Effect of Variations in Protein Intake on Enzymes of Amino Acid Metabolism

Abstract

Changes in the protein intake of animals result in changes in the activities of illany enzymes of nitrogen metabolism. Effects of starvation on enzymes of amino acid metabolism are frequently quite different from effects of ingestion of a protein-free diet. The nature of the changes is such as to suggest that the lability of body proteins depends not on protein intake as such but on the metabolic requirements of the organism. Many of the enzymes of nitrogen metabolism are elevated in starved animals, animals fed a high protein diet, and animals injected with adrenal glucocorticoid hormones. These changes appear to be associated with increasing dependence of the organism on protein metabolism. Many of these enzyme responses to dietary changes can be interpreted as adaptations contributing to the maintenance of homeostasis in the face of alterations in the external environment-alterations that might otherwise affect the internal environment of the organism detrimentally.

Modulation of rat skeletal muscle branched-chain alpha-keto acid dehydrogenase in vivo. Effects of dietary protein and meal consumption.

“…Chronic consumption of high protein diets results in induction ofrate-limiting degrading enzymes for most essential amino acids (1,2). Hence, the concentrations of most amino acids do not increase markedly in body fluids after long-term high protein feeding and may actually decrease as more protein is consumed (3).…”

“…Hence, the concentrations of most amino acids do not increase markedly in body fluids after long-term high protein feeding and may actually decrease as more protein is consumed (3). When dietary protein intake is restricted to less than the requirement for growth and maintenance these enzymes decrease (1,2), resulting in conservation of essential amino acids. The catabolic system for the branched-chain amino acids (BCAA)'-leucine, isoleucine, and valine-appears to be an exception to this homeostatic scheme for essential amino acids in that (a) 1.…”

Evolution de l'adaptation métabolique des rats nourris avec des régimes hyperprotéiques

“…De nombreux travaux montrent les effets des régimes riches en protéines sur l'activité des enzymes impliquées dans différentes voies métaboliques (Harper, 1965 ; Szepesi, 1971 (Munro, 1964;Peret et al, 1975 Bucolo, 1974). En outre, elles dépendent de la concentration en glucose ou en glucose-6-phosphate (G6P) des cellules hépatiques, l'ensemble du système étant contrôlé par différentes hormones (insuline, glucagon, catécholamines, etc.)…”

Activity of several enzymes of amino acid catabolism in the liver of rats fed protein as a meal

“…Since the meal is provided as a protein concentrate (70 % casein), its digestion causes portal aminoacidemia to rise to values usually found only when high-protein diets are fed ; however, this elevation is only temporary and not constant as in the case of protein-rich mixed-feeding (Bourdel et al, 1981a ; R6m6sy, Demign! and Aufrère, lack of effect was partly due to the induction of enzymes of amino acid catabolism in the liver (Harper, 1965 ;Szepesi and Freedland, 1968 ;Pestana, 1969 ;Mauron, Mottu and Spohr, 1973 ;Yanagi, Campbell and Potter, 1975) and to an increase in the capacity for ureogenesis (Schimke, 1962). On the contrary, even after a 4-week adaptation period, the ingestion of a protein meal fed separately can induce an important increase in liver protein synthesis (over 3-fold within 1 h) (Lardeux, Bourdel and Girard-Globa, 1978 (De Rosa and Swick, 1975).…”