Catalyzes the formation of L-carnitine from gamma-butyrobetaine. 2-oxoglutarate + 4-(trimethylamino)butanoate + O2 = carnitine + CO2 + succinate Binds 1 Fe(2+) ion per subunit. Amine and polyamine biosynthesis; carnitine biosynthesis. Expressed in the liver and in some extend in the testis and the epididymis. Undetectable during the perinatal period. During development, expression appears after the weaning of the young rat and reaches a maximal expression at the adult stage. Belongs to the gamma-BBH/TMLD family.