Histamine Homologs as Potential Mechanism-Based Irreversible Inhibitors of Mammalian Diamine Oxidase (Suicide Inhibitors).

Abstract

Diamine oxidase is an enzyme involved in two important metabolic pathways: polyamine metabolism via oxidation of putrescine and histamine metabolism via oxidation of the side chain amino group of histamine. In addition to its presence in some normal tissues, diamine oxidase has been found in significantly elevated levels in certain types of tumors such as medullary carcinoma of the thyroid and small cell carcinoma of the lung. The factors responsible for the increased diamine oxidase activity in such tumors have not been determined, nor is it known what physiological role, if any, this enzyme is playing in the tumors where it is so highly elevated. The goal of this investigation is the preparation of selective irreversible inhibitors of diamine oxidase for use in investigating the roles of this enzyme in normal and neoplastic tissue. Diamine oxidase is known to contain a carbonyl prosthetic group. Considerable circumstantial evidence indicates that the cofactor is a pyridoxal phosphate but this remains controversial. The mechanism is clearly pyridoxal-like, in which case, based on the extensive literature concerning suicide inhibition of pyridoxal enzymes, one would expect (beta)-fluoro and (beta),(gamma)-unsaturated amines to be suicide inhibitors of diamine oxidase. Furthermore, the necessary degree of selectivity required for these inhibitors might be achieved through a structural similarity to histamine, a natural substrate of diamine oxidase. Thus, we have prepared nine homologs of histamine containing acetylenic, mono-fluoro, vinyl, and vinyl halide moieties in the side chain. These compounds were tested in vitro with diamine oxidase and all were found to be reversible (competitive and non-competitive) inhibitors, with apparent K(,i)'s which range from .17-7.1 (mu)M. The following is a list of the histamine analogs which were prepared: (1) E-1{4(5)-imidazolyl}-3-aminopropene; (2) Z-1-{4(5)-imidazolyl}-3-aminopropene; (3) (beta)-fluorohistamine; (4) 1-{4(5)-imidazolyl}-2-fluoro-3-aminopropane; (5) 1-{4(5)-imidazolyl}-3-aminopropyne; (6) 2-{4(5)-imidazolyl}-3-aminopropene; (7) Z-1-bromo-2-{4(5)-imidazolyl}-3-aminopropene; (8) E-1-{4(5)-imidazolyl}-1-fluoro-3-aminopropene; (9) Z-1-{4(5)-imidazolyl}-1-fluoro-3-aminopropene.