Enzyme kinetics of cytochrome P450-mediated reactions

The most common drug-drug interactions may be understood in terms of alterations of metabolism, associated primarily with changes in the activity of cytochrome P450 (CYP) enzymes. Kinetic parameters such as Km, Vmax, Ki and Ka, which describe metabolism-based drug interactions, are usually determined by appropriate kinetic models and may be used to predict the pharmacokinetic consequences of exposure to one or multiple drugs. According to classic Michaelis-Menten (M-M) kinetics, one binding site models can be employed to simply interpret inhibition (pure competitive, non-competitive and uncompetitive) or activation of the enzyme. However, some cytochromes P450, in particular CYP3A4, exhibit unusual kinetic characteristics. In this instance, the changes in apparent kinetic constants in the presence of inhibitor or activator or second substrate do not obey the rules of M-M kinetics, and the resulting kinetics are not straightforward and hamper mechanistic interpretation of the interaction in question. These unusual kinetics include substrate activation (autoactivation), substrate inhibition, partial inhibition, activation, differential kinetics and others. To address this problem, several kinetic models can be proposed, based upon the assumption that multiple substrate binding sites exist at the active site of a particular P450, and the resulting kinetic constants are, therefore, solved to adequately describe the observed interaction between multiple drugs. The following is an overview of some cytochrome P450-mediated classic and atypical enzyme kinetics, and the associated kinetic models. Applications of these kinetic models can provide some new insights into the mechanism of P450-mediated drug-drug interactions.

Drug transport by reconstituted P-glycoprotein in proteoliposomes. Effect of substrates and modulators, and dependence on bilayer phase state

The P-glycoprotein multidrug transporter (Pgp) is an active efflux pump for chemotherapeutic drugs, natural products and hydrophobic peptides. Pgp is envisaged as a 'hydrophobic vacuum cleaner', and drugs are believed to gain access to the substrate binding sites from within the membrane, rather than from the aqueous phase. The intimate association of both Pgp and its substrates with the membrane suggests that its function may be regulated by the biophysical properties of the lipid bilayer. Using the high affinity fluorescent substrate tetramethylrosamine (TMR), we have monitored, in real time, transport in proteoliposomes containing reconstituted Pgp. The TMR concentration gradient generated by Pgp was collapsed by the addition of either the ATPase inhibitor, vanadate, or Pgp modulators. TMR transport by Pgp obeyed Michaelis--Menten kinetics with respect to both of its substrates. The Km for ATP was 0.48 mM, close to the K(m) for ATP hydrolysis, and the K(m) for TMR was 0.3 microM. TMR transport was inhibited in a concentration-dependent fashion by verapamil and cyclosporin A, and activated (probably by a positive allosteric effect) by the transport substrate colchicine. TMR transport by Pgp reconstituted into proteoliposomes composed of two synthetic phosphatidylcholines showed a highly unusual biphasic temperature dependence. The rate of TMR transport was relatively high in the rigid gel phase, reached a maximum at the melting temperature of the bilayer, and then decreased in the fluid liquid crystalline phase. This pattern of temperature dependence suggests that the rate of drug transport by Pgp may be dominated by partitioning of drug into the bilayer.

Herpes simplex virus virion host shutoff protein requires a mammalian factor for efficient in vitro endoribonuclease activity

The virion host shutoff protein (vhs) of herpes simplex virus (HSV) triggers global shutoff of host protein synthesis and accelerated mRNA turnover during virus infection and induces endoribonucleolytic cleavage of exogenous RNA substrates when it is produced in a rabbit reticulocyte (RRL) in vitro translation system. Although vhs induces RNA turnover in the absence of other HSV gene products, it is not yet known whether cellular factors are required for its activity. As one approach to addressing this question, we expressed vhs in the budding yeast Saccharomyces cerevisiae. Expression of vhs inhibited colony formation, and the severity of this effect varied with the carbon source. The biological relevance of this effect was assessed by examining the activity of five mutant forms of vhs bearing previously characterized in-frame linker insertions. The results indicated a complete concordance between the growth inhibition phenotype in yeast and mammalian host cell shutoff. Despite these results, expression of vhs did not trigger global mRNA turnover in vivo, and cell extracts of yeast expressing vhs displayed little if any vhs-dependent endoribonuclease activity. However, activity was readily detected when such extracts were mixed with RRL. These data suggest that the vhs-dependent endoribonuclease requires one or more mammalian macromolecular factors for efficient activity.

beta-Amyloid efflux mediated by p-glycoprotein

A large body of evidence suggests that an increase in the brain beta-amyloid (Abeta) burden contributes to the etiology of Alzheimer's disease (AD). Much is now known about the intracellular processes regulating the production of Abeta, however, less is known regarding its secretion from cells. We now report that p-glycoprotein (p-gp), an ATP-binding cassette (ABC) transporter, is an Abeta efflux pump. Pharmacological blockade of p-gp rapidly decrease extracellular levels of Abeta secretion. In vitro binding studies showed that addition of synthetic human Abeta1-40 and Abeta1-42 peptides to hamster mdr1-enriched vesicles labeled with the fluorophore MIANS resulted in saturable quenching, suggesting that both peptides interact directly with the transporter. Finally, we were able to directly measure transport of Abeta peptides across the plasma membranes of p-gp enriched vesicles, and showed that this phenomenon was both ATP- and p-gp-dependent. Taken together, our study suggests a novel mechanism of Abeta detachment from cellular membranes, and represents an obvious route towards identification of such a mechanism in the brain.

