Free amino acid pool in the rat skin and the skin morphological structure in acute allergic contact dermatitis

The concentration of free amino acids in the skin of rats in experimental acute allergic contact dermatitis (ACD) was studied. Free AAs were determined by reverse-phase high-performance liquid chromatography. Acute ACD was simulated using 2.4-dinitrochlorobenzene. The levels of asparagic and glutamic acids, valine, leucine, phenylalanine, lysine, and taurine increased and methionine, tyrosine, tryptophan, α-aminobutyric acid and anserine decreased in the rat skin during acute ACD development. An imbalance of amino acid metabolism was expressed in an increase in the ratio of aromatic amino acids/branched-chain amino acids by a decrease in the sum of aromatic amino acids. An increase in the Phe/Tyr ratio may indicate a decrease in the activity of the hydroxylase pathway of phenylalanine metabolism in the rat skin. Only in the skin of rats with acute ACD, the levels of valine, isoleucine and lysine were negatively correlated with the level of blood TNF-1α, which confirms their anti-inflammatory effect. Histological examination revealed an inflammatory reaction expressed by lymphocytic infiltration of various skin layers (ectoderm, subepidermal layer). Perivascular proliferation of fibroblasts and formation of subepidermal fibrosis were observed in subepithelial sections. The upper layer of the thorny layer was necrotic, the granular layer with necrosis and pronounced karyopiknosis of nuclei was observed. The epidermis thickness increased, infiltration of lymphocytes and segmented neutrophils into epidermis was noted up to the cells of the thorny layer (exocytosis). The data obtained indicate a significant metabolic imbalance of amino acids in the skin of rats in acute ACD, manifested by depletion of the amino acid pool of aromatic and sulfur-containing amino acids, impaired metabolism of phenylalanine, as well as increased levels of immunogenic amino acids that play a functional role in the regulation of anti-inflammatory, antioxidant system and proliferative activity of immune system cells on the background of a significant inflammatory process.