Promiscuous activity of arginine:glycine amidinotransferase is responsible for the synthesis of the novel cardiovascular risk factor homoarginine

► We studied biosynthesis of the cardiovascular risk factor homoarginine in humans. ► Arginine:glycine amidinotransferase (AGAT) is a key enzyme in creatine synthesis. ► Healthy lymphoblasts can synthesize homoarginine from arginine and lysine. ► In AGAT-deficient lymphoblasts homoarginine synthesis was undetectable. ► Promiscuous activity of AGAT plays a pivotal role in homoarginine synthesis.