Nutrient requirements Protein, amino acids

The determination of the structure of the iron transporter, ferric-binding, protein (hFBP)t from Haemophilus influenzae (Bruns et ah, 1997) at 0.16 nm resolution shows that it is a member of the transferrin superfamily, which includes both the transferrins and a number of periplasmic binding proteins (PBP). The PBPs transport a wide variety of nutrients, including sugars, amino acids and ions, across the periplasm from the outer to the inner (plasma) membrane in bacteria (see Chapter 3). Iron binding by transferrins (see below) requires concomitant binding of a carbonate anion, which is located at the N-terminus of a helix. This corresponds to the site at which the anions are specifically bound in the bacterial periplasmic sulfate- and... 

In cells treated with poisons that inhibit the aerobic production of ATP (e.g., 2,4-dinitrophenol in aerobic cells), the ion concentrations inside the cell gradually approach those of the exterior environment as ions move through channels in the plasma membrane down their electrochemical gradients. Eventually treated cells die partly because protein synthesis requires a high concentration of ions and partly because in the absence of a Na gradient across the cell membrane, a cell cannot import certain nutrients such as amino acids. Studies on the effects of such poisons provided early evidence for the existence of ion pumps. 

Absorption of nutrients occurs mainly in the small intestine. Carbohydrates are absorbed as monosaccharides by active transport, a process involving carrier proteins. Amino acids and fatty acids are also absorbed by active transport, but emulsified triglycerides are absorbed by passive diffusion. Large molecules, especially the immunoglobulins present in colostrum, are absorbed by a process known as pinocytosis. Many minerals and vitamins require special processes of absorption. 

As the egg production of hens falls during the period of laying from the peak of 90 per cent to perhaps 75 per cent, their requirements for amino acids (and other nutrients) decline. It is therefore possible to employ what is termed phase feeding to reduce the protein content of their diet from an initial 170 g/kg to about 150 g/kg. 

Intravenous requirements for energy, protein, amino acids, vitamins, minerals, and trace metals have not been definitely established for the newborn infant, although significant advances have been made in the past 5 years. Current estimates of intravenous requirements continue to be based on recommended oral intakes and a few nutrient balance studies. In this chapter, it is not possible to provide a detailed review on the termination of intravenous requirements for all the essential nutrients which are recommended for inclusion in TPN regimens. This topic has been the subject of several recent reviews (Jeejeebhoy, 1983 Heim et al, 1981 Zlotkin and Anderson, 1985 Winters et al, 1983). This section will deal with some of the newer issues associated with parenteral nutrition. 

Nutritional Requirements. The nutrient requirements of mammalian cells are many, varied, and complex. In addition to typical metaboHc requirements such as sugars, amino acids (qv), vitamins (qv), and minerals, cells also need growth factors and other proteins. Some of the proteins are not consumed, but play a catalytic role in the cell growth process. Historically, fetal calf semm of 1—20 vol % of the medium has been used as a rich source of all these complex protein requirements. However, the composition of semm varies from lot to lot, introducing significant variabiUty in manufacture of products from the mammalian cells. 

Patient case A patient s daily nutritional requirements have been estimated to be 100 g protein and 2,000 total kcal. The patient has a central venous access and reports no history of hyperlipidemia or egg allergy. The patient is not fluid restricted. The PN solution will be compounded as an individualized regimen using a single-bag, 24-hour infusion of a 2-in-1 solution with intravenous fat emulsion (IVFE) piggybacked into the PN infusion line. Determine the total PN volume and administration rate by calculating the macronutrient stock solution volumes required to provide the desired daily nutrients. The stock solutions used to compound this regimen are 10% crystalline amino acids (CAA), 70% dextrose, and 20% IVFE. 

Some of the more interesting examples of nutrient - nonnutrient interactions include some of the compounds that are analogs of nutrients. Mattson et al (16) found that cholesterol absorption decreased when various plant sterols were added to the diets of rats. A number of plant amino acids are not ordinarily required by herbivores and are usually not incorporated into proteins. For example, the structure of 3,4-dihydroxyphenyl-alanlne (L-dopa) is similar to that of tyrosine. L-Dopa may play a role in favism (17), as well as having a number of other deleterious effects (18, 19, 20). Essential amino acids themselves can be deleterious if they are ingested in excessive quantities or if they are not in balance with other amino acids... 

Water accounts for over half the body mass (55%) of the average human. Of the remaining 45%, 19% is protein, 19% is lipid, less than 1% is carbohydrate, and 7% is inorganic material. Nutrients must contain the raw materials that go into the construction of the components of the human body. In addition, nutrients must supply the necessary chemical energy and enzyme cofactors (vitamins and trace metal elements) that are required for the maintenance and growth of the human body. The human body requires nutrients such as water, amino acids, fats, carbohydrates, and major minerals in large amounts. Vitamins and trace metal elements are required in smaller amounts. 

Understanding the total nutritional needs of the human body requires a more detailed consideration of the chemical composition of different nutrients. As we have seen in Chapter 22, the body cannot synthesize many of the amino acids needed to build proteins. For this reason, the body requires proteins that contain the essential amino acids in sufficient quantities. Too little animal protein will lead to nutritional deficiencies because plant proteins do not contain an adequate supply of all the amino acids that active humans must receive from their diets. 

The barrier to paracellular diffusion potentially isolates the brain from many essential polar nutrients such as glucose and amino acids that are required for metabolism and, therefore, the BBB endothelium must contain a number of specific solute carriers (transporters) to supply the CNS with its requirements for these substances. The formation of tight junctions essentially confers on the BBB the properties of a continuous cell membrane, both in terms of the diffusional characteristics imposed by the lipid bilayer, and the directionality and properties of the specific transport proteins, and solute carriers (SLC) that are present in the cell membrane. Examples of BBB solute carriers (SLC transporters) are listed in Table 27.2. 

Nitrogen is a component of amino acids that make up proteins chlorophyll (the molecule that captures the sun s energy) enzymes and the genetic material, nucleic acids. Therefore, this nutrient is required in large amounts by all plants and forms one of three primary nutrients. Although nitrogen is available in abundance... 

We are concerned here with the enzymatic liberation of products that convert chemicals to less utilizable forms. The idea is to employ plant enzymes to strike directly at the insect s most critical needs, essential nutrients. In general, plant tissues are low in total nitrogen and pose a "nutritional hurdle" for insect herbivores (94). Plant proteins normally have low sulfhydryl and lysine content (95), which are nutritional requirements for insects (90,91). The nucleophilic properties of these limiting amino acids make them particularly susceptible to covalent binding by strongly electrophilic molecules such as o-quinones, hydroperoxides, ben-zoxazinones, and isothiocyanates formed in damaged plant tissues. 

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