A Role of TMAO and Urea in Salt Adaptation in Marine Fishes

Abstract

TMAO (trimethylamine oxide) and urea are stable end products and cannot be metabolized further to provide skeletons for biosynthesis. These substances are widely utilized in various marine fishes. Particularly, in cartilaginous fishes, a large amount of urea and methylamines including TMAO are accumulated as osmolytes in their tissues. In marine teleostean fishes, a considerable amount of TMAO also maintain in their bodies, although the levels are far less than those of cartilaginous fishes. However, urea levels in teleostean tissues are generally low, and only a few species utilize urea as a device for ammonia detoxification. In this paper multiple functions of TMAO and urea on marine fishes are discussed.