Biosynthetic dihydroorotate dehydrogenase from Lactobacillus bulgaricus: partial characterization of the enzyme

Two different dihydroorotate dehydrogenases in Lactococcus lactis

The pyrimidine de novo biosynthesis pathway has been characterized for a number of organisms. The general pathway consists of six enzymatic steps. In the characterization of the pyrimidine pathway of Lactococcus lactis, two different pyrD genes encoding dihydroorotate dehydrogenase were isolated. The nucleotide sequences of the two genes, pyrDa and pyrDb, have been determined. One of the deduced amino acid sequences has a high degree of homology to the Saccharomyces cerevisiae dihydroorotate dehydrogenase, and the other resembles the dihydroorotate dehydrogenase from Bacillus subtilis. It is possible to distinguish between the two enzymes in crude extracts by using different electron acceptors. We constructed mutants containing a mutated form of either one or the other or both of the pyrD genes. Only the double mutant is pyrimidine auxotrophic.

Pyrimidine metabolism in Giardia lamblia trophozoites

The pyrimidine metabolism of Giardia lamblia trophozoites (Portland I strain) was studied using whole trophozoites and trophozoite homogenates. Pyrimidines and pyrimidine nucleosides were readily incorporated into nucleic acids. Orotic and aspartic acid incorporations were below the level of detection. Enzymes of the pyrimidine salvage pathway (i.e., thymidine and uridine phosphorylases and thymidine and uridine kinases) were detected in trophozoite homogenates, but the activities of de novo pyrimidine synthesis enzymes (i.e., carbamoyl-phosphate synthase, aspartate transcarbamoylase, dihydroorotase and dihydroorotate dehydrogenase) were below the level of detection in these same homogenates. The evidence presented supports the conclusion that G. lamblia trophozoites appear incapable of synthesizing pyrimidines de novo but are capable of salvaging preformed pyrimidines and pyrimidine nucleosides from the growth medium and the enzymes of this pyrimidine salvage pathway are not organelle associated.

Purification of properties of dihydroorotase, a zinc-containing metalloenzyme in Clostridium oroticum

Dihydroorotase +4,5-L-dihydro-orotate amidohydrolase [EC 3.5.2.3]), which catalyzes the reversible cyclization of N-carbamyl-L-aspartate to L-dihydroorotate, has been purified from orotate-grown Clostridium oroticum. The enzyme is homogeneous when subjected to polyacrylamide gel electrophoresis and is stable at pH 7.6 in 0.3 M NaCl containing 10 muM ZnSO4. The enzyme has a molecular weight of approximately 110,000. Sodium dodecyl sulfate gel electrophoresis, using three different buffer systems, indicated the enzyme is composed of two subunits, each having a molecular weight of 55,000. Dihydroorotase is shown by atomic absorption spectroscopy to be a zinc-containing metalloenzyme with 4 g-atoms of zinc per 110,000 g of protein. The pH optima for the conversion of N-carbamyl-L-aspartate to L-dihydroorotate and for L-dihydroorotate to N-carbamyl-L-aspartate are pH 6.0 and 8.2, respectively. The Km values for N-carbamyl-L-aspartate and for L-dihydroorotate are 0.13 and 0.07 mM, respectively. Inhibitor studies indicate that zinc may be involved in the catalytic activity of the enzyme.