Cost of illness of HER2-positive and metastatic and recurrent HER2-positive breast cancer - a Danish register-based study from 2005 to 2016.

BACKGROUND: Information and knowledge about cost of illness and labour productivity in patients with HER2-positive early-stage and metastatic breast cancer treated with trastuzumab is limited. The aim of this study was to estimate the direct and indirect costs associated with treatment of HER2-positive breast cancer among patients with early-stage and metastatic breast cancer, treated with trastuzumab, in a 10-year period after diagnosis. MATERIALS AND METHODS: This study included all Danish HER2-positive breast cancer patients (≥ 18 years) treated with trastuzumab between 2005 and 2016 identified in The Danish Patient Register and the Danish Cancer Register. Furthermore, we identified patients experiencing metastatic or recurrent breast cancer. For the study populations, we estimated total direct costs and indirect costs for one year prior to the breast cancer diagnosis and up to 10 years after diagnosis compared with a group of matched controls free of breast cancer. In addition to The Danish Patient Register and Cancer Register, we applied patient level data from The Civil Registration System, The National Pathology Register, National Health Service Register for Primary Care, Register of Medicinal Product Statistics, Register of Municipal Services, The DREAM database, and Population's Education Register. RESULTS: We identified 4,153 HER2-positive breast cancer patients, whereof 27% were identified with metastatic or recurrent breast cancer. During the follow-up period of 10 years, we estimated excess direct costs of EUR 115,000 among the total study population compared to controls; EUR 211,000 among patients with metastases or recurrence; and EUR 89,000 among patients without metastases or recurrence. Direct costs were found to be highest in the first year after diagnosis and also peaked in the year after recurrence. Labour productivity was significantly lower among patients with recurrence 10 years after breast cancer diagnosis compared with controls. CONCLUSIONS: In this study, we estimated the direct and indirect cost associated with HER2-positive breast cancer. The costs were significantly higher during the 10 years after diagnosis compared to the control group, specifically among patients experiencing metastases or recurrence of breast cancer.

Microfluidic jet injection for delivering macromolecules into cells.

We present a microfluidic based injection system designed to achieve intracellular delivery of macromolecules by directing a picoliter-jet of a solution towards individual cells. After discussing the concept, we present design specification and criteria, elucidate performance and discuss results. The method has the potential to be quantitative and high throughput, overcoming limitations of current intracellular delivery protocols.

Microchemical systems for discovery and development.

Applications of silicon-based microreactors are summarized starting with systems for single-phase organic transformations and progressing through multiphase catalytic systems to microsystems for multistep chemical synthesis. The latter systems involve extraction and gas-liquid separation processes designed to take advantage of the dominance of surface tension effects in microfluidic devices. Integration of physical sensors (e.g., for pressure, temperature, and flow) and measurements of chemical species further enhances the utility of microreactors by enabling chemical kinetic studies and optimization of optimal operating conditions. A brief description of synthesis and handling of solid particulates is included, with particular emphasis on multistep processing of colloidal nanoparticles. Finally, scale-up issues and challenges to the adoption of microreaction technology are discussed.

Specific inhibition of a family 1A dihydroorotate dehydrogenase by benzoate pyrimidine analogues.

Dihydroorotate dehydrogenases (DHODs) catalyze the conversion of dihydroorotate to orotate in de novo pyrimidine biosynthesis. We have found that 3,4-dihydroxybenzoate and 3,5-dihydroxybenzoate are competitive inhibitors vs dihydroorotate with the prototypical family 1A DHOD from Lactococcus lactis. The dissociation constants of these compounds, determined by spectral titrations, were similar to the dissociation constant of orotate, the enzymatic reaction product, suggesting that hydroxybenzoates could be developed into useful drugs for treating infections by certain protozoan parasites.

Selenomethionine substitution of orotidine-5'-monophosphate decarboxylase causes a change in crystal contacts and space group.

