Methionine is sulphur containing amino acid. Cysteine(another sulphur containing amino acid) is synthesized endogenously from methionine, and production of cysteine accounts for approximately half of the methionine requirement in the diet. In this pathway methionine is converted to homocysteine, which in turn donates a sulphur group to serine (a non-essential amino acid) to ultimately form cysteine(another non-essential amino acid).
Methionine is a neutral amino acid, while cysteine is basic; both methionine and cysteine are gluconeogenic. Cysteine readily reacts with itself and other thiols (i.e. sulphur containing compounds) and cystine is formed from a disulphide bond between two cysteine molecules. Most of the plasma cysteine is actually found as cystine.
Dietary methionine is absorbed by a neutral amino acid transporter in the small intestine (particularly the jejunum) and plasma methionine is actively reabsorbed in the proximal tubule of the kidney; dietary cysteine and cystine are absorbed in the small intestine and actively reabsorbed in the proximal tubule of the kidney via a dibasic amino acid transporter.

Both methionine and cysteine are incorporated into structural protein and are required for normal growth. The sulphur side chains help stabilize secondary and tertiary protein structures. Methionine is part of the coenzyme S-adenosyl methionine, which influences and regulates the activity of a number of enzymatic and cellular replication processes.
Inherited defects in the transporter for dibasic amino acids can result in poor absorption of cysteine (as well as the other dibasic amino acids lysine, ornithine, and arginine) from the intestinal mucosa and poor reabsorption in the renal tubule. Unlike lysine, ornithine and arginine, cystine is not soluble in urine and readily forms crystals and stones. Cystinuria and related dysuria and urinary obstructions due to cystine urolithiasis have been described in Newfoundlands, English bulldogs, and Dachshunds. ncreased intake of DL-methionine (either in the diet or as a supplement) has been used as a therapeutic treatment for sterile struvite crystalluria and urolithiasis. The oxidation of dietary sulphur increases urinary excretion of ammonium (NH4+) resulting in a more acidic urinary pH.

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.
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 choline, an essential vitamin-like nutrient.

Puppies fed a methionine deficient diet experience decreased food intake, weight loss and evidence of dermatitis. In puppies, methionine deficiency in combination with excess cysteine resulted in hyperkaratotic, necrotic foot pad lesions, that resolved with reintroduction of methionine. Inadequate intake of sulphur amino acids without supplemental taurine has also been associated with development of taurine deficient cardiomyopathy, and pigmented gallstones in adult dogs.

Cysteine excess with methionine deficiency in puppies resulted in foot pad lesions. Excess methionine intake in adult dogs can result in ataxia, disorientation, lethargy, vomiting and ptylism