[Interaction of placental diamine oxidase with carboxyl-substituted lysine]

The interaction between diamine oxidase (DAO) of human placenta and carboxyl-substituted lysines, including N-terminal lysine containing peptides, occurs at rather a high rate and is characterized by the following features. First, the enzyme catalyzes the oxidative deamination of one amino group in the N-terminal lysine at a rate which is inversely proportional to the peptide length. Second, the bound derivatives induce a noncompetitive reversible inhibition of DAO which is enhanced during their coincubation. The inhibiting capacity of this compound is directly proportional to the peptide length; therefore, the tripeptides with the N-terminal lysine can be effective inhibitors that are not practically deaminated in the presence of DAO. Third, the binding of carboxyl-substituted lysines to DAO as well as the inhibition reaction are reversible processes and, with some limitations, can be used for enzyme purification. An analysis of the total activity of DAO in the placenta before and after fractionation of tissue extracts on molecular sieves showed that part of the enzyme is in a blocked state in vivo which does not exclude the possibility that N-terminal lysine containing peptides are related to natural DAO inhibitors.