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Abstract
Lysine is the limiting amino acid in many animal diets. Improving the efficiency of lysine use for protein synthesis can reduce feed cost and potentially the nitrogen and phosphorus output of agricultural production systems. Lysine not used for protein synthesis is oxidized, primarily by lysine α-ketoglutarate reductase (LKR). The mechanism(s) responsible for alterations in hepatic LKR activity in rodents and poultry are still unclear. For this reason, studies in both species were conducted to evaluate possible mechanisms responsible for alterations of LKR activity. Mice were fed either a high or adequate protein diet. Mice fed the high protein diet had a greater (P<0.05) LKR activity. Mice fed high and adequate protein diets for 8 days showed a significant difference (P<0.05) in aminoadipate semialdehyde synthase (AASS) mRNA expression in 2 out of the 4 replicates. However, no difference (P>0.1) in AASS protein abundance was detected in any study. Chickens were fed either a lysine deficient or lysine adequate diet. Neither LKR nor L-amino acid oxidase (AAOX) activities were significantly different between the two treatments; however, there was a tendency for both LKR (P>0.1) and AAOX (P=0.08) activities to be lower in chicks fed the lysine-deficient diet. Chicks fed lysine-deficient diet had greater (P<0.05) AASS and tended (P=0.09) to have greater lysyl oxidase mRNA expression than chicks fed a lysine-adequate diet. AAOX mRNA expression was similar to the AAOX activity and tended (P=0.1) to decrease in chicks fed the lysine-deficient diet. In chicks consuming the lysine-deficient diets, the increased AASS mRNA was not translated into greater AASS protein abundance. The results in both mice and chickens support a model in which post-translational modification of LKR regulates its activity.
