Lysine α-ketoglutarate reductase, but not saccharopine dehydrogenase, is subject to substrate inhibition in pig liver

The activity of lysine α-ketoglutarate reductase (LKR), the initial enzyme in the principal pathway of lysine catabolism, is a primary determinant of whole-body lysine status. Past research indicated that LKR activity was predominantly hepatic; recent in vivo data suggest that other tissues can also catabolize lysine. The hypothesis of this investigation was that lysine catabolism takes place in extrahepatic tissues in pigs and that the enzymes involved may be subject to inhibition or activation. Using mitochondria from various tissues of market-age pigs, the activities of LKR and saccharopine dehydrogenase were measured. Liver mitochondria had the highest LKR activity, and the enzyme was subject to substrate inhibition. Mitochondria from the muscle, kidney, heart, and intestinal epithelial cells all had measurable LKR activity. The LKR activity was significantly inhibited by a variety of compounds including saccharopine, α-aminoadipate, α-ketoadipate, 5-hydroxy-l-lysine, and several metals. Oxidation of 14C-lysine to 14CO2 was demonstrated in mitochondria isolated from the liver, muscle, and intestinal epithelial cells. Western blotting confirmed the presence of the α-aminoadipate δ-semialdehyde synthase protein in some extrahepatic tissues. These data show a significant capacity for lysine degradation in these extrahepatic tissues, most notably in cells of the intestine and muscle. These tissues should be considered important contributors to whole-body lysine catabolism.