Overt signs of toxicity to dogs and cats of dietary deoxynivalenol.

Studies were conducted to determine the dietary amounts of deoxynivalenol (DON; vomitoxin) in dog and cat food that are required to produce overt signs of toxicity (e.g., vomiting or reduced food intake). Wheat naturally contaminated with 37 mg of DON/kg was used to manufacture pet foods containing 0, 1, 2, 4, 6, 8, and 10 mg of DON/kg. Deoxynivalenol concentration in pet food following manufacture was unchanged, indicating that the toxin was stable during conventional extrusion processing. Dogs previously fed DON-contaminated food were able to preferentially select uncontaminated food. Dogs not previously exposed to DON-contaminated food consumed equal quantities of contaminated and uncontaminated food. There was no effect of 6 mg of DON/kg on dog food digestibility. Food intake of dogs was significantly reduced by DON concentrations greater than 4.5 +/- 1.7 mg/kg, and DON greater than 7.7 +/- 1.1 mg/kg reduced cat food intake. Vomiting by dogs and cats was commonly observed at the 8 and 10 mg DON levels.

Rates of lysine catabolism are inversely related to rates of protein synthesis when measured concurrently in adult female rats induced to grow at different rates.

To test the effect of changes in the rate of protein synthesis on amino acid oxidation, both were studied concurrently in individual 200-g female Sprague-Dawley rats. In a growth trial (Experiment 1), recombinant bovine somatotropin (rbST) was injected subcutaneously (0, 2 or 12 mg/d) over 6 d (n = 4 rats per rbST level). Weight gain increased with rbST level (P < 0.01); 1.96 +/- 0.8, 4.24 +/- 0.8 and 8.67 +/- 0.8 g/d, respectively. After treatment with rbST (0 or 12 mg/d) for 4 d (Experiment 2), rats were injected via a tail vein catheter with valine (400 mmol, 4.07 mBq L-[3,4(n)-3H]valine) at 0, 4, 10, 13 or 16 h after the daily rbST injection and killed 20 min later. This flooding dose was 5 to 6 times, not 10 times, the free pool as hoped. Protein synthesis in rbST-treated rats increased 46% in muscle (P < 0.001) and 36% in liver (P < 0.01). The ks was unaltered with time after rbST injection (0-16 h, P > 0.05). When 600 mmol valine (4.4 mBq L-[3,4(n)-3H]valine) was used in Experiment 3, specific activity (SA) of free valine was constant over 20 min and was 94 +/- 4% of that injected. Finally, in Experiment 4, protein synthesis and amino acid oxidation rates measured in the same rat revealed a 35% increase (P < 0.01) in protein synthesis in hind leg muscle and a 29% increase in liver (P < 0.05) from rbST-injected (12 mg/d) rats (n = 6). Lysine oxidation was estimated by continuous (12 h) infusion of L-[1-14C]lysine via the opposite tail vein catheter. Expired CO2 was collected over 20-min intervals and SA at plateau was estimated by fitting an exponential model. Lysine oxidation was reduced (P < 0.05) by 44% in rbST-treated rats. The idea that an increase in protein synthesis results in decreased amino acid oxidation remains tenable.

Efficiency of lysine or threonine retention in growing rats fed diets limiting in either lysine or threonine.

Over a 21-d experiment, the efficiency of lysine and threonine retention was determined in 80 male Sprague-Dawley rats (65.9 +/- 0.3 g, means +/- SE) fed purified diets containing an amino acid mix limiting in either lysine or threonine. With additional increments of the first limiting amino acid, lysine concentration in total body protein (g/16 g N) increased (P < 0.01) in rats fed lysine-limiting diets but, when fed threonine-limiting diets, lysine concentration in body protein first increased and then decreased (P < 0.01). As increments of the first limiting amino acid were added, the threonine concentration in total body protein increased then decreased when both lysine- (P < 0.01) and threonine- (P < 0.06) limiting diets were fed. Lysine and threonine retention were calculated based on comparative slaughter. Sixteen rats were killed on d 0 to estimate the grams of amino acid in the body. Retention responses were analyzed using a logistic equation in which lysine or threonine intake was used to predict retention. The maximum marginal efficiency (dr/dI, retention/intake) was observed at <40% of maximum retention. For lysine retention, it was 81% when lysine was limiting and 70% when threonine was limiting. For threonine retention, it was 58% when threonine was limiting and 49% when lysine was limiting. The maximum cumulative efficiency (retention adjusted for maintenance relative to cumulative intake) for lysine retention was 62% when lysine was limiting or 58% when threonine was limiting. For threonine retention, it was 51% when threonine was limiting and 35% when lysine was limiting. Thus, amino acid concentration in body protein is not constant, and amino acids are used with higher efficiency when first limiting.

Recombinant bovine somatotropin decreases hepatic amino acid catabolism in female rats.

