CTP synthetase and enzymes of pyrimidine ribonucleotide metabolism in Giardia intestinalis.

The presence of the enzyme, CTP synthetase was detected in G. intestinalis and appears to be the major route by which the parasite obtains its cytidine nucleotides, though a low phosphoribosyltransferase activity which directly converted cytosine to CMP, was also detected. The giardial CTP synthetase was substantially purified and appeared to be a dimer of molecular weight of approximately 260,000. The enzyme was activated by the purine ribonucleotide, GTP, as was uracil phosphoribosyltransferase (UPRTase), an earlier enzyme in the pyrimidine ribonucleotide pathway. Detection of other enzyme activities in the present study together with results from previous studies has allowed the delineation of the pathway by which G. intestinalis synthesizes its major pyrimidine ribonucleotides.