Mini Reviews in Medicinal Chemistry, 2002, 2, 553-563 553 Polyamine Metabolism as Chemotherapeutic Target in Protozoan Parasites C.J. Bacchi *1,2 and N. Yarlett 1,3 1 Haskins Laboratories and Departments of Biology 2 and Chemistry 3 , Pace University, 41 Park Row, New York, NY 10038, USA Abstract : Polyamines are essential cell constituents for all organisms. The present review highlights important differences in the synthesis, degradation, and interconversion of polyamines between the protozoan parasites (Trypanosoma brucei, Trypanosoma cruzi, Cryptosporidium parvum and Trichomonas vaginalis ) and their mammalian hosts. Approaches include development of mono- and di-substituted polyamine analogs targeting polyamine interconversion, as well as more traditional targeting of synthetic enzymes and related pathways. Polyamines are naturally-occurring cations which are found universally in cells. The most common polyamines are putrescine (diaminobutane), spermidine (N 1 - aminopropyl-putrescine) and spermine (N 1 ,N 4- bis(aminopropyl)putrescine). Since they are cations at physiological pH, and because they are conformationally flexible and reversibly bind to negatively-charged molecules, polyamines function in a wide range of biochemical roles including: as cofactors for synthesis of macromolecules, cell division and differentiation, and as conformational stabilizers for nucleic acids [1]. In mammalian cells, polyamine content correlates with the rate of cell division. Polyamines are unique as cations since their intracellular content is carefully regulated through synthesis, uptake, interconversion and excretion [1]. Because of the essential nature of polyamines to cell division and other processes, intervention of this pathway "Fig. (1)" has been the focus of significant antitumor and antiparasite studies. acetylspermine and N 1 -acetylspermidine, are excretable, the backconversion pathway serves to regulate intracellular polyamine levels by preventing over-synthesis or uptake. Acetaminoproprionaldehyde (APA) and H 2 O 2 are byproducts of the interconversion reactions [4]. POLYAMINE METABOLISM IN PROTOZOAN PARASITES A number of considerations set protozoan parasites apart from their mammalian hosts and give reason to hope that parasite polyamine metabolism may be selectively different from that of the host, so that differential sensitivity to various agents may be present and exploitable chemotherapeutically. 1. African Trypanosomes Synthesis of polyamines in mammalian cells "Fig. (1)" is initiated through ornithine decarboxylase (ODC) which decarboxylates ornithine to form putrescine. In mammals this enzyme is inducible, with a t1/2 of ~ 10 min. Aminopropyl groups are added from decarboxylated S- adenosylmethionine (dAdoMet) via specific spermidine and spermine aminopropyl transferases. dAdoMet is supplied by AdoMet decarboxylase, a putrescine-activated inducible enzyme with a short half-life [2]. Aminopropyl groups are donated from dAdoMet by the synthase, and methylthioadenosine (MTA) is formed as a byproduct. This is normally recycled to methionine and adenine in a 5-step pathway [3]. African trypanosomes make spermidine but not spermine "Fig. (2A)"; Table (1). Instead, spermidine is combined with 2 molecules of glutathione to form bis(glutathionyl) spermidine or trypanothione [5]. Glutathione reductase has not been detected, but trypanothione reductase, which uses only trypanothione as substrate, is present instead. Thus African trypanosomes as well as Leishmania spp and Trypanosoma cruzi, have a unique mechanism for handling oxidative stress [5]. African trypanosomes do not easily assimilate exogenous polyamines, and do not have the spermine to spermidine interconversion pathway [6]. Hence they rely on de novo synthesis for polyamine supplies. Mammalian cells take up spermine and spermidine, interconverting them to spermidine and putrescine, respectively "Fig. (1)". The enzymes responsible are spermidine/spermine N 1 -acetyltransferase (SSAT) and polyamine oxidase (PAO) [4]. Since SSAT is inducible and the intermediates in the backconversion pathway, N 1 - A major difference between polyamine metabolism in African trypanosomes and mammals lies in the half-lives of the major synthetic enzymes ODC and AdoMet decarboxylase. In mammalian cells, these have very short half-lives (10 -20 min) [2] while in trypanosomes, both have extended half-lives [7, 8]. The idiosyncratic non-regulatory nature of parasite polyamine metabolism also extends to AdoMet metabolism: AdoMet synthetase, which is heavily regulated in mammalian cells by its product, remains active in African trypanosomes even in the presence of high levels of AdoMet and dAdoMet [9]. Thus inhibition of polyamine synthesis leads to accumulation of AdoMet. MTA, the *Address correspondence to this author at the Haskins Laboratories and Departments of Biology, Pace University, 41 Park Row, New York, NY 10038, USA; Tel: (212) 346-1246; Fax: (21)346-1586; E-mail: cbacchi@pace.edu 1389-5575/02 $35.00+.00 © 2002 Bentham Science Publishers, Ltd.