Orotidine 5'-monophosphate decarboxylase (ODCase) catalyses the decarboxylation of orotidine 5'-monophosphate to uridine 5'-monophosphate, the last step in the de novo biosynthesis of uridine 5'-monophosphate. In order to determine the structure of ODCase from Escherichia coli by the multi-wavelength anomalous dispersion technique, both native and SeMet-substituted proteins have been produced and purified. During the production of SeMet ODCase, it was observed that SeMet was the only amino acid that it was necessary to add to the defined medium during expression. SeMet-substituted ODCase in complex with the inhibitor 1-(5'-phospho-beta-D-ribofuranosyl)barbituric acid crystallizes under similar conditions as the native enzyme. In contrast to the native enzyme, where the crystals belong to the orthorhombic space group P2(1)2(1)2(1), the SeMet-substituted enzyme crystallizes in the monoclinic space group P2(1), with a quadrupling of the volume of the asymmetric unit. Despite the drastic difference in symmetry, the overall crystal packing is effectively identical in the two crystal forms. The change in space group appears to originate in differences in the crystal contacts near the SeMet and Met residues. These differences can be rationalized in terms of SeMet's larger size and hydrophobicity.

Dihydrooxonate is a substrate of dihydroorotate dehydrogenase (DHOD) providing evidence for involvement of cysteine and serine residues in base catalysis.

The flavoprotein dihydroorotate dehydrogenase (DHOD) catalyzes the oxidation of dihydroorotate to orotate. Dihydrooxonate is an analogue of dihydroorotate in which the C5 carbon is substituted by a nitrogen atom. We have investigated dihydrooxonate as a substrate of three DHODs, each representing a distinct evolutionary class of the enzyme, namely the two family 1 enzymes from Lactococcus lactis, DHODA and DHODB, and the enzyme from Escherichia coli, which, like the human enzyme, belongs to family 2. Dihydrooxonate was accepted as a substrate although much less efficiently than dihydroorotate. The first half-reaction was rate limiting according to pre-steady-state and steady-state kinetics with different electron acceptors. Cysteine and serine have been implicated as active site base residues, which promote substrate oxidation in family 1 and family 2 DHODs, respectively. Mutants of DHODA (C130A) and E. coli DHOD (S175A) have extremely low activity in standard assays with dihydroorotate as substrate, but with dihydrooxonate the mutants display considerable and increasing activity above pH 8.0. Thus, the absence of the active site base residue in the enzymes seems to be compensated for by a lower pK(a) of the 5-position in the substrate. Oxonate, the oxidation product of dihydrooxonate, was a competitive inhibitor versus dihydroorotate, and DHODA was the most sensitive of the three enzymes. DHODA was reinvestigated with respect to product inhibition by orotate. The results suggest a classical one-site ping-pong mechanism with fumarate as electron acceptor, while the kinetics with ferricyanide is highly dependent on the detailed reaction conditions.

Insight into the chemistry of flavin reduction and oxidation in Escherichia coli dihydroorotate dehydrogenase obtained by rapid reaction studies.

Dihydroorotate dehydrogenase (DHOD) oxidizes dihydroorotate (DHO) to orotate in the only redox reaction of pyrimidine biosynthesis. The enzyme from Escherichia coli is a membrane-bound FMN-containing enzyme that is thought to use ubiquinone as the oxidizing substrate. The chemistry of the reduction of the flavin in DHOD from E. coli by the substrate dihydroorotate (DHO) was studied at 4 degrees C in anaerobic stopped-flow experiments conducted over a broad range of pH values. A Michaelis complex that was characterized by a approximately 20 nm red-shift of the oxidized flavin absorbance formed within the dead-time of the stopped-flow instrument ( approximately 1 ms) upon mixing with DHO. The flavin of the intermediate was reduced by DHO, forming a reduced flavin-orotate charge-transfer complex. The rate constant for the flavin reduction reaction increased with pH, from a value of 1 s(-1) at pH 6.5 to approximately 360 s(-1) at pH values greater than an observed pK(a) of 9.5 which was ascribed to Ser175, the active-site base. At all pH values, the reduced flavin-orotate charge-transfer complex dissociated too slowly to be catalytically relevant. Therefore, the oxidizing quinone substrate must bind to the reduced enzyme-orotate complex at a site distinct from the substrate binding site, in agreement with steady-state kinetic studies [Björnberg, O., Grüner, A.-C., Roepstorff, P., and Jensen, K. F. (1999) Biochemistry 38, 2899-2908]. Menadione was used as a model quinone substrate to oxidize dithionite-reduced DHOD. The reduced enzyme-orotate complex reacted rapidly with menadione (180 s(-1)), demonstrating that the reduced enzyme-orotate complex is a catalytically competent intermediate.

