs. The feasibility of the PNS sensor combined with endoscopy system is further investigated using mouse models for direct colonic mucus sampling and verifying noninvasive label-free classification of CRC from normal controls. A logistic regression-based machine learning method is employed and successfully differentiates CRC and normal mice, achieving 100% sensitivity, 93.33% specificity, and 96.67% accuracy. Moreover, Raman profiling of metabolites and their correlations with Raman signals of mucus samples are analyzed using the Pearson correlation coefficient, offering insights for identifying potential cancer biomarkers. The developed PNS-assisted endoscopy technology is expected to advance the early screening and diagnosis approach of CRC in the future.

Implications of hydrogen sulfide in colorectal cancer: Mechanistic insights and diagnostic and therapeutic strategies

Hydrogen sulfide (H₂S) is an important signaling molecule in colorectal cancer (CRC). It is produced in the colon by the catalytic synthesis of the colonocytes' enzymatic systems and the release of intestinal microbes, and is oxidatively metabolized in the colonocytes' mitochondria. Both endogenous H₂S in colonic epithelial cells and exogenous H₂S in intestinal lumen contribute to the onset and progression of CRC. The up-regulation of endogenous synthetases is thought to be the cause of the elevated H₂S levels in CRC cells. Different diagnostic probes and combination therapies, as well as tumor treatment approaches through H₂S modulation, have been developed in recent years and have become active area of investigation for the diagnosis and treatment of CRC. In this review, we focus on the specific mechanisms of H₂S production and oxidative metabolism as well as the function of H₂S in the occurrence, progression, diagnosis, and treatment of CRC. We also discuss the present challenges and provide insights into the future research of this burgeoning field.

Nutrition and sulfur

Sulfur is unusual in that it is a mineral that may be taken into the body in both inorganic and organic combinations. It has been available within the environment throughout the development of lifeforms and as such has become integrated into virtually every aspect of biochemical function. It is essential for the nature and maintenance of structure, assists in communication within the organism, is vital as a catalytic assistant in intermediary metabolism and the mechanism of energy flow as well as being involved in internal defense against potentially damaging reactive species and invading foreign chemicals. Recent studies have suggested extended roles for sulfur-containing molecules within living systems. As such, questions have been raised as to whether or not humans are receiving sufficient sulfur within their diet. Sulfur appears to have been the "poor relation" with regards to mineral nutrition. This may be because of difficulties encountered over its multifarious functions, the many chemical guises in which it may be ingested and its complex biochemical interconversions once taken into the body. No established daily requirements have been determined, unlike many minerals, although suggestions have been proposed. Owing to its widespread distribution within dietary components its intake has almost been taken for granted. In the majority of individuals partaking of a balanced diet the supply is deemed adequate, but those opting for specialized or restrictive diets may experience occasional and low-level shortages. In these instances, the careful use of sulfur supplements may be of benefit.

The role of fecal sulfur metabolome in inflammatory bowel diseases

Sulfur metabolism and sulfur-containing metabolites play an important role in the human digestive system, and sulfur compounds and pathways are associated with inflammatory bowel diseases (IBD). In fact, cysteine metabolism results in the production of taurine and sulfate, and gut microbes catabolize them into hydrogen sulfide, a signaling molecule with various biological functions. Besides metabolites originating from sulfur metabolism, several other sulfur-containing metabolites of different classes were detected in human feces, consisting of non-volatile and volatile compounds. Sulfated steroids and bile acids such as taurine-conjugated bile acids are the major classes along with sulfur amino acids and sulfur-containing peptides. Indeed, sulfur-containing metabolites were described in stool samples from healthy subjects, patients suffering from colorectal cancer or IBD. In metabolomics-driven studies, around 50 known sulfur-containing metabolites were linked to IBD. Taurine, taurocholic acid, taurochenodeoxycholic acid, methionine, methanethiol and hydrogen sulfide were regularly reported in IBD studies, and most of them were elevated in stool samples from IBD patients. We summarized from this review that there is strong interplay between perturbed gut microbiota in IBD, and the consistently higher abundance of sulfur-containing metabolites, which potentially represent substrates for sulfidogenic bacteria such as Bilophila or Escherichia and promote their growth. These bacteria might shift their metabolism towards the degradation of taurine and cysteine and therefore to a higher hydrogen sulfide production.

