Lysine oxidation by growing pigs receiving diets containing free and protein-bound lysine

Knowledge of amino acid availability in feedstuffs is central to accurate diet formulation. Dietary lysine oxidation was evaluated as a means of predicting dietary lysine availability. Growing pigs (30 kg) were offered control (1.43% lysine), free lysine-supplemented, soybean meal-, or cottonseed meal-supplemented diets. Supplemented diets provided equivalent total lysine (approximately 24.2 g/d at 30 kg BW) but availabilities of lysine, determined by slope-ratio assay, in the free lysine, soybean meal, and cottonseed meal were 100, 90, and 30%, respectively. Feed was offered in eight equally spaced meals per day to achieve three times maintenance energy intake. Following a meal containing L[U-l4C]lysine (1 microCi/kg BW), lysine oxidation, as 14CO2 expired, was lower (P 0.05) for the control diet but not different between the other three diets, contrary to the hypothesis. Lysine oxidation following an intravenous bolus dose was lowest (P 0.05) for pigs fed the control, highest (P 0.05) for pigs fed the cottonseed diet, and intermediate (P 0.05) for pigs fed the free lysine-supplemented diet (27.1, 80.2, and 47.5, dpm/kg X 10-2, respectively). Plasma lysine concentration was lower (P 0.05) and lysine specific radioactivity tended to be higher ( P 0.10) following a meal containing cottonseed than following a meal containing free lysine, indicating that the lysine pool was smaller in pigs receiving the cottonseed-meal diet. Pattern of plasma lysine concentration in pigs receiving the cottonseed diet was unique, being low, constant, and unaffected by the meal. The relationships between plasma lysine, specific radioactivity, and oxidation produced by the control and lysine diets were consistent with existing paradigms, whereas the relationships produced by feeding the intact protein diets were not, suggesting that lysine availability in intact protein sources is a function not only of ileal digestibility but also of metabolic influences on lysine catabolism