Digestion and absorption of dietary protein in man

A focus on leucine in the nutritional regulation of human skeletal muscle metabolism in ageing, exercise and unloading states

Muscle protein synthesis (MPS) and muscle protein breakdown (MPB) are influenced through dietary protein intake and physical (in)activity, which it follows, regulate skeletal muscle (SKM) mass across the lifespan. Following consumption of dietary protein, the bio-availability of essential amino acids (EAA), and primarily leucine (LEU), drive a transient increase in MPS with an ensuing refractory period before the next MPS stimulation is possible (due to the "muscle full" state). At the same time, MPB is periodically constrained via reflex insulin actions. Layering exercise on top of protein intake increases the sensitivity of SKM to EAA, therefore extending the muscle full set-point (∼48 h), to permit long-term remodelling (e.g., hypertrophy). In contrast, ageing and physical inactivity are associated with a premature muscle full set-point in response to dietary protein/EAA and contractile activity. Of all the EAA, LEU is the most potent stimulator of the mechanistic target of rapamycin complex 1 (mTORC1)-signalling pathway, with the phosphorylation of mTORC1 substrates increasing ∼3-fold more than with all other EAA. Furthermore, maximal MPS stimulation is also achieved following low doses of LEU-enriched protein/EAA, negating the need for larger protein doses. As a result, LEU supplementation has been of long term interest to maximise muscle anabolism and subsequent net protein accretion, especially when in tandem with resistance exercise. This review highlights current knowledge vis-à-vis the anabolic effects of LEU supplementation in isolation, and in enriched protein/EAA sources (i.e., EAA and/or protein sources with added LEU), in the context of ageing, exercise and unloading states.

Complex of Proline-Specific Peptidases in the Genome and Gut Transcriptomes of Tenebrionidae Insects and Their Role in Gliadin Hydrolysis

A detailed analysis of the complexes of proline-specific peptidases (PSPs) in the midgut transcriptomes of the larvae of agricultural pests Tenebrio molitor and Tribolium castaneum and in the genome of T. castaneum is presented. Analysis of the T. castaneum genome revealed 13 PSP sequences from the clans of serine and metal-dependent peptidases, of which 11 sequences were also found in the gut transcriptomes of both tenebrionid species' larvae. Studies of the localization of PSPs, evaluation of the expression level of their genes in gut transcriptomes, and prediction of the presence of signal peptides determining secretory pathways made it possible to propose a set of peptidases that can directly participate in the hydrolysis of food proteins in the larvae guts. The discovered digestive PSPs of tenebrionids in combination with the post-glutamine cleaving cysteine cathepsins of these insects effectively hydrolyzed gliadins, which are the natural food substrates of the studied pests. Based on the data obtained, a hypothetical scheme for the complete hydrolysis of immunogenic gliadin peptides by T. molitor and T. castaneum digestive peptidases was proposed. These results show promise regarding the development of a drug based on tenebrionid digestive enzymes for the enzymatic therapy of celiac disease and gluten intolerance.

The Effect of a Compound Protein on Wound Healing and Nutritional Status

Proteins provide the main building blocks for tissue growth, cell renewal, and repair during wound healing. We aimed to examine the effect of a compound protein on wound healing, nutritional status, and underlying mechanisms. We first performed a preliminary experiment to identify the appropriate wound healing assessment points. In the formal experiment, there were five groups (control group: 8.3750 g/kg/day saline solution; model group: 8.3750 g/kg/day saline solution; whey protein group: 8.3750 g/kg/day whey protein; low-dose compound protein group: 4.1875 g/kg/day compound protein; and high-dose compound protein group: 8.3750 g/kg/day compound protein) with eight rats in each group. At each turning point, we observed the wound healing rate and nutritional status of the different groups of rats. In addition, biochemical assays were used to determine the mechanisms underlying the effects of the compound protein. In the preliminary experiment, the third day after modeling was the turning point between the inflammatory and proliferation phases, and the eighth day was the turning point between the proliferation and remodeling phases. The formal experiment evaluated wound healing condition, inflammatory response, angiogenesis, collagen deposition, and nutritional status. A pathological report showed increased vascularization, collagen deposition, and epithelialization in compound protein-treated groups. Protein-treated mice showed decreased interleukin (IL)-6, IL-8, neutrophils, and lymphocytes and increased IL-10, albumin, prealbumin, total protein levels, insulin-like growth factor 1 (IGF-1), fibroblast growth factor 2 (FGF-2), and vascular endothelial growth factor (VEGF) expressions. All parameters were significant (p < 0.05) compared to the model group. There was a dose-dependent effect of the compound protein. The accelerated wound healing mechanism may be that the compound protein accelerates the whole wound healing process, making wounds transition from the inflammatory phase to the proliferation phase faster, entering the remodeling phase earlier. Administration of a compound protein can accelerate wound healing and improve the nutritional status.

