The effect of pH on the kinetics of arylsulphatases A and B

Kinetic consequences of the endogenous ligand to molybdenum in the DMSO reductase family: a case study with periplasmic nitrate reductase

The molybdopterin enzyme family catalyzes a variety of substrates and plays a critical role in the cycling of carbon, nitrogen, arsenic, and selenium. The dimethyl sulfoxide reductase (DMSOR) subfamily is the most diverse family of molybdopterin enzymes and the members of this family catalyze a myriad of reactions that are important in microbial life processes. Enzymes in the DMSOR family can transform multiple substrates; however, quantitative information about the substrate preference is sparse, and, more importantly, the reasons for the substrate selectivity are not clear. Molybdenum coordination has long been proposed to impact the catalytic activity of the enzyme. Specifically, the molybdenum-coordinating residue may tune substrate preference. As such, molybdopterin enzyme periplasmic nitrate reductase (Nap) is utilized as a vehicle to understand the substrate preference and delineate the kinetic underpinning of the differences imposed by exchanging the molybdenum ligands. To this end, NapA from Campylobacter jejuni has been heterologously overexpressed, and a series of variants, where the molybdenum coordinating cysteine has been replaced with another amino acid, has been produced. The kinetic properties of these variants are discussed and compared with those of the native enzyme, providing quantitative information to understand the function of the molybdenum-coordinating residue.

The effect of pH on the initial rate kinetics of the dimeric biliverdin-IXalpha reductase from the cyanobacterium Synechocystis PCC6803

Biliverdin-IXalpha reductase from Synechocystis PCC6803 (sBVR-A) is a stable dimer and this behaviour is observed under a range of conditions. This is in contrast to all other forms of BVR-A, which have been reported to behave as monomers, and places sBVR-A in the dihydrodiol dehydrogenase/N-terminally truncated glucose-fructose oxidoreductase structural family of dimers. The cyanobacterial enzyme obeys an ordered steady-state kinetic mechanism at pH 5, with NADPH being the first to bind and NADP(+) the last to dissociate. An analysis of the effect of pH on k(cat) with NADPH as cofactor reveals a pK of 5.4 that must be protonated for effective catalysis. Analysis of the effect of pH on k(cat)/K(m)(NADPH) identifies pK values of 5.1 and 6.1 in the free enzyme. Similar pK values are identified for biliverdin binding to the enzyme-NADPH complex. The lower pK values in the free enzyme (pK 5.1) and enzyme-NADPH complex (pK 4.9) are not evident when NADH is the cofactor, suggesting that this ionizable group may interact with the 2'-phosphate of NADPH. His84 is implicated as a crucial residue for sBVR-A activity because the H84A mutant has less than 1% of the activity of the wild-type and exhibits small but significant changes in the protein CD spectrum. Binding of biliverdin to sBVR-A is conveniently monitored by following the induced CD spectrum for biliverdin. Binding of biliverdin to wild-type sBVR-A induces a P-type spectrum. The H84A mutant shows evidence for weak binding of biliverdin and appears to bind a variant of the P-configuration. Intriguingly, the Y102A mutant, which is catalytically active, binds biliverdin in the M-configuration.

Microtiter plate cellular assay for human steroid sulfatase with fluorescence readout

Steroid sulfatase (STS; E.C. 3.1.6.2) is an enzyme involved in the local production of estrogens and androgens in target organs. Inhibitors of steroid sulfatase activity are considered novel therapeutic agents for the treatment of different pathologic conditions, including cancers of breast, endometrium, and prostate and disorders of the pilosebaceous unit. Evaluation of steroid sulfatase inhibition in cells up to now has been a cumbersome process, involving the extraction of a radioactive cleavage product into organic solvents. Here, we describe a rapid, nonradioactive cellular assay in microtiter plate format, using 4-methylumbelliferyl sulfate as a substrate. The reaction product, 4-methylumbelliferone, is read in a fluorescence microtiter plate reader. Several cell lines were assayed for sulfatase activity. To increase the sensitivity of the assay, we developed a Chinese hamster ovary (CHO) cell line stably transfected with a cDNA encoding the human steroid sulfatase. The steroid sulfatase activity in transfected cells correlated with the presence of the enzyme in these cells, as determined by immunofluorescence. For most STS inhibitors tested, including estrone-3-O-sulfamate, the results from the CHO cellular assay were in good agreement with those from a standard cell-free assay.

