ABSTRACT
The liver contains considerable activities of the enzymes glutaminase and glutamine synthetase and has the capacity for either the net synthesis or net degradation of glutamine. Further, while glutamine at high concentrations was a good substrate for gluconeogenesis and urea synthesis, glutamine at physiological concentrations was not metabolized. These findings demonstrated clearly that glutamine metabolism in the liver must be subject to metabolic regulation. The liver obtains glutamine from the portal circulation as a result of absorption from the gut. A second source of glutamine is that produced as an end-product of amino acid metabolism by the muscle and other extrahepatic tissues. Other pathways of glutamine metabolism such as the aminotransferase pathway are not of primary importance in glutamine degradation in the absence of exogenously added ketoacids. Glutamate is a poor primary substrate for gluconeogenesis in the liver, and has been assumed to penetrate the liver membrane relatively slowly.
