Current Protein and Peptide Science - Volume 20, Issue 2, 2019

Dietary protein intake as a critical regulatory factor of bone metabolism is a vital element to regulate nutritional status of mammals. Under the action of protease, dietary protein is digested into peptides and free amino acids (FAAs). Then, the metabolites are absorbed by enterocytes and metabolized in various organs of mammals. The dietary protein intake regulates bone metabolism generally via two aspects, dietary itself and signaling transduction. At the dietary level, different kinds of amino acids (AAs) of dietary protein may affect various protein metabolism of bone by regulating proteasome depending on proteolysis and protein synthesis. In addition, dietary protein from multiple sources such as animal, vegetal and healthcare products, presents distinct influences on bone metabolism via regulating calcium balance; At the cellular level, these products can regulate several biological functions via regulating signaling transduction. For example, the significant member of growth hormone/insulin-like growth factor (GH/IGF) axis can be regulated by dietary protein, which has an influence on bone metabolism through different approaches. This review mainly discusses the relationship between dietary protein and GH/IGF axis and illustrates the regulation of bone metabolism in mammals by dietary protein and its signaling transduction.

Protein is essential to growth and metabolism. Many factors influence dietary protein digestion and utilization in the gastrointestinal tract. Probiotics have attracted increasing attention in recent years owing to their broad health benefits, which may include a positive influence on the digestion and utilization of proteins. Several observations support their potential role in protein digestion. For example, probiotics can regulate the intestinal microflora and thereby influence intestinal bacteria related to proteolysis. Probiotics can also induce host digestive protease and peptidase activity, and some can release exoenzymes involved in the digestion of proteins. In addition, probiotics can improve the absorption of small peptides and amino acids by improving the absorption ability of the epithelium and enhancing transport. Furthermore, probiotics can reduce harmful protein fermentation and thus decrease the toxicity of metabolites. In this review, the roles of probiotics in intestinal protein digestion and utilization and the potential mechanisms underlying these effects are discussed.

The high inclusion of dietary protein and the imbalance of amino acid (AA) composition in animal husbandry result in inefficient utilization of protein resources and increased nitrogen excretion. Therefore, an efficient approach to alleviate the nitrogen excretion and increase the utilization of protein resources is to formulate the AA-balance protein-restricted diet with crystalline AA supplementation. Nowadays, it is essential to thoroughly understand the regulatory mechanisms of AAs on body metabolism and their vital roles in protein-restricted diets. Besides, an establishment of the amino acid balanced protein-restricted diet system is beneficial for the maintenance of healthy environment and sustainable animal husbandry. This review focused on the recent advances on functional roles of AAs and development of a protein-restricted diet system in animal husbandry.

Lactoferrin (lactotransferrin; Lf) is an iron-binding glycoprotein and one of the most important bioactivators in milk and other external secretions. It has numerous biological roles, including the regulation of iron absorption and modulation of immune responses, and has anti-microbial, anti-viral, antioxidant, anti-cancer, and anti-inflammatory activities. Lf regulates the quantity of iron absorbed in the intestine via its role in iron transport and can also chelate iron, directly or indirectly. Notably, it has been used as an adjuvant therapy for some intestinal diseases. It is now used in nutraceuticalsupplemented infant formula and other food products. This article reviews the content, distribution, physiologic functions and current applications of Lf, and aims to shed light on future prospects for additional applications of Lf.

Dietary protein and its metabolites, amino acids, are essential nutrients for humans and animals. Accumulated research has revealed that the gut microbiota mediate the crosstalk between protein metabolism and host immune response. Gut microbes are involved in the digestion, absorption, metabolism and transformation process of dietary protein in the gastrointestinal tract. Amino acids can be metabolized into numerous microbial metabolites, and these metabolites participate in various physiological functions related to host health and diseases. The components of dietary protein impact the gut microbiota composition and microbial metabolites. The source, concentration, and amino acid balance of dietary protein are primary factors which contribute to the composition, structure and function of gut microbes. A suitable ratio between protein and carbohydrate or even a low protein diet is recommended over a diet with protein in excess of requirements. Greater levels and undigested protein lead to an increase of pathogenic microorganism with associated higher risk of metabolic diseases. Herein, the crosstalk between dietary protein and gut microbiota composition and function is summarized, which will help to reveal the potential mechanism of gut microbes on the gastrointestinal tract health.