Molecular basis of C-S bond cleavage in the glycyl radical enzyme isethionate sulfite-lyase

Desulfonation of isethionate by the bacterial glycyl radical enzyme (GRE) isethionate sulfite-lyase (IslA) generates sulfite, a substrate for respiration that in turn produces the disease-associated metabolite hydrogen sulfide. Here, we present a 2.7 Å resolution X-ray structure of wild-type IslA from Bilophila wadsworthia with isethionate bound. In comparison with other GREs, alternate positioning of the active site β strands allows for distinct residue positions to contribute to substrate binding. These structural differences, combined with sequence variations, create a highly tailored active site for the binding of the negatively charged isethionate substrate. Through the kinetic analysis of 14 IslA variants and computational analyses, we probe the mechanism by which radical chemistry is used for C-S bond cleavage. This work further elucidates the structural basis of chemistry within the GRE superfamily and will inform structure-based inhibitor design of IsIA and thus of microbial hydrogen sulfide production.

Diet posttranslationally modifies the mouse gut microbial proteome to modulate renal function

Associations between chronic kidney disease (CKD) and the gut microbiota have been postulated, yet questions remain about the underlying mechanisms. In humans, dietary protein increases gut bacterial production of hydrogen sulfide (H2S), indole, and indoxyl sulfate. The latter are uremic toxins, and H2S has diverse physiological functions, some of which are mediated by posttranslational modification. In a mouse model of CKD, we found that a high sulfur amino acid-containing diet resulted in posttranslationally modified microbial tryptophanase activity. This reduced uremic toxin-producing activity and ameliorated progression to CKD in the mice. Thus, diet can tune microbiota function to support healthy host physiology through posttranslational modification without altering microbial community composition.

Campylobacter jejuni: collective components promoting a successful enteric lifestyle

Campylobacter jejuni is the leading cause of bacterial diarrhoeal disease in many areas of the world. The high incidence of sporadic cases of disease in humans is largely due to its prevalence as a zoonotic agent in animals, both in agriculture and in the wild. Compared with many other enteric bacterial pathogens, C. jejuni has strict growth and nutritional requirements and lacks many virulence and colonization determinants that are typically used by bacterial pathogens to infect hosts. Instead, C. jejuni has a different collection of factors and pathways not typically associated together in enteric pathogens to establish commensalism in many animal hosts and to promote diarrhoeal disease in the human population. In this Review, we discuss the cellular architecture and structure of C. jejuni, intraspecies genotypic variation, the multiple roles of the flagellum, specific nutritional and environmental growth requirements and how these factors contribute to in vivo growth in human and avian hosts, persistent colonization and pathogenesis of diarrhoeal disease.

Activity of methylgerambullin from Glycosmis species (Rutaceae) against Entamoeba histolytica and Giardia duodenalis in vitro

Entamoeba histolytica and Giardia duodenalis are widespread intestinal protozoan parasites which both spread via cysts that have to be ingested to infect a new host. Their environment, the small intestine for G. duodenalis and the colon for E. histolytica, contains only very limited amounts of oxygen, so both parasites generate energy by fermentation and substrate level phosphorylation rather than by oxidative phosphorylation. They both contain reducing agents able to reduce and activate nitroimidazole drugs such as metronidazole which is the gold standard drug to treat Entamoeba or Giardia infections. Although metronidazole works well in the majority of cases, it has a number of drawbacks. In animal models, the drug has carcinogenic activity, and concerns about a possible teratogenic activity remain. In addition, the treatment of G. duodenalis infections is hampered by emerging metronidazole resistance. Plant-derived drugs play a dominant role in human medicine, therefore we tested the activity of 14 isolated plant compounds belonging to seven different classes in vitro against both parasites. The tests were performed in a new setting in microtiter plates under anaerobic conditions. The compound with the highest activity was methylgerambullin, a sulphur-containing amide found in Glycosmis species of the family Rutaceae with an EC₅₀ of 14.5 μM (6.08 μg/ml) after 24 h treatment for E. histolytica and 14.6 μM (6.14 μg/ml) for G. duodenalis. The compound was successfully synthesised in the laboratory which opens the door for the generation of new derivatives with higher activity.

•

Fourteen plant compounds of seven classes were isolated.

•

The activity against Entamoeba histolytica and Giardia duodenalis was tested.

•

Methylgerambullin had the highest activity against both parasites.

