IMP dehydrogenase from the intracellular parasitic protozoan Eimeria tenella and its inhibition by mycophenolic acid

IMP dehydrogenase: inhibition by the anti-leukemic drug, tiazofurin

Tiazofurin through its active metabolite thiazole-4-carboxamide adenine dinucleotide (TAD) inhibits IMP dehydrogenase, the rate-limiting enzyme of GTP biosynthesis. IMP dehydrogenase activity in human leukemic cell extracts (33.4 +/- 0.1 nmol/h/mg protein) was increased 11-fold compared to normal leukocytes (3.1 +/- 0.5). Km values for IMP and NAD+ of leukemic IMP dehydrogenase were 22.7 and 44.0 microM, respectively. XMP inhibited competitively with IMP and noncompetitively with NAD+. NADH exerted mixed type inhibition with respect to both IMP and NAD+. The inhibitory pattern of TAD was quite similar to that of NADH; however, the affinity of TAD to leukemic IMP dehydrogenase (Ki = 0.1 microM) was three orders of magnitude higher than the natural product NADH (Ki = 150 microM). These results contribute to an understanding of the mechanism of action of tiazofurin in the treatment of leukemia.

Action of the active metabolites of tiazofurin and ribavirin on purified IMP dehydrogenase

The inhibitory mechanisms of ribavirin 5'-monophosphate (RMP) and thiazole-4-carboxamide adenine dinucleotide (TAD), the active forms of the antimetabolites ribavirin and tiazofurin, were investigated in IMP dehydrogenase purified to homogeneity from rat hepatoma 3924A. The hepatoma IMP dehydrogenase has a tetrameric structure with a subunit molecular weight of 60,000. For the substrates IMP and NAD+, Km's were 23 and 65 microM, respectively. Product-inhibition patterns showed an ordered Bi-Bi mechanism for the enzyme reaction where IMP binds to the enzyme first, followed by NAD+; NADH dissociates from the ternary complex first and then XMP is released. XMP interacts with the free enzyme and competes for the ligand site with IMP, while NADH binds to the enzyme-XMP complex. RMP exerted the same inhibitory mechanisms as XMP, and the inhibition by TAD was similar to that by NADH. However, the Ki values for RMP (0.8 microM) and TAD (0.13 microM) were orders of magnitude lower than those of XMP (136 microM) and NADH (210 microM). Thus, the drugs interact with IMP dehydrogenase with higher affinities than the natural substrates and products, RMP with the IMP-XMP site and TAD with the NADH site. Preincubation of the purified enzyme with RMP enhanced its inhibitory effect in a time-dependent manner. The enzyme was protected from this inactivation by IMP or XMP. These results provide a biochemical basis for combination chemotherapy with tiazofurin and ribavirin targeted against the two different ligand sites of IMP dehydrogenase.

Purification, characterization, and kinetic analysis of inosine 5'-monophosphate dehydrogenase of Tritrichomonas foetus

The IMP dehydrogenase of Tritrichomonas foetus, a parasitic protozoan incapable of de novo biosynthesis of purine nucleotides, has been purified about 1000-fold to apparent homogeneity. The purified enzyme demonstrated a 20-fold higher substrate turnover rate than the pure IMP dehydrogenase from sarcoma ascites tumor cells. It has a subunit molecular weight of 58,000, aggregates to a size of 380,000 at low ionic strength, and partly dissociates to a molecular weight of 270,000 in high salt concentrations. Unlike the IMP dehydrogenase of bacteria and mammals, the T. foetus enzyme does not require K+ for activity. The analysis of initial velocity and product inhibition data is consistent with a sequential, ordered bi bi kinetic mechanism for the parasite enzyme-catalyzed reaction, in which IMP binds before NAD+ and NADH is released before XMP. This is in contrast to the partially random mechanism of the bacterial enzyme which involves the formation of an enzyme-K+-(IMP) complex. Mycophenolic acid inhibits T. foetus IMP dehydrogenase uncompetitively versus both IMP and NAD+ with an apparent Ki of 9 microM. This value, which is several hundred-fold higher than that for mammalian IMP dehydrogenase, suggests significantly different binding properties of the mycophenolic acid site in T. foetus IMP dehydrogenase, which might be amenable to specific inhibitor design.