Stress shows both direct- and indirect-effects on the functions of the gastrointestinal tract, in particular on the mucus physiology and the composition of microbiota. Mucus mainly consists of heavily glycosylated proteins called mucins, which are secreted by goblet cells. The gut mucus layer is a pivotal part of the intestinal protection and colonized by commensal microbes, essential for the development and health of the host. There is a symbiotic interaction between intestinal microbiota and the host cells. On the one hand, mucus provides nutrients for the growth and adhesion of microbes; on the other hand, mucin-degrading bacteria generate energy sources for the host epithelium. However, the mucusmicrobial interaction has rarely been considered in the context of stress exposure. Therefore, this paper principally reviews the effects of stress on both mucus secretion and gut microbiota and is hoped to provide a new perspective for future study.

Cattle supply important amounts of nutritious products such as beef and milk for human consumption. However, cattle excrete large amounts of feces and urine with low utilization rate of dietary crude protein (CP). These not only negatively affect the global environment by emissions of ammonia (NH3) and nitrous oxide (N2O) and bleaching the soil and underground water, but also increase the feed cost. The low nitrogen (N) utilization rate of cattle could possibly result from the activity of rumen microorganisms degrading feed CP. Many studies indicate that it is possible to manipulate the N metabolism to improve the N utilization rate of cattle through nutritional approaches, such as dietary supplementation of rumen protected essential amino acids (EAA) including methionine (Met), lysine (Lys) and EAA analogs or feeding rations with relatively low N concentration. It is necessary to study the microbial synthesis of EAA in the rumen, the requirements of EAA of cattle under different feeding regimes, and to develop products which are more efficient and less costly to improve the N utilization rate of cattle.

There are some disparities between pharmacological and toxicological xenobiotic receptor (xenosensors) pathways. These variations include receptor models that indicate several toxic patterns. Such models have demanded some update from traditional medical receptor relations studied by pharmacologists. These may include the response time, the molecular level, and unclear directions of toxicological metabolism. Xenosensors activities were affected by many factors that include genetic elements, physiological status, xenobiotic complication, and species-specific variations. Thus, this review aims to highlight the most advanced features of xenosensors related to toxicant biotransformations and other patterns such as characteristics, recognition, and the relations between different xenosensors.

L-Homoarginine (hArg) ((2S)-amino-6-Carbamimidamidohexanoic acid) is a non-essential cationic amino acid that may be synthesised from the lysine catabolism or the transamination of its precursor (Arginine: Arg). These processes involve the use of the ornithine transcarbamoylase (OTC), an enzyme from the urea cycle or the arginine: glycine amidinotransferase (AGAT), an enzyme from the creatine biosynthesis pathway. These enzymes are tissue-specific, hence they synthesised L-hArg in animals and human organs such as the liver, kidneys, brains, and the small intestines. L-hArg plays some important roles in the pathophysiological conditions, endothelial functions, and the energy metabolic processes in different organs. These functions depend on the concentrations of the available LhArg in the body. These different concentrations of the L-hArg in the body are related to the different disease conditions such as the T2D mellitus, the cardiovascular and the cerebrovascular diseases, the chronic kidney diseases, the intrauterine growth restriction (IUGR) and the preeclampsia (PE) in pregnancy disorders, and even mortality. However, the applications of the L-hArg in both human and animal studies is in its juvenile stage, and the mechanism of action in this vital amino acid is not fully substantiated and requires more research attention. Hence, we review the evidence with the perspective of the LhArg usage in the monogastric and human nutrition and its related health implications.