Methods to identify and characterize developmental neurotoxicity for human health risk assessment. II: neuropathology.

Neuropathologic assessment of chemically induced developmental alterations in the nervous system for regulatory purposes is a multifactorial, complex process. This calls for careful qualitative and quantitative morphologic study of numerous brains at several developmental stages in rats. Quantitative evaluation may include such basic methods as determination of brain weight and dimensions as well as the progressively more complex approaches of linear, areal, or stereologic measurement of brain sections. Histologic evaluation employs routine stains (such as hematoxylin and eosin), which can be complemented by a variety of special and immunohistochemical procedures. These brain studies are augmented by morphologic assessment of selected peripheral nervous system structures. Studies of this nature require a high level of technical skill as well as special training on the part of the pathologist. The pathologist should have knowledge of normal microscopic neuroanatomy/neuronal circuitry and an understanding of basic principles of developmental neurobiology, such as familiarity with the patterns of physiologic or programmed cell de

Photochemical reactions and on-line UV detection in microfabricated reactors.

This work presents an application of microfabricated reactors and detectors for photochemical reactions. Two fabrication schemes were demonstrated for the integration of the reaction and the detection modules: coupling individually packaged chips, and monolithic integration of the two functions. In the latter fabrication scheme, we have succeeded in bonding quartz wafers to patterned silicon wafers at low temperature using a Teflon-like polymer-CYTOP[trade mark sign]. Using quartz substrates allows reaction and detection with UV light of lower wavelengths than Pyrex substrates permit. The pinacol formation reaction of benzophenone in isopropanol was the model reaction to demonstrate the performance of the microreactors. The extent of reaction was controlled by varying the flow rate and therefore the on-chip residence time. Crystallization of the product inside the microreactors was avoided by the continuous-flow design. Instead, crystallization was observed in the effluent storage device. Off-chip analysis using HPLC confirms the results obtained from the on-line UV spectroscopy. The quantum yield estimated suggests that the reactor design is effective in improving the overall efficiency of the reactor unit.

Altered expression of neurofilament subunits in diisopropyl phosphorofluoridate-treated hen spinal cord and their presence in axonal aggregations.

Diisopropyl phosphorofluoridate (DFP) is an organophosphorus ester, which produces organophosphorus ester-induced delayed neuropathy (OPIDN) in hen and other sensitive species. A single dose of DFP (1.7 mg/kg, sc.) produces mild ataxia in 7-14 days in hens, which develops into severe ataxia or paralysis with the progression of disease. OPIDN is associated with axonal swellings and degeneration of axons. This study was carried out to investigate the expression of neurofilament (NF) subunits in the spinal cord of DFP-treated hens. Hens were treated with a single dose of DFP and sacrificed 1, 5, 10, and 20 days post-treatment. Western blot analysis showed increased expression of middle molecular weight neurofilament protein (NF-M), and decreased expression of high molecular weight (NF-H) and low molecular weight (NF-L) neurofilament proteins in the 2 M urea extracts of spinal cord particulate fraction. These changes were observed within 24 h of DFP administration and persisted for 10-20 days. Thus, there was increase in the stoichiometry of NF-M:NF-L in the spinal cord of DFP-treated hens. Immunoprecipitation, cross-linking, and two-dimensional polyacrylamide gel electrophoresis showed the presence of heterodimers, but not heterotetramers, in the hen spinal cord extract. Immunohistochemical staining revealed the presence of all three NF subunits in the cytoskeletal inclusions in DFP-treated hen spinal cord cross-sections. The results suggested that each NF subunit might be accumulated by a different mechanism in the axonal aggregations of DFP-treated hen.

Structural basis for the catalytic mechanism of a proficient enzyme: orotidine 5'-monophosphate decarboxylase.

Orotidine 5'-monophosphate decarboxylase (ODCase) catalyzes the decarboxylation of orotidine 5'-monophosphate, the last step in the de novo synthesis of uridine 5'-monophosphate. ODCase is a very proficient enzyme [Radzicka, A., and Wolfenden, R. (1995) Science 267, 90-93], enhancing the reaction rate by a factor of 10(17). This proficiency has been enigmatic, since it is achieved without metal ions or cofactors. Here we present a 2.5 A resolution structure of ODCase complexed with the inhibitor 1-(5'-phospho-beta-D-ribofuranosyl)barbituric acid. It shows a closely packed dimer composed of two alpha/beta-barrels with two shared active sites. The orientation of the orotate moiety of the substrate is unambiguously deduced from the structure, and previously proposed catalytic mechanisms involving protonation of O2 or O4 can be ruled out. The proximity of the OMP carboxylate group with Asp71 appears to be instrumental for the decarboxylation of OMP, either through charge repulsion or through the formation of a very short O.H.O hydrogen bond between the two carboxylate groups.

