Amino acids assimilation mechanisms

Since in mammalian species metals first need to be assimilated from dietary sources in the intestinal tract and subsequently transported to the cells of the different organs of the body through the bloodstream, we will restrict ourselves in this section to the transport of metal ions across the enterocytes of the upper part of the small intestine (essentially the duodenum), where essentially all of the uptake of dietary constituents, whether they be metal ions, carbohydrates, fats, amino acids, vitamins, etc., takes place. We will then briefly review the mechanisms by which metal ions are transported across the plasma membrane of mammalian cells and enter the cytoplasm, as we did for bacteria, fungi and plants. The specific molecules involved in extracellular metal ion transport in the circulation will be dealt with in Chapter 8. 

These results indicate it may be possible to improve the efficiency of absorption and assimilation by altering the process of regulation. However the mechanisms governing regulation are poorly understood. It is not known whether the regulation is linked to the concentration of NH4+ or NOs" itself or to the concentrations of products of N assimilation downstream from NH4+ or NOs", such as particular amino acids. Nor is it known what the targets of the resulting feedback mechanisms are. 

The toxic effects of ozone in plant systems have been studied for some time, yet the actual mechanisms of injury are not fully understood. In addition to visible necrosis which appears largely on upper leaf surfaces, many other physiological and biochemical effects have been recorded ( ). One of the first easily measurable effects is a stimulation of respiration. Frequently, however, respiration may not increase without concomitant visible injury. Furthermore, photosynthesis in green leaves as measured by CO2 assimilation, may decrease. It is well known that ozone exposure is accompanied by a dramatic increase in free pool amino acids ( ). Ordin and his co-workers ( ) have clearly shown the effect of ozone on cell wall biosynthesis. In addition, ozone is known to oxidize certain lipid components of the cell ( ), to affect ribosomal RNA (16) and to alter the fine structure of chloroplasts (7 ). 

Molecular mechanisms of nitrate accumulation depend not only on the nitrate reductase system, but also on the ability of roots to take from the soil, nitrate or ammonium ions, and on the plant s capacity for their conversion by assimilation processes to higher products. Besides this, the assimilation depends on the ability of a given genotype to transport substances necessary for the synthesis. It was shown that genotype differences of the nitrate reductase level do not depend on the nitrate content in tissues [25]. Nitrates are accumulated in plant organisms at high concentrations when aU the nitrogen accepted cannot be utilized for the production of amino acids and for subsequent protein synthesis [26]. This occurs when the plant, in the course of its metabolism, is unable to reduce the accepted nitrates into the assimilable ammonia form. 

All mechanisms would be followed by the pull from ATP-dependent saturable assimilation into amino acids. 

In summary, there appears to be evidence for both mechanisms of NH3/NH4 transport in bacterial cells. Passive difiFusion of NH3 would not generally lead to net accumulation by cells with an alkaline interior pH relative to the medium pH. Passive diffusion would be rate-limited by the low level of NH3 available at physiological pH and by the solubility properties of the cell membrane. NH4 active transport would be required for net ammonium uptake. However, net nitrogen accumulation could be accomplished by rapid assimilation of internal ammonia into amino acids. Net accumulation might be limited by the balance of the two processes uptake by active transport vs. loss by passive diffusion. 

The success of asymmetric hydrogenation as a route to amino acid derivatives does not find more general applicability. To appreciate why this is so, an analysis of the various factors involved is required. The limitations will only be overcome when a full appreciation of mechanism and of the source of stereoselectivity has been assimilated. 

Although yeast cells were considered to incorporate up to 50% of wort amino acids directly into protein [62], analysis of the utilization of and labelled amino acids by brewers yeast show that negligible assimilation of complete amino acid occurs [63]. Thus, when amino acids enter the cell their amino groups are removed by a transaminase system and their carbon skeletons assimilated. Transaminases catalyse readily reversible reactions dependent upon the presence of the cofactor pyridoxal phosphate. The general mechanism of the reaction is depicted in Fig. 17.14. 

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