Effect of acute deficiency of dietary proline on proline balance in the rat small intestine and liver

Abstract

Introduction Proline is widely found in all types of mammalian tissue, and accounts for about 20% of the amino acids that constitute collagen (A dams 1970). Proline is nutritionally nonessential but biologically it is an important amino acid and consequently mammalian organisms synthesize the required amounts of proline even in the absence of sufficient proline consumption via food.The metabolism of higher animals is unique, and amino acid metabolism differs from one tissue to the next. Some organs are capable of synthesizing nonessential amino acids for use throughout the body. For example, arginine is primarily synthesized in the kidney and then released and distributed throughout the body.Pyrroline‐5‐carboxylate reductase is the enzyme responsible for the final stage of proline synthesis, and its activity has been confirmed in many important organs and tissue such as the cartilage, liver, small intestine, kidney and thymus gland (H erzfeld et al. 1977; S mith and P hang 1978). However, the different levels of pyrroline‐5‐carboxylate reductase activity among these organs has led to the belief that different amounts of proline are synthesized in these organs.In order to ascertain biological responses to dietary proline deficiency, it is important to identify the organs that release and distribute proline throughout the body when insufficient proline is consumed through the diet, thus reducing the blood proline concentration. Few studies have investigated this issue, but when ascertaining biological responses to dietary proline deficiency, it is more important to elucidate the effect of dietary proline deficiency on the metabolism of proline and other amino acids that are closely related metabolically to proline, in proline synthesizing organs.One of the most effective ways to assess amino acid metabolism in a target tissue of higher animals is to measure the difference between the arterial and venous concentrations of amino acids. I shikawa (1974) measured arteriovenous differences in order to examine the release of proline from the kidney and small intestine of fasted rats and the uptake of proline by the liver. In a previous study, it was found that when the plasma proline concentration was reduced to the fasting level by the consumption of a proline‐deficient diet, proline was released from the kidney (W atanabe et al. 1995, 1997).In the present study, to ensure the induction of dietary proline deficiency, a completely purified diet containing all amino acids except for proline was prepared and fed to rats under experimental conditions. To investigate the role of the small intestine and liver in supplying and ingesting proline when the uptake of proline through food is restricted, the release and uptake of amino acids in the small intestine and liver were assessed by measuring carotid artery–portal vein and portal–hepatic vein differences in proline in rats.