Crystallization and preliminary X-ray studies of membrane-associated Escherichia coli dihydroorotate dehydrogenase.

Dihydroorotate dehydrogenases (DHODs) are flavin-containing enzymes which catalyse the conversion of (S)-dihydroorotate to orotate, the fourth step in the de novo biosynthesis of pyrimidine nucleotides. Two major families of DHODs have now been identified based on their amino-acid sequence similarities. The two families differ in their reaction mechanisms, but structures are only known of enzymes belonging to family 1. DHOD from Escherichia coli is a typical member of family 2, which contains the membrane-associated enzymes from Gram-negative bacteria and eukaryotes. Yellow crystals grown of this enzyme belong to the space group P4(1)2(1)2 or P4(3)2(1)2. The unit-cell parameters are a = b = 119.2, c = 294.3 A. Owing to the rather large c axis, the currently available resolution of data is 2.2 A.

Neurotoxicity of ethyl methacrylate in rats.

Ethyl methacrylate (ethyl 2-methyl-2-propenoate, EMA) has been implicated in the development of neurologic impairment following occupational exposure. The potential of EMA to produce neurotoxicity was investigated in adult male Sprague-Dawley rats in two experiments. In the first experiment, animals were administered 100, 200, 400, or 800 mg/kg by daily intraperitoneal (i.p.) injections for 60 d. Control rats received daily i.p. injections of 1 ml saline/kg. Clinical observations, spontaneous motor activity, and performance in the Morris water maze were assessed. Alterations in clinical parameters in the higher dose groups included lethargy, impaired breathing, decreased weight gain, and increased mortality. Alterations in motor activity were observed at 100 mg/kg, a dose that did not cause alterations in clinical parameters, body weight gain, or mortality. There was also a dose-dependent impairment in performance in the Morris water maze. In the second experiment, animals were administered EMA in drinking water at concentrations of 0.1, 0.2, or 0.5% for 60 d. Control rats were administered tap water. Animals were perfused at the termination of exposure and samples of brain, spinal cord, and sciatic nerve were prepared for histological examination. Spongiform alterations were observed in fiber tracts of the forebrain, brainstem, and spinal cord. Clusters of axonal swellings were scattered throughout the dorsal, ventral, and lateral columns of the spinal cord, and typically involved internodal segments of two or three neighboring axons. Shrunken axons with separated myelin lamellae and large axons with thinner than normal myelin sheaths were apparent in the sciatic nerve. The patterns of alterations in the white matter of the spinal cord and the sciatic nerve are consistent with myelinopathy, but additional experiments are necessary to confirm whether oligodendroglia and Schwann cells are the primary sites of injury. In addition to the alterations associated with myelin, there was a decrease in the density of neurons in the ventral horn of the spinal cord. While the observed effects of EMA on the nervous system of rats are consistent with neurologic symptoms of workers exposed to EMA, additional experiments are necessary to determine if the level and route of exposures associated with occupational use produce these impairments in experimental animals.

Structure of dihydroorotate dehydrogenase B: electron transfer between two flavin groups bridged by an iron-sulphur cluster.

BACKGROUND: The fourth step and only redox reaction in pyrimidine de novo biosynthesis is catalyzed by the flavoprotein dihydroorotate dehydrogenase (DHOD). Based on their sequences, DHODs are grouped into two major families. Lactococcus lactis is one of the few organisms with two DHODs, A and B, belonging to each of the two subgroups of family 1. The B enzyme (DHODB) is a prototype for DHODs in Gram-positive bacteria that use NAD+ as the second substrate. DHODB is a heterotetramer composed of two different proteins (PyrDB and PyrK) and three different cofactors: FMN, FAD, and a [2Fe-2S] cluster. RESULTS: Crystal structures have been determined for DHODB and its product complex. The DHODB heterotetramer is composed of two closely interacting PyrDB-PyrK dimers with the [2Fe-2S] cluster in their interface centered between the FMN and FAD groups. Conformational changes are observed between the complexed and uncomplexed state of the enzyme for the loop carrying the catalytic cysteine residue and one of the lysines interacting with FMN, which is important for substrate binding. CONCLUSIONS: A dimer of two PyrDB subunits resembling the family 1A enzymes forms the central core of DHODB. PyrK belongs to the NADPH ferredoxin reductase superfamily. The binding site for NAD+ has been deduced from the similarity to these proteins. The orotate binding in DHODB is similar to that in the family 1A enzymes. The close proximity of the three redox centers makes it possible to propose a possible electron transfer pathway involving residues conserved among the family 1B DHODs.

