Transport of Amino Acids and Sugars

Although the transport of amino acids and sugars has been studied for a number of years, the mechanisms by which this movement occurs remain unknown. In mammalian cells such as those of the kidney, intestine, and muscle, and in erythrocytes, transport of most amino acids is an active process involving a proposed mobile carrier that binds its substrate before translocating it across the membrane. Sugars are usually actively transported only in the kidney and intestine, while 

Meister (1973) has suggested that transport of many amino acids in the kidney and probably in other tissues could be mediated by a cycle of enzymatic reactions involving the membrane-bound enzyme y-glu-tamyl transpeptidase as a binding site for amino acids. Meister does not, however, exclude the possibility of other transport pathways for the active transport of amino acids, nor is he able to define the extent to which the cycle could function. At the present time, the identification of the y-glutamyl transpeptidase cycle as a mediator of amino acid transport represents only an interesting hypothesis, and attempts to correlate y-glutamyl transpeptidase activity with amino acid transport activity have been fruitless. 

On the basis of evidence obtained both in vivo and in vitro. Young and Freedman (1971) have proposed the existence of at least four different systems for amino acid transport in mammalian kidney, according to the amino acid structure, i.e., for neutral, basic, and acidic amino acids, and for imino acids and glycine. Using Ehrlich ascites tumor cells, Oxender and Christensen (1963) have characterized several trans- 

Isolated rabbit kidney brush border vesicles 

Isolated rat kidney brush border membrane vesicles 

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Stevens, B.R., Kaunitz, J.D., Wright, E.M. (1984). Intestinal transport of amino acids and sugars Advances using membrane vesicles. Ann. Rev. Physiol. 46,417-433. 

Curran, P. F., 1972, Active transport of amino acids and sugars. Arch. Intern. Med. 129 258. 

Transport of amino acids and most hexose sugars into the blood from the intestine... 

The penetration of amino acids and sugars into the yeast activates membrane transport systems called permeases. The general amino acid... 

Sialic acid residues were found to be important in the transport of amino acids and proteins in cancer cells, since neuraminidase treatment of HeLa cells decreased the net accumulation of a-aminoisobutyric acid without altering the rate of efflux of preloaded cells (Brown and Michael, 1969), and the same treatment of L1210 leukemia cells inhibited the outward flow of proteins without influencing lysis or the release of nucleosides or sugars. Some relative specificity in the release of proteins was shown by disk-gel electrophoresis (Click et aL, 1966). 

The gradients of H, Na, and other cations and anions established by ATPases and other energy sources can be used for secondary active transport of various substrates. The best-understood systems use Na or gradients to transport amino acids and sugars in certain cells. Many of these systems operate as symports, with the ion and the transported amino acid or sugar moving in the same direction (that is, into the cell). In antiport processes, the ion and the other transported species move in opposite directions. (For example, the anion transporter of erythrocytes is an antiport.) Proton symport proteins are used by E. coU and other bacteria to accumulate lactose, arabinose, ribose, and a variety of amino acids. E. coli also possesses Na -symport systems for melibiose as well as for glutamate and other amino acids. 

Tsuchiya W, Okada Y. 1982. Differential effects of cadmium and mercury on amino acid and sugar transport in the bullfrog small intestine. Experientia (Basel) 38 1073-1075. 

Membrane permeability is one of the most difficult problems for protocells to solve. Electrolytes, amino acids and sugars permeate liposomes at a very small rate. Today we find highly evolved transport mechanisms, resting on evolved proteins, which solve this problem for the cell. Protocells must have resorted to more basic, but sufficiently functional tricks. 

Although the tegument contains specific systems for molecular and ion transport - especially amino acids, hexose sugars, vitamins, purines, pyrimidines, nucleotides, and lipids - it probably also serves a number of other vital functions (647) (a) it is a major site of catalytic activity and... 

The present chapter will deal mainly with coupled transport of hydrophilic nutrients such as amino acids and sugars, and their derivatives by animal cells, as too little is known whether the uptake of lipophilic solutes, such as fatty acids, is also coupled. 

Ouabain inhibits sodium extrusion from the cell and thereby obliterates its critical concentration differences. Ouabain acts on the energy utilization step for the active extrusion of sodium ions, as evidenced by its inhibition of transport adenosine triphosphatase (20). The inhibition of bile salt transport by ouabain (21,22) resembles similar actions against other transport systems (e.g., those for amino acids and sugars). The omission of cations other than sodium from the incubation media does not greatly impair taurocholate transport (22). 

Active transport occurs in nearly all cells of the body, but most notably in the kidney, liver, and the intestine. The species transported are primarily ions, Na" in particular, and also include amino acids and sugars. The sketch in Figure 1.4b shows the basic mechanism of active Na+ transport in the kidney, which is crucial to its proper fxmction and the production of urine. The process shown, a very crude approximation of the actual intricacies, involves a protein carrier C picking up Na+ from the low concentration (urine) side and tiien delivering and releasing it on the high concentration (peritubular) side. Both the release of Na+ and the return trip of the carrier... 

Active transport mechanisms have been observed for a large number of substances. A variety of ions are transported actively, for example, Na+, K+, Ca++, and even H+ ions (for the production of gastric juice). Among organic substances, the transport of sugars is particularly important (the flow of glucose into cells), as well as that of amino acids and other low molecular weight substan< es which arc reabsorbed by the kidney tubules. In several specialized tissues, active transport is a prerequisite for their function, especially in nerves (cf. Chapt. XXIII-7). Also in the muscle, excitability cannot be maintained if active transport has come to a standstill (Chapt. XXIII-6). 

Crawhall, J. C., and Segal, S., 1968, Transport of some amino acids and sugars in rat-liver slices, Biochim. Biophys. Acta 163 163. 

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