Uptake of Amino Acids and Glucose

Concentrations, turnover, and uptake of free amino acids probably have been examined more extensively than any other DOM component. For several decades it has been possible to measure turnover of dissolved free amino acids (DFAA) from the uptake of 14C- or3H-labeled amino acids (e.g., Azam and Holm-Hansen, 1973). Equally important was the development 

Several studies have measured DFAA concentrations and turnover (see Chapter 4 and Munster, 1993), but here we concentrate on those that compare DFAA uptake with bacterial production. The fraction of bacterial production supported by DFAA is one index for the relative importance of amino acids, not only in supporting bacterial growth but also in the overall flux of DOM. ( Flux is used here to indicate both production and uptake in a quasi-steady state.) If DOM concentrations are constant, DOM production will equal total uptake rates by microbes there is no evidence of photo-oxidation of amino acids and of the other compounds discussed here (see Chapter 10). Total uptake includes respiration and assimilation into biomass. Here assimilation is defined as the appearance of a radioactive compound in cells (both cellular LMW and HMW pools) respiration is excluded. 

An HPLC method suitable for measuring low concentrations of glucose and other monosaccharides in natural waters was introduced to aquatic ecologists more than 10 years after the HPLC method for DFAA (Mopper et al., 1992). Like DFAA, monosaccharide concentrations are quite low, but 

TABLE I Summary of Aquatic Studies That Compared Free Amino Acid Uptake and Bacterial Production 

Regime Location Percentage of bacterial production Comments Reference 

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During prolonged storage in the laboratory under conditions of nutrient starvation, facultatively oliogotrophic bacteria may be isolated and these display transport systems for the uptake of amino acids and glucose that are coregulated. 

Arathoon WR, Telling CR (1982), Uptake of amino acid and glucose by BHK-21 clone 13 suspension cells during cell growth, Dev. Biol. Stand. 50 145-154. 

Kawanaka, M., Hayashi, S. and Carter, C.E. (1986) Uptake and excretion of amino acids and utilization of glucose by Schistosoma japonicum eggs. Japanese Journal of Medical Sciences and Biology 39, 199-206. 

Several carrier systems have been shown to be present in the brain endothelium, allowing for the selective transport of a group of common substrates (Table 13.1). The most common system is the one that mediates the transport of glucose, which provides the brain with virtually all its energy. Carrier-mediated mechanisms are also responsible for the absorption of two other energy sources ketone bodies, which are derived from lipids, and lactic acid, a by-product of sugar metabolism. Carrier-mediated transport systems are also involved in the uptake of amino acids by the brain. The brain can manufacture its own small neutral and acidic amino acids however, large neutral and basic amino acids are obtained from the bloodstream. 

The key role of insuhn in protein metabolism usually is evident only in diabetic patients with persistently poor glycemic control. Insulin stimulates amino acid uptake and protein synthesis and inhibits protein degradation in muscle and other tissues. The increased conversion of amino acids to glucose also results in increased production and excretion of urea and ammonia. In addition, there are increased circulating concentrations of branched-chain amino acids as a result of increased proteolysis, decreased protein synthesis, and increased release of branched-chain amino acids from the liver. 

Other effects of iodide include the inhibition of glucose transport, presumably by reduction of the number of available glucose carriers in pig cells (Filetti et al, 1986), and the inhibition of amino acid and uridine uptake Kleiman de Pisarev et al., 1978 Pisarev and Itoiz, 1972). The relationship of these effects to the previously described effects is not known. 

Glyphosate has been found to retard uptake of amino acids, nucleotides, and glucose in isolated cells (99), Other studies have indicated that glyphosate-caused inhibition of uptake of amino acids by intact plant roots (71) or excised buds (72) is not severe. 

This metabolic activity is achieved by a turnover of amino acids and proteins that is as rapid as that of lipids and carbohydrates. In an adult human -400 g of body proteins is turned over each day. Of this -50 g replaces digestive enzymes and -15 g replaces hemoglobin. The amino acid concentration in plasma is small (total 3.2 mM, of which -25% is glutamine) but the turnover of -400 g day of protein is equal to the uptake and release back into the plasma of 4.6 mol of a-amino-iV, so that the average lifetime of an amino acid in the plasma is -5 min. Plasma amino acids are turned over with the same kind of rapidity as plasma glucose or free fatty acids. Like plasma glucose, the plasma amino acid concentration is remarkably constant, but it is not understood how this is regulated. 

Altered metabolism in the muscles. Lack of insulin results in reduced muscle cell uptake of amino acids, fatty acids, and glucose, followed by deterioration of muscles. 

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