Amino acid disorders and urea cycle disorders
Steve Hannigan in Inherited Metabolic Diseases: A Guide to 100 Conditions, 2018
Amino acid disorders can be caused by problems with transport of the amino acids into the cells or by impairment of the breakdown of amino acids. Amino acids are the building blocks of proteins. Some amino acids can be synthesised by the body, while others must be obtained through protein from the diet. The latter are known as the essential amino acids. A deficiency in one of the enzymes needed to break down amino acids means that the body is unable to use them for growth and repair. Newborn babies are routinely screened for several metabolic disorders. Of these screening tests, the most commonly known is the heel prick test, which is used to diagnose phenylketonuria (PKU).
Nutrient Interactions and Glucose Homeostasis
Emmanuel Opara in NUTRITION and DIABETES, 2005
Fatty acids not resynthesized into triglycerides for storage as lipids are oxidized and, by so doing, generate substrates also used for de novo glucose synthesis (gluconeogenesis) and storage as glycogen. In the postabsorptive state, normal blood glucose is maintained by a combination of gluconeogenesis and glycogenolysis. During short-term fasting, blood glucose is primarily maintained by hepatic glycogen breakdown. With extended periods of fasting, fatty-acid oxidation and amino-acid catabolism become predominant sources of energy, and through these processes, substrates are generated for gluconeogenesis. As noted earlier, glucose is a major fuel that also occupies a central position in the metabolism of other nutrients in the body. It is a precursor molecule that is capable of providing many metabolic intermediates for various biosynthetic reactions (3). Hence, the metabolism of glucose is normally regulated by a well-coordinated system among the different tissues in the body. For instance, in the muscle, glycolytic degradation of glucose produces ATP, and the rate of glycolysis increases as the muscle contracts more intensely, thereby demanding more ATP. On the other hand, as previously noted, the liver and the kidneys serve to keep a constant level of glucose in the blood by producing and exporting glucose when the tissues demand it, while the liver takes up and stores glucose when it is available in excess (3). The turnover of muscle protein occurs slowly with little or no diurnal changes in the size of the protein pool in response to feeding and fasting (4). There are 20 standard amino acids in proteins, with variations in their carbon skeletons. Consequently, there are many different catabolic pathways for the degradation of amino acids for energy production. Altogether, the energy from these pathways accounts for only 10 percent to 15 percent of the body’s energy production (3). Although much of the catabolism of amino acids takes place in the liver, six amino acids, namely leucine, isoleucine, valine, asparagine, aspartate, and glutamate, are metabolized in the resting muscle (4). However, the three branched-chain amino acids, leucine, isoleucine, and valine, are only oxidized as metabolic fuels in the muscle, adipose tissue, kidney, and brain. These extra-hepatic tissues have a single aminotransferase that is not present in the liver and acts on all three branched-chain amino acids to produce the corresponding keto-acids (3). The overwhelming majority of amino acids are glucogenic. Hence, their carbon skeleton generates intermediates of tricaboxylic acid (TCA) cycle that are used for de novo glucose synthesis.
Nutrition
Barbara Smith, Linda Field in Nursing Care, 2019
Every body cell contains protein, and about three-quarters of our body solids are protein (Kozier et al., 2004, 2014). Proteins are organic substances consisting of amino acids, the chemical subunits of proteins. There are 20 different amino acids; eight of these cannot be produced in the body and are therefore an essential aspect of the diet – they are known as essential amino acids. Non-essential amino acids can be produced in the body from amino acids derived from the diet. Proteins may be complete or incomplete. Complete proteins contain all the essential amino acids and also some non-essential amino acids. Examples of complete proteins include meat, poultry, fish, dairy products and eggs. Some animal proteins are described as partially complete proteins, as they contain less than the required amount of essential amino acids and thus cannot support continued body growth. Examples of partially complete proteins include gelatine and the milk protein casein. Incomplete proteins contain one or more essential amino acids and are usually found in vegetables. It is important that patients are given complete proteins in their diet, especially if a high-protein diet is ordered, as in the case study of Mrs Jones at the start of this chapter.
