Divergent polyamine metabolism in the Apicomplexa

Abstract

The lead enzymes of polyamine biosynthesis, i.e. ornithine decarboxylase (ODC) and arginine decarboxylase (ADC), were not detected in Toxoplasma gondii [the limit of detection for ODC and ADC was 5 pmol min "1 (mg protein) "1 ], indicating that T. gondii lacks a forward-directed polyamine biosynthetic pathway, and is therefore a polyamine auxotroph. The biochemical results were supported by results obtained from data-mining the T. gondii genome. However, it was possible to demonstrate the presence of a highly active backconversion pathway that formed spermidine from spermine, and putrescine from spermidine, via the combined action of spermidine/ spermine N 1 -acetyltransferase (SSAT) or spermidine N 1 -acetyltransferase (SAT) and polyamine oxidase (PAO). With spermine as the substrate, T. gondii SSAT had a specific activity of 1.84 nmol min "1 (mg protein) "1 , and an apparent K m for spermine of 180 mM; with spermidine as the substrate, the SAT had a specific activity of 3.95 nmol min "1 (mg protein) "1 , and a K m for spermidine of 240 mM. T. gondii PAO had a specific activity of 10.6 nmol min "1 (mg protein) "1 , and a K m for acetylspermine of 36 mM. Furthermore, the results demonstrated that T. gondii SSAT was 50 % inhibited by 30 mM di(ethyl)norspermine. The parasite actively transported arginine and ornithine, which were converted via the arginine dihydrolase pathway to citrulline and carbamoyl phosphate, resulting in the formation of ATP via carbamate kinase. The lack of polyamine biosynthesis by T. gondii is contrasted with polyamine metabolism by other apicomplexans.

Toxoplasma gondii Development of Its Replicative Niche: in Its Host Cell and Beyond

“….) that are then transported into the parasite (27,28). Like tryptophan and other amino acids that the parasite scavenges from its host, arginine and polyamine transporters remain to be identified.…”

“…Indeed, arginine limitation reduces growth of wild-type type 1 parasites but not ROP16 knockouts (35). But arginase 1 expression would also act to reduce polyamine levels in the host cell, and how parasites handle decreased avail-ability of these nutrients, which also must be scavenged, remains to be determined (27).…”

An Inside Job: Hacking into Janus Kinase/Signal Transducer and Activator of Transcription Signaling Cascades by the Intracellular Protozoan Toxoplasma gondii

“…Two biologically opposing outcomes are possible following induction of arginase 1 in infected cells. On the one hand, Toxoplasma is a polyamine auxotroph; therefore, induction of arginase 1 could promote growth of type 1 parasites by supplying essential polyamine nutrients, as is seen in some cases (15,36). Alternatively, the parasite is also an arginine auxotroph, and therefore induction of arginase 1 could act to limit growth of parasites by limiting the availability of this essential amino acid, which is also seen in some cases (8,25).…”

Inhibition and Structure of Toxoplasma gondii Purine Nucleoside Phosphorylase

“…Toxoplasma gondii lives in nucleated mammalian cells. As polyamines are abundant in these cells, it is believed that polyamine salvage replaces de novo polyamine synthesis (37,38). Under conditions of polyamine abundance, no 5=-methylthioadenosine, a by-product of polyamine synthesis, is formed in T. gondii.…”

“…Due to a lack of forward-directed polyamine biosynthetic enzymes, T. gondii is unable to generate 5=-methylthiopurines (37,38). Examination of the structure of TgPNP indicates that the residues in the active site cannot accommodate 5=-methylthioinosine, but the solvent-filled pocket surrounding the 5=-OH group of immucillin-H suggests that the active site is able to accept a larger 5= group.…”