Functions and Metabolism of Amino Acids in the Hair and Skin of Dogs and Cats

The hairHair and skinSkin of domestic catsCat or dogsDog account for 2% and 12–24% of their body weight, respectively, depending on breedBreeds and age. These connective tissues contain proteinProtein as the major constituent and provide the first line of defenseDefense against external pathogens and toxins. MaintenanceMaintenance of the skinSkin and hairHair in smooth and elastic states requires special nutritional support, particularly an adequate provision of amino acidsAmino acid (AAs). KeratinKeratin (rich in cysteineCysteine, serineSerine and glycineGlycine) is the major proteinProtein both in the epidermisEpidermis of the skinSkin and in the hairHair. FilaggrinFilaggrin [rich in some AAs (e.g., serineSerine, glutamateGlutamate, glutamineGlutamine, glycineGlycine, arginineArginine, and histidineHistidine)] is another physiologically important proteinProtein in the epidermis of the skinEpidermis. CollagenCollagen and elastinElastin (rich in glycineGlycine and prolineProline plus 4-hydroxyproline) are the predominant proteinsProtein in the dermisDermis and hypodermis of the skinHypodermis. TaurineTaurine and 4-hydroxyproline are abundant free AAs in the skinSkin of dogsDog and catsCat, and 4-hydroxyproline is also an abundant free AA in their hairHair. The epidermisEpidermis of the skinSkin synthesizes melanin (the pigment in the skinSkin and hairHair) from tyrosineTyrosine and produces trans-urocanate from histidineHistidine. Qualitative requirementsRequirement for proteinogenic AAs are similar between catsCat and dogsDog but not identical. Both animalAnimal species require the same AAs to nourish the hairHair and skinSkin but the amounts differ. Other factors (e.g., breedsBreeds, coat colorColor, and age) may affect the requirementsRequirement of catsCat or dogsDog for nutrients. The developmentDevelopment of a healthy coat, especially a black coat, as well as healthy skinSkin critically depends on AAs [particularly arginineArginine, glycineGlycine, histidineHistidine, prolineProline, 4-hydroxyproline, and serineSerine, sulfur AAs (methionineMethionine, cysteineCysteine, and taurineTaurine), phenylalaninePhenylalanine, and tyrosine] and creatine. Although there are a myriad of studies on AA nutritionNutrition in catsCat and dogsDog, there is still much to learn about how each AA affects the growthGrowth, developmentDevelopment and maintenanceMaintenance of the hairHair and skinSkin. AnimalAnimal-sourced foodstuffsFoodstuff (e.g., feather mealFeather meal and poultryPoultry by-product mealMeal) are excellent sources of the AAs that are crucial to maintain the normal structureStructure and healthHealth of the skinSkin and hairHair in dogsDog and catsCat.