The effect of recombinant bovine somatotropin (rbST) on hepatic amino acid catabolism in female rats was investigated. Daily injections of rbST for 5 d decreased liver homogenate lysine alpha-ketoglutarate reductase (EC 1.5.1.8) activity (P < 0.05) and liver homogenate lysine oxidation (P < 0.05) approximately 35%. Liver homogenate methionine and valine oxidation were depressed approximately 20 (P = 0.13) and 35% (P < 0.05), respectively. These data show a decrease in hepatic capacity to oxidize amino acids in rats administered rbST. Whether depressed liver amino acid degrading enzyme activity plays a role in amino acid oxidation in vivo remains to be evaluated.

Diminishing returns in weight, nitrogen, and lysine gain of pigs fed six levels of lysine from three supplemental sources.

Diminishing returns responses to increments of lysine intake were evaluated in 82 crossbred barrows (10.2 +/- SE = .31 kg) fed one of six concentrations of lysine expressed as 55, 80, 90, 100, 115, or 140% of a concentration (10.9 g/kg of diet) that supported maximum weight gain. Supplemental lysine was provided from either soybean meal (SBM), SBM+synthetic lysine (SBM + L), or SBM+corn gluten meal (SBM + CGM) additions to a basal diet consisting of corn, minerals, and vitamins. A logistic equation was used to describe weight (kilograms/day), nitrogen (grams/day) and lysine (grams/day) gain as a function of lysine intake from each diet. The parameter Rmax, asymptotic maximum response at infinite intake, was shared for SBM and SBM+L (.70 +/- .02, 17.6 +/- .4 and 7.6 +/- .5, respectively) but was different (P < .05) for SBM + CGM (.62 +/- .02, 15.7 +/- .4 and 6.4 +/- .4, respectively) diets. A plateau in weight gain response was observed at approximately 100% of our estimate of the lysine requirement (9.8 g/kg, based on analyzed values), but nitrogen and lysine gain responses did not approach a plateau until 120 or 145% of the requirement. No differences were detected among pigs fed the SBM and SBM+L sources; however, the efficiency of lysine use by pigs fed the SBM+CGM diet was lower. Maximum marginal efficiency (dr/dI) occurred at 45, 40, and 35% of Rmax for weight, nitrogen, and lysine gain, respectively. The maximum marginal efficiency of lysine gain was 81% for pigs fed the SBM and SBM+L diets but only 68% for pigs fed the SBM+CGM diet. Consequently, diminishing returns were apparent for at least the upper 55 to 65% of the response curve. Whole-body lysine content increased (P < .03) from 5.5 to 6.0 g/16 g of N and glycine (9.6 to 7.9) and proline (6.4 to 5.4) content decreased (P < .001) as lysine intake increased. No changes were detected in body threonine content (pooled average = 2.8).

Role of protein synthesis in amino acid catabolism.

Except for branched chain amino acids, the site of indispensable amino acid degradation is the liver. Location of amino acid degradation capacity in a single organ may play an important role in the reutilization of amino acids derived from protein turnover. The importance of preferential utilization of amino acids for protein synthesis on catabolism of amino acids is demonstrated in two ways. First, by minimal oxidation of an amino acid at dietary concentrations below that required for maximum gain followed by a near proportionate oxidation with increased dietary level, and second, by increased oxidation of an indispensable amino acid when another amino acid limits protein synthesis. A direct effect of protein synthesis on amino acid catabolism can be shown by a marked increase in amino acid catabolism when protein synthesis is inhibited.

Use of a four-parameter logistic equation to evaluate the response of growing rats to ten levels of each indispensable amino acid.

Over a 21-d period, 400 [four rats/level, 10 levels/amino acid, 10 indispensable amino acids (IAA)] male weanling rats (65.9 +/- 0.3 g; mean +/- SEM) were fed diets with one of 10 levels of each of the 10 IAA. In addition, four rats were fed an amino acid-free diet and 16 rats were killed on d 0 for individual body composition. With the exception of the limiting amino acid (LAA), an increment (35% of the requirement) of each IAA was added to the mixture to insure that the LAA remained first limiting. A four-parameter logistic equation was used to describe the nitrogen and weight gain responses of rats to each IAA. Conservation of nitrogen, defined as a predicted y-intercept value greater than the value observed for rats fed an amino acid-free diet (-0.304 +/- 0.023 g N/21 d), was seen when diets devoid of total aromatic amino acids or lysine (-0.062 +/- 0.013 g N/21 d) or histidine, leucine, tryptophan or valine (-0.115 +/- 0.011 g N/21 d) were fed. When total sulfur amino acids were first limiting, diminishing returns (a decrease in the first derivative) was evident from zero intake to Rmax (estimated asymptotic response maximum). In contrast, when other IAA were limiting, diminishing returns were apparent after approximately the first third of the full response. Based on the first derivative of the response curves, the efficiency of nitrogen gain depends on the LAA. The dietary LAA would be expected to influence the shape of the response curve and therefore influence the quantitative aspects of diminishing returns.