Solubilization and reconstitution of hepatic System A-mediated amino acid transport. Preparation of proteoliposomes containing glucagon-stimulated transport activity.

System A-mediated amino acid transport activity from rat liver plasma membrane vesicles has been solubilized and reconstituted into proteoliposomes using a freeze-thaw-dilution technique. The presence of cholate, at a cholate to protein ratio of 1:1, during the freeze-thaw step resulted in an enhancement in recoverable transport activity. The carrier required both phosphatidylcholine and phosphatidylethanolamine for optimal activity, but the addition of cholesterol to the reconstitution procedure appeared to have no significant effect on the resulting activity. A lipid to protein ratio of 20:1 yielded maximal transport activity. Sonication of the proteoliposomes provided some improvement in the accuracy of replicate assays for a given proteoliposome preparation. Isolated liver plasma membrane vesicles prepared from rats treated in vivo with glucagon in combination with dexamethasone contained stimulated System A activity. This enhanced transport activity could be solubilized and recovered in proteoliposomes generated from these plasma membranes. The data support the proposal that hormone regulation of the hepatic System A gene results in the de novo synthesis and plasma membrane insertion of the carrier protein itself.

Amino acid-dependent inactivation of glucagon-induced System A transport activity in cultured rat hepatocytes.

Hepatocytes isolated from glucagon-treated rats contain stimulated System A activity. If these cells are placed in primary culture, the enhanced transport decays rapidly provided the culture medium contains substrate amino acids. This amino acid-dependent inactivation can be composed of trans-inhibition (protein synthesis-independent), repression (protein synthesis-dependent), or both depending on the particular substrate tested. Repression was most prominently observed with a group of small neutral amino acids that are commonly found in proteins. A strong trans-inhibition response was induced by a variety of amino acid analogs. Amino acids showing no reactivity with System A produced neither trans-inhibition nor repression. Repression of System A activity in culture was blocked by inhibitors of both RNA and protein synthesis. In contrast to inhibitors of RNA biosynthesis such as actinomycin and alpha-amanitin, inhibitors of poly(A) polymerase (cordycepin and adenine-9-beta-D-arabinopyranoside) did not prevent the inactivation of the transport activity. These results demonstrate that both the stimulation of activity and the turnover of the hepatic System A activity are controlled at the transcriptional level.

Cis-inhibition, trans-inhibition, and repression of hepatic amino acid transport mediated by System A. Substrate specificity and other properties.

Substrate regulation of System A-mediated amino acid transport was investigated in primary cultures of rat hepatocytes. Studies on the substrate specificity of trans-inhibition and repression revealed considerable differences between the two processes. Those data along with a difference in temperature sensitivity suggest that the two phenomena are not related or inter-dependent in any direct way. However, kinetic analysis indicates that both trans-inhibition and repression decrease the number of functional carriers within the plasma membrane. Cis-inhibition tests show that the hepatic System A carrier exhibits a wide degree of tolerance with regard to modification of the alpha-amino and alpha-carboxyl groups. In general, the amino acids that cause the greatest degree of trans-inhibition are only moderate cis-inhibitors of System A-mediated transport (40-60% inhibition of Na+-dependent 2-aminoisobutyric acid uptake). The substrate specificity of amino acid-induced repression appears to be similar to that for System A-mediated transport, although an amino acid can exhibit cis-inhibition of System A activity without causing repression (or trans-inhibition). For example, S-methyl-L-cysteine serves as a competitive inhibitor of System A, yet it does not cause trans-inhibition or repression to a considerable degree. However, through its cis-inhibition of transport, S-methyl-L-cysteine blocks asparagine-dependent repression, apparently by suppressing the cytoplasmic accumulation of asparagine.