•

Methylgerambullin is a sulphur-containing amide from Glycosmis spp. (Citrus plants).

•

High cysteine concentrations interfered with methylgerambullin activity.

Sulfur Cycling and the Intestinal Microbiome

In this review, we focus on the activities transpiring in the anaerobic segment of the sulfur cycle occurring in the gut environment where hydrogen sulfide is produced. While sulfate-reducing bacteria are considered as the principal agents for hydrogen sulfide production, the enzymatic desulfhydration of cysteine by heterotrophic bacteria also contributes to production of hydrogen sulfide. For sulfate-reducing bacteria respiration, molecular hydrogen and lactate are suitable as electron donors while sulfate functions as the terminal electron acceptor. Dietary components provide fiber and macromolecules that are degraded by bacterial enzymes to monomers, and these are fermented by intestinal bacteria with the production to molecular hydrogen which promotes the metabolic dominance by sulfate-reducing bacteria. Sulfate is also required by the sulfate-reducing bacteria, and this can be supplied by sulfate- and sulfonate-containing compounds that are hydrolyzed by intestinal bacterial with the release of sulfate. While hydrogen sulfide in the intestinal biosystem may be beneficial to bacteria by increasing resistance to antibiotics, and protecting them from reactive oxygen species, hydrogen sulfide at elevated concentrations may become toxic to the host.

Microbial pathways in colonic sulfur metabolism and links with health and disease

Sulfur is both crucial to life and a potential threat to health. While colonic sulfur metabolism mediated by eukaryotic cells is relatively well studied, much less is known about sulfur metabolism within gastrointestinal microbes. Sulfated compounds in the colon are either of inorganic (e.g., sulfates, sulfites) or organic (e.g., dietary amino acids and host mucins) origin. The most extensively studied of the microbes involved in colonic sulfur metabolism are the sulfate-reducing bacteria (SRB), which are common colonic inhabitants. Many other microbial pathways are likely to shape colonic sulfur metabolism as well as the composition and availability of sulfated compounds, and these interactions need to be examined in more detail. Hydrogen sulfide is the sulfur derivative that has attracted the most attention in the context of colonic health, and the extent to which it is detrimental or beneficial remains in debate. Several lines of evidence point to SRB or exogenous hydrogen sulfide as potential players in the etiology of intestinal disorders, inflammatory bowel diseases (IBDs) and colorectal cancer in particular. Generation of hydrogen sulfide via pathways other than dissimilatory sulfate reduction may be as, or more, important than those involving the SRB. We suggest here that a novel axis of research is to assess the effects of hydrogen sulfide in shaping colonic microbiome structure. Clearly, in-depth characterization of the microbial pathways involved in colonic sulfur metabolism is necessary for a better understanding of its contribution to colonic disorders and development of therapeutic strategies.

Mechanisms of microbial hydrogen disposal in the human colon and implications for health and disease

In the human gastrointestinal tract, dietary components, including fiber, that reach the colon are fermented principally to short-chain fatty acids, hydrogen, and carbon dioxide. Microbial disposal of the hydrogen generated during anaerobic fermentation in the human colon is critical to optimal functioning of this ecosystem. However, our understanding of microbial hydrogenotrophy is fragmented and, at least as it occurs in the colon, is mostly theoretical in nature. Thorough investigation and integration of knowledge on the diversity of hydrogenotrophic microbes, their metabolic variation and activities as a functional group, as well as the nature of their interactions with fermentative bacteria, are necessary to understand hydrogen metabolism in the human colon. Here, we review the limited data available on the three major groups of H(2)-consuming microorganisms found in the human colon [methanogens, sulfate-reducing bacteria (SRB), and acetogens] as well as evidence that end products of their metabolism have an important impact on colonic health.

