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 ...
