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Comparative studies of alanine and alpha amino iso butyric acid uptake by freshly isolated rat liver cells

, : Comparative studies of alanine and alpha amino iso butyric acid uptake by freshly isolated rat liver cells. Journal of Biological Chemistry 254(5): 1653-1658

The transport of radiolabeled alanine and its nonmetabolizable analog, .alpha.-aminoisobutyric acid (AIB), by freshly isolated rat hepatocytes is compared. Alanine transport was separated from its metabolism by inhibition of alanine aminotransferase activity with aminooxyacetate. When cells were incubated in medium containing 142 mM Na and 1 mM amino acid, the steady state intracellular/extracellular distribution ratio for both amino acids was approximately 10, but the rate of accumulation was 4 times faster for alanine than for AIB. 2,4-Dinitrophenol lowered the distribution ratios for alanine and AIB to those found in a medium devoid of Na, 1.7 and 1.0, respectively, but had no discernible effect on the transport of either amino acid in Na-free medium. The Na-dependent portions of the uptake of both alanine and AIB were saturable as measured by 1 min rates of uptake. Na-independent uptake of alanine exhibited both a small saturable component and a nonsaturable component and Na-independent AIB uptake was entirely nonsaturable. Thus, the Na-dependent uptake of both alanine and AIB is mediated by active transport, the Na-independent uptake of alanine by both carrier-mediated transport and diffusion, and all of the Na-independent uptake of AIB by diffusion. The inhibitory effects of system-selective amino acids on alanine and AIB uptake were studied to discern the role of different carrier-mediated neutral amino acid transport systems. Leucine produced a 15% reduction in the rate of alanine uptake from Na-free medium but had no discernible effect on AIB uptake, indicating that the L system transport alanine but not AIB. N-Methylalanine and N-methyl-AIB inhibited the rate of Na-dependent alanine uptake by 65-70% while the rate of Na-dependent AIB uptake was inhibited by more than 90%. Thus, the A system accounts for only 70% of Na-dependent alanine uptake but is responsible for nearly all of Na-dependent AIB uptake. Serine and cysteine inhibited that portion of Na-dependent alanine uptake not inhibited by N-methylalanine, thereby defining a role for the ASC system in mediating alanine uptake. When Na in the medium was replaced by Li, the rate of alanine uptake was reduced by an amount comparable to that produced by N-methylalanine. The addition of N-methylalanine to the Li-containing medium did not affect the rate of alanine uptake further, but serine and cysteine lowered this rate to that found in the choline medium. Li may be used to directly elicit the contribution of the ASC [alanine serine cysteine] system in liver cells.


PMID: 762163

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