Gastrointestinal Interaction between Dietary Amino Acids and Gut Microbiota: With Special Emphasis on Host Nutrition

The gastrointestinal tract (GIT) of humans and animals is host to a complex community of different microorganisms whose activities significantly influence host nutrition and health through enhanced metabolic capabilities, protection against pathogens, and regulation of the gastrointestinal development and immune system. New molecular technologies and concepts have revealed distinct interactions between the gut microbiota and dietary amino acids (AAs) especially in relation to AA metabolism and utilization in resident bacteria in the digestive tract, and these interactions may play significant roles in host nutrition and health as well as the efficiency of dietary AA supplementation. After the protein is digested and AAs and peptides are absorbed in the small intestine, significant levels of endogenous and exogenous nitrogenous compounds enter the large intestine through the ileocaecal junction. Once they move in the colonic lumen, these compounds are not markedly absorbed by the large intestinal mucosa, but undergo intense proteolysis by colonic microbiota leading to the release of peptides and AAs and result in the production of numerous bacterial metabolites such as ammonia, amines, short-chain fatty acids (SCFAs), branched-chain fatty acids (BCFAs), hydrogen sulfide, organic acids, and phenols. These metabolites influence various signaling pathways in epithelial cells, regulate the mucosal immune system in the host, and modulate gene expression of bacteria which results in the synthesis of enzymes associated with AA metabolism. This review aims to summarize the current literature relating to how the interactions between dietary amino acids and gut microbiota may promote host nutrition and health.

Prolidase is a critical enzyme for complete gliadin digestion in Tenebrio molitor larvae

Prolidase is a proline-specific metallopeptidase that cleaves imidodipeptides with C-terminal Pro residue. Prolidase was purified and characterized from the Tenebrio molitor larval midgut. The enzyme was localized in the soluble fraction of posterior midgut tissues, corresponding to a predicted cytoplasmic localization of prolidase according to the structure of the mRNA transcript. Expression of genes encoding prolidase and the major digestive proline-specific peptidase (PSP)-dipeptidyl peptidase 4-were similar. The pH optimum of T. molitor prolidase was 7.5, and the enzyme was inhibited by Z-Pro, indicating that it belongs to type I prolidases. In mammals, prolidase is particularly important in the catabolism of a proline-rich protein-collagen. We propose that T. molitor larval prolidase is a critical enzyme for the final stages of digestion of dietary proline-rich gliadins, providing hydrolysis of imidodipeptides in the cytoplasm of midgut epithelial cells. We propose that the products of hydrolysis are absorbed from the luminal contents by peptide transporters, which we have annotated in the T. molitor larval gut transcriptome. The origin of prolidase substrates in the insect midgut is discussed in the context of overall success of grain feeding insects.

Human gut microbiota: does diet matter?

