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Diet Induced Alterations and Distribution of Indices of Lysine Catabolism Stephanie K. Gatrell Lysine is thought to be oxidized primarily by lysine α-ketoglutarate reductase (LKR) and saccharopine dehydrogenase (SDH), which are restricted to the mitochondrial matrix. Lysine is presumably transported through the plasma membrane by isoforms(s) of a cationic amino acid transporter (CAT). Although the mechanism(s) of lysine transport through the inner mitochondrial membrane is not known, it is presumptively transported by one or both mitochondrial “ornithine transporters” (ORC-1/ORC-2). Tissue distribution of LKR and the mechanism(s) responsible for alterations in hepatic lysine catabolism in swine are unclear. Also, the roles of the alternative pathways of lysine degradation, the L-amino acid oxidase-dependent (LAAO), in poultry, and the lysyl oxidase-dependent pathway (LO), in mammals and poultry, have not been established. Genetic effects on lysine degradation were evaluated throughout the production-cycle in 2 strains of commercial turkey. Hepatic LKR activity (P<0.05), LKR mRNA (P<0.01), SDH activity (P<0.05), lysine oxidation (LOX, P<0.0001), LAAO activity (P<0.05), LO activity (P<0.01) L-amino acid oxidase (P<0.0001), and CAT1 mRNA abundance (P<0.05) differed throughout the production cycle. Differences in indices of lysine catabolism due to strain in these 2 commercial lines of turkey were not detected. The LAAO and LO activities represented only 0.21 and 0.03% of the activity of enzymes involved in the saccharopine-dependent pathway. These data support that the saccharopine-dependent pathway is the predominant pathway of lysine degradation in turkey liver and that indices of hepatic lysine catabolism vary throughout the production cycle. Next, studies were conducted in swine to characterize the tissue distribution and evaluate possible mechanisms of alterations in lysine degradation. In growing pigs, LKR activity (P<0.001) was highest in liver, intestine and kidney samples and SDH activity (P<0.0001) and LKR (P<0.0001) and SDH mRNA (P<0.0001) were highest in liver. Interestingly, tissue distribution of LKR activity was correlated with ORC-1 and ORC-2 mRNA (r 2 =0.32, P<0.05 and r 2 =0.41, P<0.05, respectively). Average LO activity across tissues represented only 0.5% of the activity of enzymes involved in the saccharopine-dependent pathway. These data indicate that extra-hepatic tissues play a role in whole-body lysine degradation, ORC transporters may play a role in the transport of lysine into the mitochondrial matrix for its catabolism, and the saccharopine-dependent pathway is the predominant pathway of lysine degradation in pig tissues. To further investigate the role of ORC in lysine catabolism and to discern the mechanisms responsible for alterations of lysine catabolism, weanling pigs (n=35) were fed either a control (C), high protein (HP), low protein (LP), high lysine (HL) or low lysine (LL) diet. Liver LKR activity (P<0.05) and AASS protein expression (P<0.01) were reduced in pigs consuming the LL diet compared to C. Liver SDH mRNA expression was reduced (P<0.08) with the consumption of the LL diet compared to C, and AASS mRNA was reduced (P<0.05) with the consumption of the LL diet compared to the HP and HL diets. No significant dietary alterations in lysine catabolism were detected in heart or kidney. There was an increase (P<0.05) in liver ORC-1 mRNA expression with the consumption of the HL diet and a dramatic decrease in ORC-1 expression when treated with the LL diet. From these data it can be concluded that diet-induced alterations in lysine catabolism do occur in pig liver and the response of the ORC-1 mRNA to low lysine diets implicates that transporter as an important molecular basis of lysine conservation.