LYSINE OXIDATION BY GROWING PIGS RECEIVING DIETS CONTAINING FREE AND PROTEIN-BOUND LYSINE

Knowledge of amino acid availability in feedstuffs is central to accur ate diet formulation. Dietary lysine oxidation was evaluated as a mean s of predicting dietary lysine availability. Growing pigs (30 kg) were offered control (1.43% lysine), free lysine-supplemented, soybean mea l-, or cottonseed meal-supplemented diets. Supplemented diets provided equivalent total lysine (approximately 24.2 g/d at 30 kg BW) but avai labilities of lysine, determined by slope-ratio assay, in the free lys ine, soybean meal, and cottonseed meal were 100, 90, and 30%, respecti vely. Feed was offered in eight equally spaced meals per day to achiev e three times maintenance energy intake. Following a meal containing L -[U-C-14]lysine (1 mu Ci/kg BW), lysine oxidation, as (CO2)-C-14 expir ed, was lower (P <.05) for the control diet but not different between the other three diets, contrary to the hypothesis. Lysine oxidation fo llowing an intravenous bolus dose was lowest (P <.05) for pigs fed the control, highest(P <.05) for pigs fed the cottonseed diet, and interm ediate (P <.05) for pigs fed the free lysine-supplemented diet (27.1, 80.2, and 47.5, dpm/kg x 10(-2), respectively). Plasma lysine concentr ation was lower (P <.05) and lysine specific radioactivity tended to b e higher (P <.10) following a meal containing cottonseed than followin g a meal containing free lysine, indicating that the lysine pool was s maller in pigs receiving the cottonseed-meal diet. Pattern of plasma l ysine concentration in pigs receiving the cottonseed diet was unique, being low, constant, and unaffected by the meal. The relationships bet ween 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 sourc es is a function not only of ileal digestibility but also of metabolic influences on lysine catabolism.