Kinetic evidence for the presence of putative germination receptors in Clostridium difficile spores

Clostridium difficile is a spore-forming bacterium that causes Clostridium difficile-associated disease (CDAD). Intestinal microflora keeps C. difficile in the spore state and prevents colonization. Following antimicrobial treatment, the microflora is disrupted, and C. difficile spores germinate in the intestines. The resulting vegetative cells are believed to fill empty niches left by the depleted microbial community and establish infection. Thus, germination of C. difficile spores is the first required step in CDAD. Interestingly, C. difficile genes encode most known spore-specific protein necessary for germination, except for germination (Ger) receptors. Even though C. difficile Ger receptors have not been identified, taurocholate (a bile salt) and glycine (an amino acid) have been shown to be required for spore germination. Furthermore, chenodeoxycholate, another bile salt, can inhibit taurocholate-induced C. difficile spore germination. In the present study, we examined C. difficile spore germination kinetics to determine whether taurocholate acts as a specific germinant that activates unknown germination receptors or acts nonspecifically by disrupting spores' membranes. Kinetic analysis of C. difficile spore germination suggested the presence of distinct receptors for taurocholate and glycine. Furthermore, taurocholate, glycine, and chenodeoxycholate seem to bind to C. difficile spores through a complex mechanism, where both receptor homo- and heterocomplexes are formed. The kinetic data also point to an ordered sequential progression of binding where taurocholate must be recognized first before detection of glycine can take place. Finally, comparing calculated kinetic parameters with intestinal concentrations of the two germinants suggests a mechanism for the preferential germination of C. difficile spores in antibiotic-treated individuals.

Metabolism of the colonic mucosa in patients with inflammatory bowel diseases: an in vitro proton magnetic resonance spectroscopy study

Metabolism of the colonic mucosa of patients with ulcerative colitis (UC; n=31) and Crohn's disease (CD; n=26) and normal mucosa (control, n=26) was investigated using in vitro high-resolution proton magnetic resonance spectroscopy. Of the 31 UC patients, 20 were in the active phase and 11 were in the remission phase of the disease. Out of 26 CD patients, 20 were in the active phase, while 6 were in the remission phase of the disease. Twenty-nine metabolites were assigned unambiguously in the perchloric acid extract of colonic mucosa. In the active phase of UC and CD, significantly lower (P<or=.05) concentration of amino acids (isoleucine, leucine, valine, alanine, glutamate and glutamine), membrane components (choline, glycerophosphorylcholine and myo-inositol), lactate and succinate were observed compared to normal mucosa of controls. Patients in the active phase of UC and CD also showed increased level of alpha-glucose compared to normal mucosa. Altered level of metabolites indicates decreased protein and carbohydrate metabolism, thereby decreased energy status and deterioration of mucosa integrity during chronic inflammation. In the remission phase of UC and CD, the concentration of most of the metabolites was similar to controls except for lower values of lactate, glycerophosphorylcholine and myo-inositol in UC and Lac in CD. Formate was significantly lower in patients with the active phase of UC compared to patients with the active phase of CD, suggesting the potential of in vitro MRS in the differentiation of these two diseases.

Do interventions which reduce colonic bacterial fermentation improve symptoms of irritable bowel syndrome?

Abnormal fermentation may be an important factor in irritable bowel syndrome (IBS). Gastroenteritis or antibiotic therapy may damage the colonic microflora, leading to increased fermentation and the accumulation of gas. Gas excretion may be measured by whole-body calorimetry but there has only been one such study on IBS to date. We aimed to assess the relationship between IBS symptoms and fermentation rates in IBS. A purpose-built, 1.4-m3, whole-body calorimeter was used to assess excretion of H2 and CH4 in IBS subjects while consuming a standard diet and, again, after open randomization on either the standard diet together with the antibiotic metronidazole or a fiber-free diet to reduce fermentation. Metronidazole significantly reduced the 24-hr excretion of hydrogen (median value compared to the control group, 397 vs 230 ml/24 hr) and total gas (H2 + CH4; 671 vs 422 ml/min) and the maximum rate of gas excretion (1.6 vs 0.8 ml/min), as did a no-fiber polymeric diet (hydrogen, 418 vs 176 ml/min; total gas, 564 vs 205 ml/min; maximum rate of gas excretion, 1.35 vs 0.45 ml/min), with a significant improvement in abdominal symptoms. IBS may be associated with rapid excretion of gaseous products of fermentation, whose reduction may improve symptoms.

Glutathione uptake after intraperitoneal administration and glutathione radiopharmacology after rectal administration, in mice

Glutathione is a biologic aminothiol radioprotector. Hydrolysis of exogenous glutathione takes place in the extracellular compartment and leads to two metabolites: gamma-glutamylcysteine and glycine. In healthy mice, after an intraperitoneal administration of glutathione, all organs absorb the gamma-glutamylcysteine and the glycine with variable kinetics according to their enzymatic equipment. The rectal administration of glutathione in mice previously irradiated at the pelvic region, increases the availability of glutathione in the rectum and in other organs at a distant from the irradiation site. This contribution could be used to protect the rectum and the uterus during therapeutic irradiation.