Structure of a human lysosomal sulfatase

BACKGROUND

. Sulfatases catalyze the hydrolysis of sulfuric acid esters from a wide variety of substrates including glycosaminoglycans, glycolipids and steroids. There is sufficient common sequence similarity within the class of sulfatase enzymes to indicate that they have a common structure. Deficiencies of specific lysosomal sulfatases that are involved in the degradation of glycosamino-glycans lead to rare inherited clinical disorders termed mucopolysaccharidoses. In sufferers of multiple sulfatase deficiency, all sulfatases are inactive because an essential post-translational modification of a specific active-site cysteine residue to oxo-alanine does not occur. Studies of this disorder have contributed to location and characterization of the sulfatase active site. To understand the catalytic mechanism of sulfatases, and ultimately the determinants of their substrate specificities, we have determined the structure of N-acetylgalactosamine-4-sulfatase.

RESULTS

. The crystal structure of the enzyme has been solved and refined at 2.5 resolution using data recorded at both 123K and 273K. The structure has two domains, the larger of which belongs to the alpha/beta class of proteins and contains the active site. The enzyme active site in the crystals contains several hitherto undescribed features. The active-site cysteine residue, Cys91, is found as the sulfate derivative of the aldehyde species, oxo-alanine. The sulfate is bound to a previously undetected metal ion, which we have identified as calcium. The structure of a vanadate-inhibited form of the enzyme has also been solved, and this structure shows that vanadate has replaced sulfate in the active site and that the vanadate is covalently linked to the protein. Preliminary data is presented for crystals soaked in the monosaccharide N-acetylgalactosamine, the structure of which forms a product complex of the enzyme.

CONCLUSIONS

. The structure of N-acetylgalactosamine-4-sulfatase reveals that residues conserved amongst the sulfatase family are involved in stabilizing the calcium ion and the sulfate ester in the active site. This suggests an archetypal fold for the family of sulfatases. A catalytic role is proposed for the post-translationally modified highly conserved cysteine residue. Despite a lack of any previously detectable sequence similarity to any protein of known structure, the large sulfatase domain that contains the active site closely resembles that of alkaline phosphatase: the calcium ion in sulfatase superposes on one of the zinc ions in alkaline phosphatase and the sulfate ester of Cys91 superposes on the phosphate ion found in the active site of alkaline phosphatase.

Increase in extracellular NCAM and amyloid precursor protein following induction of long-term potentiation in the dentate gyrus of anaesthetized rats

The extracellular concentrations of the amyloid precursor protein (APP) and neural-cell adhesion molecule (NCAM) in the dentate gyrus of the anaesthetized rat were assayed before and after the induction of long-term potentiation (LTP) in vivo. Levels of high molecular weight neurofilament protein and activity of the lysosomal enzyme arylsulphatase were measured as internal controls and indicators of neuronal damage. Ninety minutes after the induction of LTP, the concentrations of NCAM and APP increased, in an NMDA-dependent manner, in the absence of changes in neurofilament and arylsulphatase levels. The delayed changes in the extracellular concentration of these molecules may reflect events leading to morphological modifications following LTP.

Aluminofluorides and beryllofluorides as inhibitors of sulphatases. Analogues of hydrogen sulphate?

The inhibition of fluoride of sulphatase A from ox liver and of the sulphatases of Helix pomatia and Aspergillus oryzae is decreased by EDTA and increased by Al3+ or Be2+, implicating aluminofluorides and beryllofluorides in the reaction. The inhibition, which is reversible, takes several minutes to develop fully and, at least for the sulphatase of H. pomatia, is of a non-linear mixed competitive-non-competitive type. It is suggested that the aluminofluorides and beryllofluorides are acting as analogues of HSO4-. If so, then this behaviour must be considered, as well as their role as analogues of phosphate, in interpreting the effects of these compounds in intact cells.

The subcellular location in rat kidney of the peripheral benzodiazepine acceptor

In rat kidney high-affinity binding sites for [3H]Ro-5-4864 and [3H]PK-11195 with the properties of the peripheral-type acceptor were found enriched in mitochondrial (M) and light-mitochondrial-lysosomal (L) fractions on differential centrifugation. When the combined M and L fractions were subjected to sucrose density gradient centrifugation, these binding sites were found enriched at a density of 1.155 g/ml coincident with a population of light mitochondria, whereas a population of heavier mitochondria (rho = 1.175 g/ml) had few or no binding sites. Transmission electron microscopy showed that whereas the heavier mitochondria appeared highly pure and intact, the lighter mitochondria appeared less intact and to be contaminated with vesicular structures. After fractionation of the light mitochondria and vesicles by centrifugation, both fractions showed the same ratio of [3H]Ro-4864 binding sites to monoamine oxidase activity consistent with the vesicles being of mitochondrial outer-membrane origin. Digitonin pre-treatment had no effect on the density of acceptor-rich fractions on sucrose density gradient centrifugation. However, pretreatment with succinate/iodophenylnitrophenylphenyltetrazolium (INT) perturbed equally the density of acceptor-rich fractions and mitochondrial marker enzymes. When mitochondrial fractions were subjected to sonication prior to density gradient centrifugation the binding sites were now found highly enriched in a much lighter fraction coincident with the monoamine oxidase activity and thus consistent with being outer-membrane vesicles. When a mitochondrial fraction was subjected to hypotonic treatment before assay no evidence for activation/unmasking of binding sites was found. The hypotonic treatment did not release any inhibitor of the binding sites. These results are consistent with the peripheral benzodiazepine acceptor having an outer-membrane location on a sub-population of rat kidney mitochondria. Those mitochondria showing high levels of the acceptor are either light mitochondria or appear more susceptible to osmotic damage than those mitochondria in which the acceptor is absent or at low levels.

