The colour of the coat depends on the presence of pheomelanin grains (yellow to red pigments) and eumelanin grains (brown to black pigments).The production of these pigments requires the presence of tyrosine and phenylalanine, two aromatic amino acids (so-termed because of their ring structure).
A deficiency of tyrosine and phenylalanine in the diet of dogs and cats with dark or black coats, will lead to reddening of the hair. Studies on Newfoundland and black Labrador puppies show that the phenylalanine and tyrosine levels needed for optimal coat pigmentation are twice that of the corresponding growth-related requirements. Tyrosine supplementation even helps increase the intensity of hair coloration.
The Siamese cat’s particular colour (" colourpoint") is due to properties of tyrosinase, the key enzyme in melanin production. In this breed, the enzyme works only at a relatively low temperature. Warmer areas like the flanks, the belly and the back remain clear, while colder areas like the head, feet and tail are coloured.
Besides its role in hair and iris pigmentation, tyrosine is also a dopamine, noradrenalin and adrenalin precursor. These molecules are involved in the proper functioning of the brain and in the reproductive function. Tyrosine supplementation therefore has a positive effect on fertility.
Tyrosine is either provided directly in the diet or synthesised from phenylalanine, an essential amino acid. Milk and dairy products are excellent sources of tyrosine and rice is the only vegetable source to contain register-able quantities of this amino acid.