Functional study on alanine aminotransferase: The synthetase of key umami compound alanine in Urechis unicinctus

Umami, the fifth basic taste alongside sour, sweet, bitter, and salty, is primarily influenced by free amino acids (FAAs), organic acids and inorganic ions, as indicated by Taste Activity Value (TAV). Marine organisms represent ideal models for investigating the origin and regulatory mechanisms of umami taste. Urechis unicinctus, valued for its distinctive flavor and nutritional value, was selected to explore the molecular basis of its umami taste. Unlike most other seafood, its umami flavor originated from high concentration of alanine (Ala) in FAAs. The results showed that variations in Ala content between different cultured populations may be associated with glutamic-pyruvic transaminase (GPT) activity and gene expression levels. Functional studies confirmed that gpt interference led to a significant decrease in Ala and key umami-related amino acids in muscle tissue. Furthermore, mutagenesis analysis revealed that the conserved (K298, S297) and evolutionary (M294, N239) residues are critical for GPT activity, with mutations reducing activity by 36%-90%. GPT evolutionary divergence (M294 → H294, N239 → C239) between invertebrates and vertebrates may underlie their Ala content differences. Taken together, our findings clarify the molecular mechanism underlying umami formation in U. unicinctus, and provide new insights into the origin of umami compounds in marine organisms.