The effects of short-term parenteral nutrition on human liver protein and amino acid metabolism during laparoscopic surgery

BACKGROUND

This study was undertaken to elucidate the specific effects of short-term artificial nutrition on human liver protein metabolism.

METHODS

Thirty patients undergoing elective laparoscopic cholecystectomy were studied: a control group (n = 16) and a group that received total parenteral nutrition (TPN; n = 14). The nutrition consisted of a balanced i.v. solution of nutrients (17.5 nonprotein kcal/kg body wt, 50% fat, 50% carbohydrates, and 0.1 gN/kg) that was discontinued when the investigation was finished, after a total infusion time of 8.6 +/- 1.0 hours. A liver biopsy specimen was taken as soon as possible after surgery was started, for the determination of the free hepatic amino acid concentrations. In 16 of the patients, L[2H5]phenylalanine was given by i.v. to determine the fractional synthesis rate of total liver protein in a second liver biopsy specimen taken approximately 30 minutes later.

RESULTS

The fractional synthesis rate of total liver protein was 15.2% +/- 4.7%/d in the TPN group (n = 7), which was not different from that of the control group (17.7% +/- 3.8%/d, n = 9). However, the free hepatic concentrations of alanine (p < .05) and the essential amino acids increased (p < .001) in the TPN group, whereas the total hepatic amino acid concentrations were comparable between the groups.

CONCLUSION

Thus short-term TPN induced specific changes of the free hepatic amino acid concentrations, whereas total liver protein synthesis remained unaffected by the nutrition.

Thiolmethyltransferase activity in the human colonic mucosa: implications for ulcerative colitis

Ulcerative colitis is associated with a selective reduction of n-butyrate oxidation by the colonic epithelial cells although the reason for this has been unclear. Colonic epithelial cell n-butyrate oxidation can be inhibited in vitro by incubation with sulphide but the role of mucosal detoxification of sulphide in the metabolic welfare of the colonic mucosa has not been examined. This study aimed to assess the role mucosal detoxification of sulphide by thiolmethyltransferase (TMT)-mediated methylation may play in protecting the healthy colonic mucosa from the adverse effects of luminal sulphide. Colonic epithelial cell suspensions from healthy human proximal (n = 9) and distal colon (n = 10) were incubated in the presence of 14C-labelled n-butyrate (5 mmol/L) alone, butyrate plus sodium hydrogen sulphide (NaHS) (1.5 mmol/L), or butyrate plus NaHS plus S-adenosyl-methionine 1,4 butane disulphonate (SAMe) (5 mmol/L). Study end points were metabolic performance (14CO2 production) and mucosal TMT activity. Incubation with NaHS induced a significant inhibition of 14CO2 production compared with control incubations (P < 0.001) which was similar for proximal and distal colonic cell suspensions. S-adenosyl-methionine 1,4 butane disulphonate reversed this effect completely in proximal but not in distal cell incubations, suggesting a greater susceptibility of the distal colon to the sulphide effect. Although median whole mucosal TMT values did not differ between proximal and distal colonic mucosa, a non-normal distribution of distal TMT values was observed. However, neither the degree of sulphide inhibition of control 14CO2 production nor the degree to which SAMe reversed this inhibition correlated with whole mucosal TMT activity. The study concluded that regional variation exists in TMT activity in the human colon but whilst methylation appears to protect colonic epithelial cells against sulphide-induced inhibition of n-butyrate oxidation, this cannot be directly correlated with mucosal TMT activity.

Methionine derivatives diminish sulphide damage to colonocytes--implications for ulcerative colitis

BACKGROUND

Bacterial production of anionic sulphide is increased in the colon of ulcerative colitis and sulphides can cause metabolic damage to colonocytes.

AIMS

To assess the reversal of the damaging effect of sulphide to isolated colonocytes by methionine and methionine derivatives.

METHODS AND SUBJECTS

Isolated colonocytes were prepared from rat colons and 12 human colectomy specimens. In cell suspensions 14CO2/acetoacetate generation was measured from [1-14C]-butyrate (5.0 mmol/l) in the presence of 0-2.0 mmol/l sodium hydrogen sulphide. The effect of 5.0 mmol/l L-methionine, S-adenosylmethionine 1,4 butane disulphonate and DL-methionine-S-methylsulphonium chloride on sulphide inhibited oxidation was observed.