Kinetic mechanism of uracil phosphoribosyltransferase from Escherichia coli and catalytic importance of the conserved proline in the PRPP binding site.

Phosphoribosyltransferases catalyze the formation of nucleotides from a nitrogenous base and 5-phosphoribosyl-alpha-1-pyrophosphate (PRPP). These enzymes and the PRPP synthases resemble each other in a short homologous sequence of 13 amino acid residues which has been termed the PRPP binding site and which interacts with the ribose 5-phosphate moiety in structurally characterized complexes of PRPP and nucleotides. We show that each class of phosphoribosyltransferases has subtle deviations from the general consensus PRPP binding site and that all uracil phosphoribosyltransferases (UPRTases) have a proline residue at a position where other phosphoribosyltransferases and the PRPP synthases have aspartate. To investigate the role of this unusual proline (Pro 131 in the E. coli UPRTase) for enzyme activity, we changed the residue to an aspartate and purified the mutant P131D enzyme to compare its catalytic properties with the properties of the wild-type protein. We found that UPRTase of E. coli obeyed the kinetics of a sequential mechanism with the binding of PRPP preceding the binding of uracil. The basic kinetic constants were derived from initial velocity measurements, product inhibition, and ligand binding assays. The change of Pro 131 to Asp caused a 50-60-fold reduction of the catalytic rate (kcat) in both directions of the reaction and approximately a 100-fold increase in the KM for uracil. The KM for PRPP was strongly diminished by the mutation, but kcat/KM,PRPP and the dissociation constant (KD,PRPP) were nearly unaffected. We conclude that the proline in the PRPP binding site of UPRTase is of only little importance for binding of PRPP to the free enzyme, but is critical for binding of uracil to the enzyme-PRPP complex and for the catalytic rate.

The activity of Escherichia coli dihydroorotate dehydrogenase is dependent on a conserved loop identified by sequence homology, mutagenesis, and limited proteolysis.

Dihydroorotate dehydrogenase catalyzes the oxidation of dihydroorotate to orotate. The enzyme from Escherichia coli was overproduced and characterized in comparison with the dimeric Lactococcus lactis A enzyme, whose structure is known. The two enzymes represent two distinct evolutionary families of dihydroorotate dehydrogenases, but sedimentation in sucrose gradients suggests a dimeric structure also of the E. coli enzyme. Product inhibition showed that the E. coli enzyme, in contrast to the L. lactis enzyme, has separate binding sites for dihydroorotate and the electron acceptor. Trypsin readily cleaved the E. coli enzyme into two fragments of 182 and 154 residues, respectively. Cleavage reduced the activity more than 100-fold but left other molecular properties, including the heat stability, intact. The trypsin cleavage site, at R182, is positioned in a conserved region that, in the L. lactis enzyme, forms a loop where a cysteine residue is very critical for activity. In the corresponding position, the enzyme from E. coli has a serine residue. Mutagenesis of this residue (S175) to alanine or cysteine reduced the activities 10000- and 500-fold, respectively. The S175C mutant was also defective with respect to substrate and product binding. Structural and mechanistic differences between the two different families of dihydroorotate dehydrogenase are discussed.

Kinetic mechanism of OMP synthase: a slow physical step following group transfer limits catalytic rate.

