Inhibition of S37 ascites cell amino acid transport systems by alpha-chloromethylketone analogs

Effectors of amino acid transport processes in animal cell membranes

Various effectors, which act upon ion gradients, protein synthesis, membrane components or cellular functional groups, have been employed to provide insights into the nature of amino acid-membrane transport processes in animal cells. Such effectors, for example, include ions, hormones, metabolites and various organic reagents and their judicious use has allowed the following list of conclusions. Sodium ion has been found to stimulate amino acid transport in a wide variety of cell systems, although depending on the tissue and/or substrate, this ion may have no effect on such transport, or even inhibit it. Amino acid transport can be stimulated in some cell systems by other ions such as K+, Li+, H+ or Cl-. Both H+ and K+ have been found to be inhibitory in other systems. Amino acid transport is dependent in many cell systems upon an inwardly directed Na+ gradient and is stimulated by a membrane potential (negative cell interior). In some cell systems an inwardly directed Cl- and H+ gradient or an outwardly directed K+ gradient can energize transport. Structurally dissimilar effectors such as ouabain, Clostridium enterotoxin, aspirin and amiloride inhibit amino acid transport presumably through dissipation of the Na+ gradient. Inhibition by certain sugars or metabolic intermediates of the tricarboxylic acid cycle may compete with the substrate for the energy of the Na+ gradient or interact with the substrate at the carrier level either allosterically or at a common site. Stimulation of transport by other sugars or intermediates may result from their catabolism to furnish energy for transport. Insulin and glucagon stimulate transport of amino acids in a variety of cell systems by a mechanism which involves protein synthesis. Microtubules may be involved in the regulation of transport by insulin or glucagon. Some reports also suggest that insulin has a direct effect on membranes. In addition, a number of growth hormones and factors have stimulatory effects on amino acid transport which are also mediated by protein synthesis. Steroid hormones have been noted to enhance or diminish transport of amino acids depending on the nature of the hormone. These agents appear to function at the level of protein synthesis. While stimulation may involve increased carrier synthesis, inhibition probably involves synthesis of a labile protein which either decreases the rate of synthesis or increases the rate of degradation of a component of the transport system.(ABSTRACT TRUNCATED AT 400 WORDS)

Uptake of L-glutamine into synaptosomes. Is the gamma-glutamyl cycle involved?

Transport of L-glutamine into rat cortical synaptosomes has been investigated by (14C)L-glutamine uptake experiments. This amino acid enters synaptosomes both by an active carrier mediated system, which may be the result of gamma-glutamyl cycle activity and by a Na+-dependent transport system. This view is supported by the following observations: a) as demonstrated previously (10), glutamine inside synaptosomes reaches concentrations higher than those of the incubation medium, and initial rates of uptake approach saturation kinetics; b) the uptake of glutamine is inhibited by uncouplers; c) the uptake is inhibited by methionine sulfoximine, a suicide-inhibitor of an enzyme of the gamma-glutamyl cycle; d) the initial rate of uptake is lowered by decreasing the Na+-level of the incubation medium or by adding ouabain. The validity of this hypothesis is discussed.