RESULTS

In rat colonocytes sodium hydrogen sulphide dose dependently reduced oxidative metabolite formation from n-butyrate, an action reversed in order of efficacy by S-adenosylmethionine 1,4 butane disulphonate > DLmethionine-S-methyl-sulphonium chloride > L-methionine. In human colonocytes S-adenosylmethionine 1,4 butane disulphonate most significantly improved 14CO2 production (p = < 0.005) suppressed by sodium hydrogen sulphide.

CONCLUSION

Sulphide toxicity in colonocytes is reversible by methyl donors. The efficiency of sulphide detoxification may be an important factor in the pathogenesis and treatment of ulcerative colitis for which S-adenosylmethionine 1,4 butane disulphonate may be of therapeutic value.

The concentrations of free amino acids in human liver tissue obtained during laparoscopic surgery

The concentrations of the free amino acids in individual tissues gives information concerning amino acid, energy and protein metabolism. In muscle and intestinal mucosa, different metabolic states are distinctly characterized by an altered free amino acid pattern. Furthermore, the patterns are quite different in individual tissues. So far, liver tissue has not been investigated systematically in this respect. The aims of this investigation were to establish a standardized sampling procedure for liver tissue during laparoscopic surgery and to characterize the free amino acid concentrations in human liver tissue. Aspartate was the most abundant amino acid in the liver, followed by taurine, glutamine, glutamate, glycine and alanine. These six, and most abundant, amino acids constitute 90% of the total hepatic amino acid concentration. In the future, liver tissue sampling during laparoscopic surgery may be used as a model for investigating the influence of nutrition and hormones on hepatic amino acid and protein metabolism in man.

Intestinal amino acid content in critically ill patients

BACKGROUND

The purpose of the study was to determine the concentrations of free amino acids and the total protein content of the human intestinal mucosa during critical illness.

METHODS

The free amino acid and protein concentrations in endoscopically obtained biopsy specimens from the duodenum and the distal colonic segments were determined on 19 critically ill patients. The free amino acids were separated by ion exchange chromatography and detected by fluorescence, and the protein content was quantified by the method of Lowry.

RESULTS

In general, the typical amino acid pattern of the intestinal mucosa was seen, with very high levels of taurine, aspartate and glutamic acid. The main difference, as compared to a reference series of healthy subjects, was the elevated glutamine concentration of the duodenal mucosa. This amino acid was unaltered in the descending colon and depressed in the rectum. At the same time, the glutamatic acid concentrations were unaltered, suggesting that the degradation of glutamine was not increased in the septic state of the majority of the patients studied. Phenylalanine and the two branched-chain amino acids, valine and leucine, were elevated in the duodenal mucosa, and in the colonic mucosa, methionine and phenylalanine were elevated; otherwise, all the other individual amino acids were unaltered or depressed.

CONCLUSIONS

The alterations seen in mucosal free amino acid and protein concentrations in connection with critical illness are different in many respects and contrast with the findings seen after starvation or moderate surgical trauma.

The content of free amino acids in the human duodenal mucosa

The free amino acid concentrations and the total protein content of the duodenal mucosa were determined in biopsy specimens obtained during endoscopic examinations in 10 healthy subjects. The amino acids were separated and quantified by ion exchange chromatography using fluorescence detection. The protein content was analysed according to Lowry. The amino acid pattern found in the duodenal mucosa was quite different from that in the plasma. The total amounts of all individual free amino acids were considerably higher in the mucosa than in the plasma (16.2 +/- 0.6 mmol/kg biopsy weight compared to 2.4 +/- 0.1 mmol/l). Taurine, glutamate and aspartate constituted more then 65% of the total content of all amino acids in the mucosa. Glutamine, the most abundant amino acid in plasma (21%), ranked only as sixth in the duodenal mucosa (4%); still, the absolute concentrations were quite similar in the mucosa and plasma (0.60 +/- 0.05 mmol/kg vs. 0.53 +/- 0.02 mmol/l). This study demonstrates the possibility of determining free amino acids in endoscopic biopsy specimens from the human duodenum. The technique is recommended for repeated sampling in clinical studies on the amino acid metabolism of the intestinal mucosa.