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The effect of increasing amounts (0%, 25%, 50%, 75%, and 100%) of dietary supplementation with an organic micromineral complex (Fe, Zn, Cu, Mn, and Se) on antioxidant defenses and mineral deposition in tissues of Nile tilapia juveniles was evaluated, where 100% supplementation represented the average adopted by the feed industry in Brazil. Fish (initial weight 23.93 ± 0.80 g) were fed until apparent satiation twice a day for 56 days. The maximum deposition of Fe and Zn in the hepatopancreas occurred in fish given approximately 50% supplementation, whereas the deposition of Mn and Se increased linearly with the inclusion of the complex. The activity of catalase and superoxide dismutase in the hepatopancreas decreased in fish fed the 50% dose, when compared to those not receiving mineral supplementation or those receiving higher doses. Glutathione peroxidase (GPx) activity in the hepatopancreas increased as the dietary Se concentration increased. However, the concentration of metallothionein in the hepatopancreas showed an inverse relationship to the increase in dietary supplementation of the organic mineral complex. There was no relationship between the doses of organic micromineral supplementation and the activities of GPx, reduced glutathione, non-protein thiols, or protein carbonylation. However, diets supplemented with 50% to 100% promoted greater GPx activity when compared to the 0% supplemented diet. Supplementation with intermediate doses of organic microminerals, approximately 50% of that used in commercial tilapia diets, promoted the homeostasis of metal metabolism, especially for Fe and Zn.

Purified diets containing six levels of methionine plus two different levels of cystine (0 and 0.45%) were fed to growing rats to determine the methionine requirements for maintenance and for growth. A model was developed to subdivide the methionine requirement for maintenance from its requirement for growth. In the absence of cystine, the methionine requirement for growth based on weight gain response was 8.41 mg/g live weight gain and the maintenance requirement was 0.38 times metabolic body size (W g 75 ); based on N retention response, the growth requirement was 0.37 mg/mg N gain and the maintenance need was 0.36 mg times metabolic body size. In the presence of 0.45% cystine, the methionine requirement for growth was 3.32 mg/g live weight gain and the maintenance requirement was 0.11 times metabolic body size (W g 75 ); based on N retention, the growth requirement was 0.16 mg/mg N gain and 0.07 times metabolic body size. As a percentage of retained protein, according to these equations, the sulfur-containing amino acids calculate to be 6.0%; the reported sulfur-containing amino acid fraction of rat muscle crude protein is 5.1%. Forty-two percent of the requirement for total sulfur-containing amino acids for growth was comprised of methionine. From these relative requirements with versus without added dietary cystine, we estimated that 59 to 61% of the methionine need for growth can be replaced with cystine whereas 71 to 81% of the methionine need for maintenance can be replaced with cystine.

Purified diets containing six levels of tryptophan were fed to growing rats to determine tryptophan requirements for growth and for maintenance. A mathematical model was employed to subdivide the tryptophan need for maintenance from its requirement for growth. Based on the model relating weight gain to tryptophan intake, the tryptophan need for growth was 2.48 mg/g weight gain and the daily tryptophan need for maintenance (mg) was 0.066 times metabolic body size (W g · 75 ). Based on the model relating nitrogen gain to tryptophan intake, the tryptophan need for growth was 0.11 mg/mg nitrogen retention and the daily tryptophan need for maintenance was 0.078 times metabolic body size (W g · 75 ). Previous tryptophan requirements for growth and for maintenance corresponded very closely with these estimates. As a percentage of deposited protein, according to these equations, tryptophan was calculated to be 1.7%; the reported tryptophan content of rat muscle crude protein is 1.8%

Purified diets containing six levels of histidine were fed to growing rats to evaluate histidine requirements for growth and for maintenance. A model was developed to subdivide the requirement for maintenance from the requirement for growth. Based on weight gain response, the histidine need for growth was 4.02 mg/g weight gain and the daily histidine need for maintenance was .130 times metabolic body size (Wg .75 ). Based on nitrogen gain response, the histidine need for growth was .168 mg/mg N gain and the daily histidine need for maintenance (mg) was .137 times metabolic body size (Wg .75 ). The plateau of plasma histidine concentration was reached at 22 mg intake/day. Previous histidine requirements for growth (for less than 5 g daily weight gain or 120 mg daily N retention) and for maintenance were in very close agreement with these estimates. As a percentage of retained protein, according to theses equations, histidine was calculated to be 2.7%; the reported histidine content of rat muscle CP is 2.85%.