Methionine is a neutral essential amino acid and a precursor for cysteine. Cysteine (Cys or C ) on the other hand is a basic amino acid that can react with itself and other compounds that have sulfur.
In blood, most of the cysteine is found as cystine which is formed when two molecules of cysteine linked together via a disulfide bond, i.e., via its sulfur-containing or thiol side chain.
Dietary sources get absorbed in the small intestine with the help of dibasic amino acid transporters and reabsorption of the amount in blood plasma occurs in the proximal tubule in the kidney.
Finally, cystine does not dissolve in urine, can easily form stones and crystals.
Providing dietary sources or supplementation is important since it will spare the agony of your cats and dogs having to convert methionine to cysteine.
Foods rich in cysteine include high protein animal foods such as meat, pork, poultry, dairy, eggs, fish as well as lentils, oatmeal, sunflower seeds, broccoli, Brussels sprouts, wheat germs, and so on.
- It is an important protein building block and helps in protein structure and correct folding to ensuring rigidity and strength required including in keratin via its disulfide inter and intrachain that can also be reshuffled.
- According to Study of keratin hair of domestic cat under methionine and cystine experimental diet using FT-Raman spectroscopy research published in Journal of Vibrational Spectroscopy, “methionine and cysteine are…important for the of the hair protein, keratin.”
- It is a component of glutathione antioxidant that cells produce. Glutathione also forms part of felinine, a cat pheromone. Other components of glutathione include glycine and glutamine.
- Donates sulfur to choline.
- It is a precursor of taurine but there might be a need for taurine if your cat has low cysteine sulphinate decarboxylase as well as hepatic cysteine dioxygenase as they are essential in its conversion to taurine.
The minimum methionine-cystine requirement for cats is 1.10% for growth and reproduction and 0.40% for adult maintenance based on dry matter. The equivalent amounts based on 1000 kcal cat food is 2.75g and 1.0g respectively, AAFCO 2014
Deficiency can be due to inappropriate diets, faulty dibasic amino acid transporter, GI disorders among other causes.
Deficiency or “excess can result in animal hair damage, such as loss, slow growth, and brittle appearance.”
Also, “high level of dietary cystine caused debilitating neurological symptoms which resulted in death of some cats.” 
This can be corrected by supplementation of methionine and cysteine and your fluffy friend’s hair features will improve.
Cystine crystalluria in cats
Genetic disorders that affect the normal renal reabsorption of dibasic amino acids like cystine has been linked to the formation of cystine crystals in cats that affects a very small percentage of felines.