Enzymes of uridine 5'-monophosphate biosynthesis in Schistosoma mansoni

In Schistosoma mansoni, the major product of in vitro orotate metabolism was orotidine 5'-monophosphate (OMP), whereas in mouse liver it was UMP. In contrast to mammalian cells, OMP appeared not to be 'channeled' from orotate phosphoribosyltransferase to OMP decarboxylase in S. mansoni, resulting in substantial degradation of OMP to orotidine. Significant differences were observed in the inhibitor specificity of phosphoribosyltransferase between S. mansoni and mouse liver, indicating that this enzyme may be a potential chemotherapeutic target in S. mansoni. Two distinct phosphoribosyltransferases were found in S. mansoni. One enzyme, having the higher molecular weight, utilized orotate, 5-fluorouracil and uracil as substrates, while the other only orotate. Both enzymes were inhibited by 5-azaorotic acid (oxonic acid) but only the 'orotate-specific' enzyme was inhibited by 4,6-dihydroxypyrimidine. OMP decarboxylase activity co-eluted with both phosphoribosyltransferases from Sephadex G-100 gel chromatography. We suggest that phosphoribosyltransferase in S. mansoni plays a role in both de novo UMP biosynthesis as well as in the salvage of uracil and uridine.     

Cytosolic protein kinase activity associated with the maturation of the malaria parasite Plasmodium berghei

Seven cytosolic phosphoproteins with relative molecular masses of 110, 58, 52, 46, 38, 36 and 34kDa and isoelectric points between 4.2 and 5.0 are identified from the rodent malaria parasite Plasmodium berghei. Similar patterns of phosphorylated proteins are obtained from parasite cytosol after incubation of intact infected erythrocytes with [32P]orthophosphate, or from parasite cytosol incubated with [gamma-32P]ATP. The characteristics of the phosphorylation reaction are similar to the previously described Plasmodium protein kinase [Wiser, M.F., Eaton, J.W. and Sheppard, J.R. (1983) J. Cell. Biochem. 21, 305-314], suggesting that the same protein kinase is involved. More protein phosphorylation activity is associated with the mature parasites than the immature forms, suggesting that these phosphoproteins may play some role in the parasite's erythrocytic stage cycle.     

Properties and substrate specificity of a purine phosphoribosyltransferase from the human malaria parasite, Plasmodium falciparum

The properties of a purine phosphoribosyltransferase from late trophozoites of the human malaria parasite, Plasmodium falciparum, are described. Enzyme activity with hypoxanthine, guanine and xanthine as substrates eluted in parallel during hydroxylapatite, size exclusion and DEAE-Sephadex chromatography as well as during chromatofocusing experiments. Furthermore, enzyme activity with all three purine substrates changed in parallel during heat inactivation of enzyme preparations and upon cold storage (4 degrees C) of the enzyme. When considered together, these results support the view that the phosphoribosyltransferase is capable of utilizing all three purine bases as substrates. Additional characterization revealed that the apparent molecular weight and isoelectric point of this enzyme are 55,500 and 6.2, respectively, and that the apparent Km for 5-phosphoribosyl-1-pyrophosphate ranges from 13.3 to 21.4 microM, depending on the purine base serving as substrate. The apparent Km values for hypoxanthine, guanine and xanthine were found to be 0.46, 0.30 and 29 microM, respectively. Other experiments showed that several divalent cations and sulfhydryl reagents produce a marked reduction of enzyme activity whereas dithiothreitol activates the enzyme. It should be noted that the ability to utilize xanthine as a substrate serves to distinguish the P. falciparum enzyme from its counterpart in the parasite's host cell, the human erythrocyte. The human enzyme shows only barely detectable activity with xanthine while the parasite enzyme displays similarly high levels of activity with all three purine substrates. Thus, the parasite enzyme might prove to be selectively susceptible to inhibition by xanthine analogs and related compounds.     

Oral administration of Nigella sativa oil ameliorates the effect of cisplatin on brush border membrane enzymes,carbohydrate metabolism and antioxidant system in rat intestine

Cisplatin (CP) is an effective chemotherapeutic agent that induces gastrointestinal toxicity. Nigella sativa oil (NSO) has been shown to be beneficial in a wide range of gastrointestinal disorders. The present study investigates the possible protective effect of NSO on CP-induced gastrointestinal toxicity. NSO administration (2 ml/kg bwt, orally), prior to and following, a single dose CP treatment (6 mg/kg bwt. ip), significantly attenuated the CP-induced decrease in brush border membrane (BBM) enzyme activities in intestinal homogenates and BBM vesicles (BBMV). NSO administration also mitigated CP induced alterations in the activities of carbohydrate metabolism enzymes and in the enzymatic and non-enzymatic antioxidant parameters in the intestine. The results suggest that NSO by empowering the endogenous antioxidant system improves intestinal redox and metabolic status and restores BBM integrity in CP treated rats. Histopathological studies supported the biochemical findings. Thus, NSO may help prevent the accompanying gastrointestinal dysfunction in CP chemotherapy.