2004 by CRC Press LLC. In this chapter we will try to define the role of the intestine in the intermediary metabolism of amino acids. For a long time it has been recognized that the intestine is not a passive receptacle that indiscriminately absorbs food, regardless of its composition and quantity. In the 1970s, several authors demonstrated that apart from its absorptive capacity, the intestine played an important role in the intermediary metabolism of macronutrients. In this chapter, we focus on the intestinal metabolism of protein and amino acids. It was found that enterocytes metabolized in vitro significant quantities of glutamine and stoichiometrically produced alanine and ammonia, regardless of its site of entry, be it the intestinal lumen or the arterial inflow. Windmueller and Spaeth performed landmark investigations regarding the contribution of amino acids and carbohydrates to the intermediary metabolism of the intestine. They and others established that ammonia, alanine, and glutamic acid were the main products of degradation of glutamine in enterocytes. Ammonia generation as an end point of protein metabolism was in the 1960s considered to arise from bacterial degradation of food protein and urea, because antibiotic therapy lowered plasma ammonia levels and improved hepatic encephalopathy. Due to the work of several authors, it became clear that the intestine of germ-free animals also produced significant quantities of ammonia, which must therefore have arisen from intermediary metabolism of protein by the gut wall itself without the interference of bacteria. This subject will be dealt with elsewhere in this book (Chapter 26).