The human oro-gastrointestinal (GI) tract is a complex system, consisting of oral cavity, pharynx, oesophagus, stomach, small intestine, large intestine, rectum and anus, which all together with the accessory digestive organs constitute the digestive system. The function of the digestive system is to break down dietary constituents into small molecules and then absorb these for subsequent distribution throughout the body. Besides digestion and carbohydrate metabolism, the indigenous microbiota has an important influence on host physiological, nutritional and immunological processes, and commensal bacteria are able to modulate the expression of host genes that regulate diverse and fundamental physiological functions. The main external factors that can affect the composition of the microbial community in generally healthy adults include major dietary changes and antibiotic therapy. Changes in some selected bacterial groups have been observed due to controlled changes to the normal diet e.g. high-protein diet, high-fat diet, prebiotics, probiotics and polyphenols. More specifically, changes in the type and quantity of non-digestible carbohydrates in the human diet influence both the metabolic products formed in the lower regions of the GI tract and the bacterial populations detected in faeces. The interactions between dietary factors, gut microbiota and host metabolism are increasingly demonstrated to be important for maintaining homeostasis and health. Therefore the aim of this review is to summarise the effect of diet, and especially dietary interventions, on the human gut microbiota. Furthermore, the most important confounding factors (methodologies used and intrinsic human factors) in relation to gut microbiota analyses are elucidated.

Small intestinal absorption of glycine and glycyl-glycine in patients with chronic renal failure

Intestinal absorption of glycine and glycyl-glycine was studied in 9 patients with chronic renal insufficiency (mean creatinine clearance 9 ml/min) and 7 healthy controls. After an oral load of the amino acid or dipeptide, plasma alpha-amino nitrogen (alpha-NH2N) was measured repeatedly for two hours and the area under curve (AUC) was calculated using the trapezoidal rule. In uremic patients, plasma alpha-NH2N was significantly lower after glycine than after glycyl-glycine at 30, 45 and 60 min and also AUC was smaller after the amino acid indicating a reduced total uptake. One patient with severe, terminal uremia had a flat curve after glycine administration. In control subjects, plasma levels were lower after glycine than after glycyl-glycine at 30 min but AUC did not differ between the amino acid and the dipeptide. Generally these results agree with those in earlier studies of non-uremic man showing that, in small intestine, dipeptides are taken up better and by a different mechanism than amino acids.

The movement of proteins across the insect and tick digestive system

The movement of intact proteins across the digestive system was shown in a number of different blood-feeding and non-blood-feeding insects in the orders Blattaria, Coleoptera, Diptera, Hemiptera, Lepidoptera, Orthoptera, Neuroptera and Siphonaptera, as well as in two tick families Ixodidae and Argasidae. Protein movement was observed for both normal dietary and xenobiotic proteins, which suggest that the mechanism for transfer is not substrate specific. The number of studies on the mechanism of movement is limited. The research so far suggests that movement can occur by either a transcellular or an intercellular pathway in the ventriculus with most of the research describing the former. Transfer is by continuous diffusion with no evidence of pinocytosis or vesicular transport common in mammalian systems. Proteins can move across the digestive system without modification of their primary or multimeric structure and with retention of their functional characteristics. Accumulation in the hemolymph is the result of the protein degradation rate in the gut and hemolymph and transfer rate across the digestive system and can be highly variable depending on species. Research on the development of delivery systems to enhance protein movement across the insect digestive system is in its infancy. The approaches so far considered with some success include the use of lipophilic-polyethylene glycol (PEG) polymers, the development of fusion proteins with lectins, reduced gut protease activity and the development of amphiphilic peptidic analogs. Additional research on understanding the basic mechanisms of protein delivery across the insect digestive system, the importance of structure activity in this transfer and the development of technology to improve movement across the gut could be highly significant to the future of protein and nucleic acid-based insecticide development as well as traditional chemical insecticidal technologies.

