Changes

Methionine is part of the coenzyme s-adenosylmethionine, which through its ability to transfer to and methylate other substrates, is able to modify the activities of a range of different metabolic processes including nucleic acids, proteins, lipds and secondary metabolites); it is also a constituent of protein, and a precursor of cysteine<ref name="Stipanuk">Stipanuk MH and Watford M. Amino acid metabolism. In Biochemical and physiologic aspects of human nutrition. 2000 Philidelphia, PA: WB Saunders Company p. 265-270.</ref>.  

Methionine is part of the coenzyme s-adenosylmethionine, which through its ability to transfer to and methylate other substrates, is able to modify the activities of a range of different metabolic processes including nucleic acids, proteins, lipds and secondary metabolites); it is also a constituent of protein, and a precursor of cysteine<ref name="Stipanuk">Stipanuk MH and Watford M. Amino acid metabolism. In Biochemical and physiologic aspects of human nutrition. 2000 Philidelphia, PA: WB Saunders Company p. 265-270.</ref>.  

Cysteine readily forms sulphide bonds with other thiol groups stabilizing secondary and tertiary structure in proteins such as hair, glutathione, and [[insulin]]; and acts as a sulphur donor to [[Vitamin B4 (Choline) - Nutrition|choline]], an essential vitamin-like nutrient<ref name="NRC">National Research Council (NRC). Protein and Amino Acids. In Nutrient Requirements for Dogs and Cats. 2006 Washington, DC: National Academies Press p. 125-126.</ref>. Cysteine is a precursor to [[Taurine - Nutrition|taurine]] in dogs, however cats have low activity of hepatic cysteine dioxygenase and cysteine sulphinate decarboxylase activity (two key enzymes in the conversion of cysteine to taurine) and require a preformed source of taurine in the diet<ref name="De la Rosa">De la Rosa J, et al. Metabolism of cysteine and cyteinesulfinate in rat and cat hepatocytes. J Nutr 1987;117:549-558.</ref>. Cysteine is also a precursor of felinine, a urine pheromone produced by cats that gives cat urine its distinctive aroma<ref name="Hendriks">Hendriks WH, et al. Importance of sulfate, cysteine and methionine as precursors to felinine synthesis by domestic cats (Felis catus). Comp Biochem Physiol C Toxicol Pharmacol 2001;129:211–216.</ref>.

Cysteine readily forms sulphide bonds with other thiol groups stabilizing secondary and tertiary structure in proteins such as [[Hair - Anatomy & Physiology|hair]], glutathione, and [[insulin]]; and acts as a sulphur donor to [[Vitamin B4 (Choline) - Nutrition|choline]], an essential vitamin-like nutrient<ref name="NRC">National Research Council (NRC). Protein and Amino Acids. In Nutrient Requirements for Dogs and Cats. 2006 Washington, DC: National Academies Press p. 125-126.</ref>. Cysteine is a precursor to [[Taurine - Nutrition|taurine]] in dogs, however cats have low activity of hepatic cysteine dioxygenase and cysteine sulphinate decarboxylase activity (two key enzymes in the conversion of cysteine to taurine) and require a preformed source of taurine in the diet<ref name="De la Rosa">De la Rosa J, et al. Metabolism of cysteine and cyteinesulfinate in rat and cat hepatocytes. J Nutr 1987;117:549-558.</ref>. Cysteine is also a precursor of felinine, a urine pheromone produced by cats that gives cat urine its distinctive aroma<ref name="Hendriks">Hendriks WH, et al. Importance of sulfate, cysteine and methionine as precursors to felinine synthesis by domestic cats (Felis catus). Comp Biochem Physiol C Toxicol Pharmacol 2001;129:211–216.</ref>.

==Consequences of Methionine and Cysteine Deficiency==

==Consequences of Methionine and Cysteine Deficiency==