Postnatal D2-CAM/N-CAM sialylation state is controlled by a developmentally regulated Golgi sialyltransferase

Golgi-enriched fractions have been isolated from rat brain of increasing postnatal age and defined by electron microscopy and distribution of marker enzymes. The expression of sialyltransferase activity associated with these fractions has been demonstrated to developmentally decrease and this appeared to be, in part, dependent on endogenous competitive inhibition. The developmental regulation of this activity paralleled the sialylation state of the neural cell adhesion molecule (D2-CAM/N-CAM) and could be demonstrated to be capable of endogenously sialylating this protein in the isolated Golgi fractions. In 12-day-old animals the majority of the transferred [14C]sialic acid was found to be associated with the high-molecular-weight [greater than 200 kilodaltons (kd)] form of D2-CAM/N-CAM, indicative of the protein having been heavily sialylated. Sialylation of the individual D2-CAM/N-CAM polypeptides was also demonstrated in both 12-day and adult animals and transfer was evident only in the 180-kd and 115-kd components and not in the 140-kd component. In contrast, Golgi-enriched fractions prepared from adult animals showed little capability of heavily sialylating D2-CAM/N-CAM to any significant extent.

Synaptosomal sialyltransferase glycosylates surface proteins that are inaccessible to the action of membrane-bound sialidase

Sialyltransferase has been characterized in P2 pellets derived from animals of increasing age. The enzyme was found to be associated with the plasma membrane and to be developmentally regulated at times coincident with cell migration and fibre outgrowth. This regulation appeared to be due, in part, to an endogenous competitive inhibitor in the P2 pellet but not in the synaptosome. Optimal transfer of [14C]N-acetylneuraminic acid to endogenous synaptosomal acceptors was achieved only in the absence of detergent. Furthermore, the transferred sialic acid was found to be inaccessible to the action of membrane-bound sialidase. The significance of these findings is discussed.

Characterization and cellular localization of a developmentally regulated rat neural sialidase

A developmentally regulated neural sialidase has been identified in particulate, subcellular fractions of rat brain. Enzyme activity, measured using a [3H]sialoganglioside substrate, was linear with time and had a pH optimum of 4.0-4.5. Protein linearity was only observed at low protein concentrations. This appeared to be caused by enzyme access to a lipophilic substrate, as activity was significantly stimulated by membrane-fluidizing agents. Enzyme activity was developmentally expressed in P2 pellets coincident with in vivo synaptogenesis. It was located on the synaptosome and was particularly high in myelin-containing fractions. Its cellular distribution was confined to neuronal cells and centrally derived oligodendrocytes.

The sulphatase of ox liver. XXV. Sulphatase A as a hysteretic enzyme

The kinetic behaviour of the system native--substrate-modified sulphatase A (arylsulphate sulphohydrolase, EC 3.1.6.1) has been investigated and it has been shown that the progress curve of the complete reaction, including both the inactivation and reactivation stages, can be treated as that of a simple hysteretic system in which the substrate-modified enzyme is activated by a product of the reaction. It has been concluded that the early suggestions that the modification of sulphatase A was accompanied by the exposure of a second ligand-binding site are incorrect. It has been shown that, in the absence of sulphate, the rate of reversion of substrate-modified to native sulphatase A is increased by 4-nitrocatechol but not by the same concentration of 2-nitrophenol. A detailed reaction sequence is proposed. This explains the kinetic behaviour of sulphatase A with nitrocatechol sulphate or 2-nitrophenyl sulphate as substrate and in the presence or absence of sulphate.

Comparative kinetics of the sulphatases A

The kinetic behaviour of the sulphatase A from kangaroo liver is that of a simple hysteretic system involving a substrate-modified form of the enzyme. The equilibrium between the native and substrate-modified forms is influenced by one of the reaction products, sulphate. The behaviour of the system differs markedly from that involving the ox enzyme and a generalised model is presented to account for the behaviour of the sulphatases A from ox, human, rat and kangaroo livers.