Orotate phosphoribosyltransferase (OMP synthase, EC 2.4.2.10) forms the UMP precursor orotidine 5'-monophophate (OMP) from orotate and alpha-D-5-phosphoribosyl-1-pyrophosphate (PRPP). Here, equilibrium binding, isotope partitioning, and chemical quench studies were used to determine rate and equilibrium constants for the kinetic mechanism. PRPP bound to two sites per dimer with a KD of 33 microM. Binding of OMP and orotate also occurred to a single class of two sites per dimer, with KD values of 3 and 280 microM, respectively. Pyrophosphate binding to two sites was weak with a KD of 960 microM, and in the presence of bound orotate, its affinity for the first site was enhanced 4-fold (KD = 230 microM). Preformed E.OMP, E.PRPP, E.PPi, and E.orotate complexes were trapped as products in isotope partitioning experiments, indicating that each was catalytically competent and confirming a random mechanism. Rapid quench experiments revealed burst kinetics for product formation in both the forward phosphoribosyltransferase and the reverse pyrophosphorolysis reactions. The steady-state rate in the forward reaction was preceded by a burst (nfwd = 1.5/dimer) of at least 300 s-1. In the pyrophosphorolysis reaction, a burst (nrev = 0.7/dimer; k >/= 300 s-1) was also noted. These results allowed us to develop a complete kinetic mechanism for OPRTase, in which a rapid phosphoribosyl transfer reaction at equilibrium is followed by a slow step involving release of product. When the microviscosity, etarel, of the reaction medium was increased with sucrose, the forward kcat decreased in proportion to etarel with a slope of 0.8. In the reverse reaction a more limited dependence of kcat (slope = 0. 3) was observed. On the basis of the known structures of OPRTase, we propose that a highly conserved, catalytically important, solvent-exposed loop descends during catalysis to shield the active site. In the accompanying paper, the slow product release step is shown to relate to movement of the solvent-exposed loop.

Selective silver staining for neural degeneration.

Silver degeneration stains are typically considered the domain of studies of neural circuitry, but have also proven useful in studies of neurotoxicity. They are particularly well suited to localizing sites of injury, and determining the extent and time-course of degeneration.

Immunohistological markers of cerebrovascular integrity.

The brain depends on other organ systems of the body for oxygen, nutrients, and the elimination of metabolic byproducts. The primary route for such transfer of these essentials is the cerebrovasculature. The cerebrovasculature also participates in metabolizing or excluding xenobiotics, segregating components of the immune response, regulating pH and osmolarity of the cerebrospinal fluid, selectively distributing hormones, and impeding pathogenic invasion. Various aspects of these diverse functions are attributed to the complex structural and molecular properties of cerebral endothelial cells collectively referred to as the blood-brain barrier (1-7). The hallmark structural specialization of the blood-brain barrier is the tight junction between the endothelial cells, which prevents diffusion of plasma proteins and molecules of a similar size or larger (8-12). Other structural specializations include close apposition of astrocytic endfeet, sparsely distributed pericytes, and extensive association with microglia. Molecular specializations include endothelial expression of transporters and enzymes, such as those involved in xenobiotic metabolism (12-27).

The crystal structure of Lactococcus lactis dihydroorotate dehydrogenase A complexed with the enzyme reaction product throws light on its enzymatic function.

Dihydroorotate dehydrogenases (DHODs) catalyze the oxidation of (S)-dihydroorotate to orotate, the fourth step and only redox reaction in the de novo biosynthesis of pyrimidine nucleotides. A description is given of the crystal structure of Lactococcus lactis dihydroorotate dehydrogenase A (DHODA) complexed with the product of the enzyme reaction orotate. The structure of the complex to 2.0 A resolution has been compared with the structure of the native enzyme. The active site of DHODA is known to contain a water filled cavity buried beneath a highly conserved and flexible loop. In the complex the orotate displaces the water molecules from the active site and stacks above the DHODA flavin isoalloxazine ring, causing only small movements of the surrounding protein residues. The orotate is completely buried beneath the protein surface, and the orotate binding causes a significant reduction in the mobility of the active site loop. The orotate is bound by four conserved asparagine side chains (Asn 67, Asn 127, Asn 132, and Asn 193), the side chains of Lys 43 and Ser 194, and the main chain NH groups of Met 69, Gly 70, and Leu 71. Of these the Lys 43 side chain makes hydrogen bonds to both the flavin isoalloxazine ring and the carboxylate group of the orotate. Potential interactions with bound dihydroorotate are considered using the orotate complex as a basis for molecular modeling. The role of Cys 130 as the active site base is discussed, and the sequence conservation of the active site residues across the different families of DHODs is reviewed, along with implications for differences in substrate binding and in the catalytic mechanisms between these families.