Amino acid transport into cells

The answer is c. (Murray, pp 505—626. Scriver, pp 4029—4240. Sack, pp 121-138. Wilson, pp 287-320.) Vasopressin, which is also called antidiuretic hormone, increases the permeability of the collecting ducts and distal convoluted tubules of the kidney and thus allows passage of water. Like the mineralocorticoid aldosterone, vasopressin results in an expansion of blood volume. However, the mode of action of aldosterone is different it causes sodium reabsorption, not water reabsorption. Sodium reabsorption indirectly leads to increased plasma osmolality and thus water retention in the blood. Cortisol is a glucocorticoid that potentiates catabolic metabolism chronically. Epinephrine stimulates catabolic metabolism acutely. Insulin acutely favors anabolic metabolism, in large part by allowing glucose and amino acid transport into cells. 

Figure 8.5 Act/Ve amino add transport into cells. Amino acids are transported into cells against their concentration gradient coupled to Na ion transport down its concentration gradient. The Na ion is transported out in exchange for K -ions, via the NayK ATPase, Chapter 5).

Transporters genes encode proteins, generally constituted by 12 transmembrane spanning regions. These mediated Na+ or H+ dependent accumulation of small molecules such as neurotransmitters, antibiotics, ions and cationic amino-acid transporters into the cells or organelles. The transport is performed by different mechanisms uniport, substrate-ion sym-port, substrate-ion antiport, substrate-substrate or ion-ion antiport, and ATP-dependent translocation. 

Amino acid transport into the mammary cell... 

Allfrey, V.G., Mirsky, A.E. Amino acid transport into the cell nucleus and reactions governing nuclear protein synthesis. In Protein biosynthesis (Harris, R.J.C., ed.), p. 49-81. London Academic Press 1961... 

Novogrodosky, A., Nehring, RE., Jr and Meister, A. (1979). Inhibition of amine acid transport into lymphoid cells by the glutamine analog L-2-amino-4-oxo-5-chloropentanoate. Proc. Natl Acad. Sci. USA 76, 4932-4935. 

There are two distinct pools of HA in the brain (1) the neuronal pool and (2) the non-neuronal pool, mainly contributed by the mast cells. The turnover of HA in mast cells is slower than in neurons it is believed that the HA contribution from the mast cells is limited and that almost all brain histaminergic actions are the result of HA released by neurons (Haas Panula, 2003). The blood-brain barrier is impermeable to HA. HA in the brain is formed from L-histidine, an essential amino acid. HA synthesis occurs in two steps (1) neuronal uptake of L-histidine by L-amino acid transporters and (2) subsequent decarboxylation of l-histidine by a specific enzyme, L-histidine decarboxylase (E.C. 4.1.1.22). It appears that the availability of L-histidine is the rate-limiting step for the synthesis of HA. The enzyme HDC is selective for L-histidine and its activity displays circadian fluctuations (Orr Quay, 1975). HA synthesis can be reduced by inhibition of the enzyme HDC. a-Fluoromethylhistidine (a-FMH) is an irreversible and a highly selective inhibitor of HDC a single systemic injection of a-FMH (10-50 mg/kg) can produce up to 90% inhibition of HDC activity within 60-120 min (Monti, 1993). Once synthesized, HA is taken up into vesicles by the vesicular monoamine transporter and is stored until released. 

The transport of amino acids at the BBB differs depending on their chemical class and the dual function of some amino acids as nutrients and neurotransmitters. Essential large neutral amino acids are shuttled into the brain by facilitated transport via the large neutral amino acid transporter (LAT) system [29] and display rapid equilibration between plasma and brain concentrations on a minute time scale. The LAT-system at the BBB shows a much lower Km for its substrates compared to the analogous L-system of peripheral tissues and its mRNA is highly expressed in brain endothelial cells (100-fold abundance compared to other tissues). Cationic amino acids are taken up into the brain by a different facilitative transporter, designated as the y system, which is present on the luminal and abluminal endothelial membrane. In contrast, active Na -dependent transporters for small neutral amino acids (A-system ASC-system) and cationic amino acids (B° system), appear to be confined to the abluminal surface and may be involved in removal of amino acids from brain extracellular fluid [30]. Carrier-mediated BBB transport includes monocarboxylic acids (pyruvate), amines (choline), nucleosides (adenosine), purine bases (adenine), panthotenate, thiamine, and thyroid hormones (T3), with a representative substrate given in parentheses [31]. 

An alternative fate for the GSH conjugate is transportation via the blood to the kidney, filtration out of the blood and in the brush border of the tubular cells glutamyltransferase, and cleavage of the conjugate by a dipeptidase to yield the cysteine conjugate. The cysteine conjugate is then taken up by the amino acid transporter system into the proximal tubular cell where toxicity occurs. The result of this is then basically the same as the other scenario. 

Gly) formed in the reaction is transported by an uncharacterized transport system (3) and cleaved by an intracellular protease (4) or cleaved by a membrane-bound protease and the free amino acids transported. The y-glutamyl cycle enzymes are found in those tissues for which the transport of glutathione into cells is an important function. This includes liver and kidney cells in animals. 

Noradrenergic neurons. The noradrenergic neuron uses NE for its neurotransmitter. Monoamine neurotransmitters are synthesized by means of enzymes, which assemble neurotransmitters in the cell body or nerve terminal. For the noradrenergic neuron, this process starts with tyrosine, the amino acid precursor of NE, which is transported into the nervous system from the blood by means of an active transport pump (Fig. 5 — 17). Once inside the neuron, the tyrosine is acted on by three enzymes in sequence, the first of which is tyrosine hydroxylase (TOH), the rate-limiting and most important enzyme in the regulation of NE synthesis. Tyrosine hydroxylase converts the amino acid tyrosine into dihydroxyphenylalanine (DOPA). The second enzyme DOPA decarboxylase (DDC), then acts, converting DOPA into dopamine (DA), which itself is a neurotransmitter in some neurons. However, for NE neurons, DA is just a precursor of NE. In fact, the third and final NE synthetic enzyme, dopamine beta-hydroxylase (DBH), converts DA into NE. The NE is then stored in synaptic packages called vesicles until released by a nerve impulse (Fig. 5—17). 

GSH is not transported into cells. For circulating GSH to increase intracellular GSH concentrations, it must first be hydrolyzed to Glu and CysGly, which are subsequently transported into the cell and serve as substrates for GSH synthesis. Thus, GSH administered orally or parenterally, and that produced by the liver and released into the circulation enhance tissue levels of GSH by providing a source of its constituent amino acids. In contrast, GSH monoesters, which are well absorbed after oral administration, as is GSH, are readily transported to cells and then hydrolyzed to GSH and the corresponding alcohol. Thus, higher cellular levels of GSH result from oral administration of GSH monoesters than from oral administration of comparable doses of GSH. 

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