Catalytic properties of monoamine oxidase from the lamprey Lampetra fluviatilis liver

Abstract

270 The substrate-inhibition specificity of liver monoamine oxidase (MAO) has been studied in mature lampreys ( Lampetra fluviatilis ) of both sexes. The catalytic properties of the enzyme have been found to resemble those of the classic mammalian MAO: the enzyme deaminates tyramine, tryptamine, serotonin, and benzylamine, but not histamine; it is sensitive to proflavin, the well-known type A MAO inhibitor; and is not inhibited by 10 ‐2 M semicarbazide. It is assumed that the MAO from the lamprey liver, as well as liver MAOs of other hydrobionts (squids and fish), has several substrate-binding sites or is represented by several isozymes. The observed specificity of the inhibitory effect of proflavin confirms this assumption. The detoxicating activity of the lamprey liver MAO is low, which may be accounted for by the anatomical and physiological characteristics of the digestive system of lampreys. The enzyme monoamine oxidase (monoamine:O 2 oxidoreductase (deaminating); EC 1.4.3.4) plays the key role in the adrenergic neurotransmitter metabolism and implements an important barrier function on inactivation of biogenic amines [1‐3]. Although this enzyme was discovered more than 80 years ago, its functional role remains obscure. It was established that, in the majority of animals, this enzyme is highly active in the liver and brain [1]. To date, the catalytic properties of liver MAO have been studied predominantly in representatives of higher terrestrial animals. In numerous representatives of the most ancient class of contemporaneous aquatic vertebrates, Cyclostomata, only the MAO from the lamprey Lampetra japonica brain [4] and the hagfish Eptatretus burgeri brain [5‐7] have been studied. Notably, the activity of lamprey liver MAO has not been studied earlier.