Four groups of five kittens each were individually fed for 6 wk either a purified control diet containing 0.5% methionine (Met) (based on 18% casein supplemented with arginine, cystine and threonine) or one of three experimental diets containing an additional 2, 3 or 4% L-Met. The diets with added Met caused a reduction in food intake (FI) on the second day and a negative body weight gain (BWG), in proportion to the level of added Met. After 10 d, kittens fed the 2 and 3% Met diets increased their FI and had positive BWG. During the last 10 d of the experiment, the control, 2% Met and 3% Met groups had BWG of 28, 15 and 0 g/d, respectively. Kittens given the 4% Met diet showed no adaptation and continued to lose weight. In these kittens plasma concentration of Met was 50-70 times and cystathionine about three times greater than in control kittens. Four male kittens were fed the same 4% Met diet for 6 wk and then switched to a diet containing 4% L-Met plus 4% glycine (Gly) for 12 d. Average daily FI was 21.4 +/- 1.3 g with 4% Met and 48.5 +/- 2.5 g after the addition of Gly, and BWG went from negative to positive. These results indicate that growing kittens are more sensitive than rats to excess Met and have a limited adaptive capacity. Kittens did not grow normally when the diet contained 2% or more dietary Met, which was equivalent to 0.6 g Met/(kg body wt.d).

I. Six purified amino acid diets containing 6.0 g cystine/kg and the following levels of L-methionine (g/kg diet): 2.1, 2.7, 3.3, 3.9, 4.5, 9.0 were presented to twelve weanling kittens (six male and six female) for six periods of 10 d each. Kittens were assigned to the diets in accordance with a 6 x 6 balanced Latin-square design.2. Body-weight gains of males and females attained apparent plateaux at 3.3 g methionine/kg diet and were respectively (mean +SEM) 22+4 and 18 + 2 g/d. Daily food intakes attained apparent plateaux at 2.7 g methionine/kg diet for male and female kittens and were 63+ 10 and 49+4 g/d respectively. Nitrogen retentions (calculated as dietary-N intake minus faecal-and urinary-N excretion) attained apparent plateaux at 3.9 g methionine/kg diet for both male and female kittens and were 0.85 k0.15 and 0.65k0.05 g/d respectively.3. Previous work has shown that the kitten's L-methionine requirement, in a diet lacking cystine, is 7.5 g/kg diet. Our results indicate that the kitten's L-methionine requirement is 3.9 g/kg diet when 6.0 g cystine/kg is provided, thus approximately 50% of the animal's sulphur amino acid requirement can be met by cystine.Methionine is an essential amino acid for most species of animals and, therefore, must be supplied in the diet for normal growth and physiological function. Cystine, conversely, is considered to be a dispensable amino acid because cysteine can be synthesized from methionine by the trans-sulphuration pathway. Cystine spares methionine by decreasing the amount of methionine required for maximal growth; the metabolic explanation for this is unclear. Finkelstein & Mudd (1967) found that the addition of cystine to a low-methionine diet resulted in a significant decrease in the hepatic activity of cystathionine synthase (EC 4.2.1.22) and, to a lesser extent, a decrease in cystathionase (EC 4.2.1.15) activity (two enzymes important in the conversion of methionine to cysteine). More recently, however, Stipanuk & Benevenga (1977) found that the addition of cystine to a diet given to rats resulted in a depression of cystathionine synthase activity by 3447% of control values. However, their findings, with regard to protein synthesis, suggested that the decrease in methionine oxidation was more likely due to a greater utilization of methionine for protein synthesis than to a decrease in cystathionine synthase activity. Methionine cannot be completely replaced by cystine since methionine cannot be synthesized from cystine; therefore, the use of a basal diet containing excess cystine permits a determination of the minimal methionine requirement. This value, combined with a value for the methionine requirement without cystine, allows a determination of the proportion of the total sulphur amino acid requirement that can be met by cystine.Although early studies of the feline's S amino acid metabolism indicated that methionine was dispensable for the adult cat (Dymsza & Miller, 1964;Rambaut & Miller, 1965), more recent findings have shown that weight loss occurs when ...

1. The nitrogen requirement of the weanling kitten was determined in a series of three experiments. In each experiment, diets were formulated to provide the growing kitten with the essential amino acids at or above the level of requirement. Expt 1 utilized a 4 x 4 balanced Latin square design with two groups of kittens (four male and four female). The crystalline L-amino acid diets were presented at four levels of dietary crude protein (N x 6.25) of 140, 160, 180 and 200 g/kg diet. The design for Expts 2 and 3 was a 6 x 6 balanced Latin square. For each of these experiments, groups of six male and six female kittens were assigned to diets. The six levels of dietary crude protein were 120, 140, 160, 180,200 and 220 g/kg diet; dietary N was supplied by crystalline L-amino acids for Expt 2 and casein plus a supplementary amino acid mix for Expt 3. Food intake, weight gain and N retention were determined in each experiment.2. A sigmoidal model y = P1+ P2/[1 +e(p3+p4. z)] was fitted to the response of weight gain and N retention to dietary N. The calculated requirement (95% of the upper asymptote, P1 +P2) for these experiments varied from 170 to 230 g protein/kg diet with the majority of these values falling between 180 and 200 g protein/kg.3. On the basis of these three experiments, the kitten's requirement for dietary crude protein is between 180 and 200 g/kg diet (28.8-32.0 gN/kg) for purified diets which provide a calculated 21 MJ metabolizable energy/kg diet.The dietary requirement for protein is based on two metabolic demands: (1) the necessity for sufficient quantities of essential amino acids in the proper proportions, (2) the necessity for adequate amounts of nitrogen to provide for the synthesis of non-essential amino acids and other nitrogenous compounds.Within the last 5 years the essential amino acid requirements for maximal kitten growth have been determined with crystalline L-amino acid diets. Protein requirements of the kitten determined before this information was available may have been inflated due to insufficient amounts of a particular amino acid. In addition, protein requirements determined using animal protein sources may be confounded by the association of the level of protein and palatability of the diet. Based on studies in which either casein or a mixture of fish and liver were used as the protein source (Dickinson & Scott, 1956; Miller & Allison, 1958;Jansen et al. 1975), the (US) National Research Council (1978) recommended that 'a protein of a quality equivalent to that derived from unprocessed mammalian, avian or fish muscle should be presented at a level of 28 % of metabolizable energy (ME) in the diet of a growing kitten. This is equivalent to 35 % protein in a diet (dry basis) providing 21 kJ (5 kcal) MEjg dry matter'. To define the N requirement more precisely we have completed three experiments in which all essential amino acids were supplied in adequate amounts and N was supplied as individual amino acids or casein. Food intake, weight gain and N retention have been determined in...

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