Methionine and Cystine Requirements of the Cat

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Abstract
Three experiments with weanling American Shorthair kittens fed a purified L-amino acid diet (4700 kcal metabolizable energy/kg diet) were conducted to determine the methionine and cystine requirements and to also estimate the portion of the total sulfur amino acid requirement that could be furnished by cystine. All diets were fortified with both taurine and inorganic sulfate. Both rate and efficiency of weight gain responded linearly to graded increments of dietary L-methionine (in the presence of excess cystine), a plateau being reached at about 0.45%. At this level of methionine, cats grow maximally when fed 0.45% L-cystine. Thus, 50% of the sulfur amino acid need of the cat can be furnished by cystine. A third experiment was carried out to establish whether cystine is a dispensable or an indispensable amino acid for feline growth. Cats grew just as well when fed their sulfur amino acids in the form of methionine alone (1.0% of the diet) as when fed an isosulfurous level of a methionine-cystine combination (0.45% methionine + 45% cystine). Hence, similar to findings with other mammalian species, cystine is a dispensable amino acid for cat growth, but it can contribute significantly to the total sulfur amino acid requirement and thereby spare dietary methionine.