Pharmacological study of TA-0910, a new thyrotropin-releasing hormone (TRH) analog, (I): Effects on the central nervous system by oral administration

Effects of orally administered TA-0910, a new thyrotropin-releasing hormone (TRH) analog, on the central nervous system (CNS) were investigated and compared with those of TRH. TA-0910 shortened the duration of pentobarbital-induced sleep and antagonized reserpine-induced hypothermia at 0.3 mg/kg or more in mice. TA-0910 enhanced the spontaneous motor activity at the higher dose of 30 mg/kg in mice. The drug also activated acute spontaneous EEGs in rabbits at 1 mg/kg. TRH produced these effects at about 100 times higher doses than TA-0910. In antagonizing pentobarbital-induced sleep, the dose ratios of i.v. versus p.o. of TA-0910 and TRH were about 1/10 and 1/100, respectively. The duration of the antagonistic action of TA-0910 on pentobarbital-induced sleep in mice was about 8 times longer than that of TRH when administered orally as well as intravenously. These potent and long-acting TA-0910 effects on the CNS are discussed in connection with its biotransformation.

Changes of the immune response due to the absorption of antigenic protein or peptides

Rabbits immunized against human gammaglobulin (HGG) were fed with either intact HGG or pepsin-digested HGG. Circulating antibodies were determined over an absorption time of 4 h. The oral application of antigenic peptides reduced circulating antibodies from 550 micrograms/ml to 120 micrograms/ml. A further parenteral antigen exposure to 50 mg HGG revealed that orally pretreated animals are protected against a marked blood pressure decrease due to antigen antibody reactions. One possible explanation for this phenomenon is that circulating antibodies are fixed in the wall of the gut by enterally applied antigenic peptides and eliminated into the lumen of the gastrointestinal tract. If this phenomenon can be extrapolated to human beings it would be a new therapeutic concept for the treatment of diseases in which circulating antibodies are of importance.

Augmented release of gastric inhibitory polypeptide into the portal vein in response to intraduodenal glucose and amino acids in anesthetized rats treated with methylprednisolone or alloxan

Thirty rats were treated with methylprednisolone, 30 rats were treated with alloxan, and 30 control rats were treated with saline alone. The levels of fasting serum insulin and blood glucose and of plasma GIP before and after duodenal instillation of glucose or amino acids were measured using an acute rat preparation that enabled multiple blood samplings from the portal vein. Treatment of the rats with methylprednisolone was followed by increased fasting levels of serum insulin, blood glucose, and plasma GIP and by an augmented GIP release in response to duodenal glucose and amino acids as compared with normal controls. Similarly, treatment with alloxan was followed by decreased fasting levels of serum insulin, by increased fasting levels of blood glucose and plasma GIP, and by an increased GIP release in response to duodenal glucose and amino acids. The augmented GIP release in response to duodenal instillation of glucose and amino acids both in methylprednisolone-treated rats and in alloxan-treated rats may be explained by an increased absorption of these nutrients owing to an increased Na+ K+ ATPase activity in the intestinal mucosa of corticosteroid- and alloxan-treated rats. The elevated fasting GIP levels, on the other hand, are difficult to explain.

Availability to pigs of amino acids in cereal grains. 1. Endogenous levels of amino acids in ileal digesta and faeces of pigs given cereal diets

1. Endogenous levels of amino acids in ileal digesta were determined as the output from pigs given protein-free diets and by extrapolation to zero intake of linear regressions of ileal amino acid output v. dietary amino acid intake. The protein-free diets included 0 or 50 g cellulose/kg and extrapolations were made from two series of four diets which contained graded levels of wheat or barley as the only source of protein. Within each series, dietary fibre level (mg/g) was maintained at approximately 140 or 190 neutral-detergent fibre (NDF) respectively. Endogenous amino acid levels in faeces were also determined. 2. Endogenous amino acid output in faeces was linearly related to dietary fibre level; endogenous ileal output increased with dietary fibre up to approximately 100 mg NDF/g, after which endogenous output no longer increased. 3. The amino acid composition of endogenous ileal protein varied little among levels of output and among different experiments. The composition appears to be determined by the predominance of mucin protein, the slow absorption of some acids and the methods commonly used to measure output. The very high levels of proline and glycine in ileal digesta seemed characteristic only of protein-free and low-protein diets. 4. The amino acid composition of endogenous faecal protein also varied little among different estimates, but was considerably different from that of endogenous ileal protein. Furthermore, the similarity of bacterial and faecal proteins suggested that much of the endogenous faecal protein was of bacterial origin.