The in vivo sythesis of serum albumin by liver has been shown to be specifically controlled by plasma levels of essential amino acids, especially tryptophan. The apparent relative stability of the albumin mRNA would suggest that these amino acid effects occur at the translational level. The overall aim of this research project is to analyze the effects of essential amino acid limitation on albumin synthesis, and to investigate the possibility of exogenous albumin as a source of essential amino acids. The focus of these studies will be a highly differentiated mouse hepatoma cell line, Hepa, which continues to secrete the serum proteins, albumin, alpha-fetoprotein, transferring, and complement C3b inactivator. The initial phase of the project will involve the systematic manipulation of culture media concentrations of tryptophan, histidine, isoleucine, leucine, and phenylalanine. Absolute rates (molecules/cells/min) of albumin secretion will be measured by radioimmunoassay. The specificity of amino acid effects on albumin synthesis will be assessed by comparison with other serum proteins and total cellular protein. Kinetic analysis of albumin and total protein synthesis will include measurements of: (1) relative and absolute (albumin) rates of synthesis; (2) ribosome transit times; and (3) number-average polyribosome sizes. From these data mRNA translational efficiencies (initiation rates) and relative and absolute (albumin) cellular contents of functional mRNAs will be calculated. The data will be interpreted in terms of general translational control models. (G)
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