Amino acid kinetics and the response to nutrition in patients with cancer
Published in International Journal of Radiation Biology, 2019
Barbara S. van der Meij, Laisa Teleni, Marielle P. K. J. Engelen, Nicolaas E. P. Deutz
Purpose: Amino acids are involved in many physiological processes in the body and serve as building blocks of proteins which are the main component of muscle mass. Often patients with cancer experience muscle wasting, which is associated with poor outcomes. The purpose of this paper is to discuss amino acid kinetics in cancer, review the evidence on the response to nutrition in patients with cancer, and to give recommendations on the appropriate level of amino acid or protein intake in cancer. Current evidence shows that amino acid kinetics in patients with cancer are disturbed, as reflected by increased and decreased levels of plasma amino acids, an increased whole body turnover of protein and muscle protein breakdown. A few studies show beneficial effects of acute and short-term supplementation of high protein meals or essential amino acid mixtures on muscle protein synthesis. Conclusions: Cancer is associated with disturbances in amino acid kinetics. A high protein intake or supplementation of amino acids may improve muscle protein synthesis. Future research needs to identify the optimal level and amino acid mixtures for patients with cancer, in particular for those who are malnourished.
Serum and plasma amino acids as markers of prediabetes, insulin resistance, and incident diabetes
Published in Critical Reviews in Clinical Laboratory Sciences, 2018
C. Gar, M. Rottenkolber, C. Prehn, J. Adamski, J. Seissler, A. Lechner
Presently, routine screening misses many cases of prediabetes and early type 2 diabetes (T2D). Therefore, better biomarkers are needed for a simple and early detection of abnormalities of glucose metabolism and prediction of future T2D. Possible candidates for this include plasma or serum amino acids because glucose and amino acid metabolism are closely connected. This review presents the available evidence of this connectivity and discusses its clinical implications. First, we examine the underlying physiological, pre-analytical, and analytical issues. Then, we summarize results of human studies that evaluate amino acid levels as markers for insulin resistance, prediabetes, and future incident T2D. Finally, we illustrate the interconnection of amino acid levels and metabolic syndrome with our own data from a deeply phenotyped human cohort. We also discuss how amino acids may contribute to the pathophysiology of T2D. We conclude that elevated branched-chain amino acids and reduced glycine are currently the most robust and consistent amino acid markers for prediabetes, insulin resistance, and future T2D. Yet, we are cautious regarding the clinical potential even of these parameters because their discriminatory power is insufficient and their levels depend not only on glycemia, but also on other components of the metabolic syndrome. The identification of more precise intermediates of amino acid metabolism or combinations with other biomarkers will, therefore, be necessary to obtain in order to develop laboratory tests that can improve T2D screening.
Levels of amino acids in human hepatocellular carcinoma and adjacent liver tissue
Published in Nutrition and Cancer, 1995
Takashi Nishizaki, Takashi Matsumata, Akinobu Taketomi, Kazuharu Yamamoto, Keizo Sugimachi
Total parenteral nutrition can be used to overcome amino acid imbalance in cancer patients. Because there is little documentation of treatment for amino acid imbalance in patients with hepatocellular carcinoma (HCC), we designed a study 1) to compare tissue levels of amino acids between HCC and the adjacent liver and 2) to determine which serum amino acids correlate to tumor volume. A significant elevation of methionine and a significant decrease of glycine and cystine were observed in HCC compared with adjacent liver tissue, and a significant correlation was found between tumor volume and serum methionine levels (x = ‐0.636, p < 0.01). Thus the tumor tissue competes successfully with host tissue for nitrogen substrates, particularly methionine, and an accelerated protein synthesis in HCC consumes large amounts of these amino acids. The possibility of methionine‐depleted treatment could be considered for patients with HCC.
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