Amino Acid Uptake Sites

Although the studies listed in Table 3 generally report an association of radioactivity with neuronal elements, none has demonstrated that receptors are not also labeled, or that diffusion from the uptake site is absent during the fixation and processing for autoradiography. This is an especially troubling concern for electron microscopic evaluation. Similarly, the majority of reports have not defined the nature of the accumulated radioactivity in terms of kinetics, pharmacology, or even chemical form. 

In Vitro Autoradiographic Studies of Amino Acid Uptake Sites m Tissue Sections and Slices 

Hokfelt and Ljungdahl, 1970 Iversen and Bloom, 1972 Hatton et al, 1973 Marshall and Voaden, 1975 Hampton and Redburn, 1983 Sarthy, 1983 

Hokfelt and Ljungdahl, 1971 Matus and Dennison, 1971 Iversen and Bloom, 1972 Wilkin et al, 1981 Sarthy, 1983 

I have presented an approach to autoradiographic localization of receptors at the light microscopic level. The important basic as- 

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Answer The similar Kt values for L-leucine and L-valine indicate that the transporter binding site can accommodate the side chains of both amino acids equally well it is probably a hydrophobic pocket of suitable size for either R group. The 20-fold higher Kt for D- than for L-leucine indicates that the binding site recognizes differences of configuration about the a carbon. Based on the lower Vm3X in the absence of Na+ for all three substrates, we know that Na+ entry is essential for amino acid uptake the transporter acts by symport of leucine (or valine) and Na+. 

The binding of insulin to the cell membrane is not random, but takes place at specific sites called receptors. For example, the adipose cell is believed to contain 10,000 overt binding sites and many more occult binding sites. The receptors are located at the periphery of the cell membrane. Experiments in which insulin is bound to plastic beads have demonstrated that ]the hormone is capable of exerting its metabolic effects on glucose penetration, amino acid uptake, protein synthesis, DNA and RNA synthesis, and lipogenesis. 

Sodium activates amino acid uptake despite much research, the mechanism of this activation remains unknown. At least two mechanisms have been proposed sodium stimulates the production of energy needed for transport in an unspecific fashion. There is coupled binding of sodium and amino acid at the carrier site, and the movement of the sodium down its concentration gradient provides the energy needed for transport. 

Insulin also plays a role in fat metabolism. In humans, most fatty acid synthesis takes place in the liver. The mechanism of action of insulin involves directing excess nutrient molecules toward metabolic pathways leading to fat synthesis. These fatty acids are then transported to storage sites, predominantly adipose tissue. Finally, insulin stimulates the uptake of amino acids into cells where they are incorporated into proteins. 

Increased uptake of branched-chain amino acids Muscle is the principal site for degradation of branched-chain amino acids (see... 

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