Excretion pattern investigation of urinary normal and modified nucleosides of breast cancer patients by RP-HPLC and factor analysis method

Modified nucleosides, formed post-transcriptionally in RNA by a number of modification enzymes, are excreted in abnormal levels in the urine of patients with malignant tumors. To test their usefulness as tumor markers, and to compare them with the conventional tumor markers, a reversed-phase high-performance liquid chromatographic (RP-HPLC) method and a factor analysis method have been used to study the excretion pattern of nucleosides of breast cancer patients. A clear cut differentiation of the breast cancer group and the healthy individuals in two clusters without overlapping was obtained.

Concentrations and flux of rare earth elements in a semifield plot as influenced by their agricultural application

The concentrations of rare earth elements (REs) in atmospheric particles, soil, soil water, surface runoff, and different parts of corn in an experimental plot in the suburb of Beijing, China were measured and the flux of REs was estimated. The concentration of REs in air particles with diameter less than 10 microm is 36 ng/m3 and, by ratio analysis, the origin of REs in the atmosphere was likely local soil. The concentration of soluble REs is 0.69 microg/L in rainwater, 5-7 microg/L in surface runoff, as well as 1-4 microg/L in soil water and the application of RE mixture has no observed influences on their concentrations. The concentration of soluble REs in surface runoff and soil water was dominated by aqueous-solid-phase partitioning. By estimation of the flux, most of the REs applied will remain in the soil phase and continued application of REs will cause an accumulation of REs in agricultural soil.

The vhs1 mutant form of herpes simplex virus virion host shutoff protein retains significant internal ribosome entry site-directed RNA cleavage activity

The virion host shutoff (vhs) protein of herpes simplex virus (HSV) triggers global shutoff of host protein synthesis and accelerated turnover of host and viral mRNAs during HSV infection. As well, it induces endoribonucleolytic cleavage of RNA substrates when produced in a rabbit reticulocyte lysate (RRL) in vitro translation system. The vhs1 point mutation (Thr 214-->Ile) eliminates vhs function during virus infection and in transiently transfected mammalian cells and was therefore previously considered to abolish vhs activity. Here we demonstrate that the vhs1 mutant protein induces readily detectable endoribonuclease activity on RNA substrates bearing the internal ribosome entry site of encephalomyocarditis virus in the RRL assay system. These data document that the vhs1 mutation does not eliminate catalytic activity and raise the possibility that the vhs-dependent endoribonuclease employs more than one mode of substrate recognition.

Accumulation of rare earth elements in corn after agricultural application

Using both pot and plot experiments, the dose-dependent accumulation of rare earth elements (REs) in corn (Zea mays L.) after application of an agricultural REs mixture was measured. In the pot experiment, the dose-dependent accumulation of REs in corn root and stem was observed, but it could not be detected in corn leaf under the dosage of 20 mg REs kg(-1) soil (oven-dry mass). The non-observed effect concentration (NOEC) for accumulation of REs in corn seedling with the pot experiment was 1.0 mg REs kg(-1). In the plot experiment, the dose-dependent accumulation was observed at an early stage after application of REs and the NOEC value of 32 mg REs m(-2) was obtained. At harvest, no dose-dependent accumulation of REs was observed in any part of the corn. These results can be confirmed by the fingerprinting analysis based on the differences between La to RE ratios in the REs mixture and in pot or plot soil. We observed that the plant shows no preference on individual RE and the results of fingerprinting indicated clearly the incorporation of exogenous REs in plant tissues, in a similar manner as that observed in the dose-dependent distribution of RE concentrations. The results indicated also a translocation process of REs from plant root to leaf when applied to soil or from leaf to root when applied to leaf. A homeostatic regulation mechanism for excessive uptake of REs in plants is suggested to regulate the concentrations of REs in the plant.

Specific p53 mutations detected in plasma and tumors of hepatocellular carcinoma patients by electrospray ionization mass spectrometry

Hepatocellular carcinoma (HCC), a common cause of cancer deaths worldwide, has several major etiological risk factors, including infection with the hepatitis viruses and exposure to aflatoxin B1. A specific missense mutation resulting from a guanine to thymine transversion at the third position of codon 249 in the p53 tumor suppressor gene has been reported in 10-70% of HCCs from areas of high dietary exposure to aflatoxin B1. Short oligonucleotide mass analysis was compared with DNA sequencing in 25 HCC samples for specific p53 mutations. Mutations were detected in 10 samples by short oligonucleotide mass analysis in agreement with DNA sequencing. Analysis of another 20 plasma and tumor pairs showed 11 tumors containing the specific mutation, and this change was detected in six of the paired plasma samples. Four of the plasma samples had detectable levels of the mutation; however, the tumors were negative, suggesting possible multiple independent HCCs. Ten plasma samples from healthy individuals were all negative. This molecular diagnostic technique has implications for prevention trials and for the early diagnosis of HCC.

Excretion pattern investigation of urinary normal and modified nucleosides of breast cancer patients by RP-HPLC and factor analysis method

Modified nucleosides, formed post-transcriptionally in RNA by a number of modification enzymes, are excreted in abnormal levels in the urine of patients with malignant tumors. To test their usefulness as tumor markers, and to compare them with the conventional tumor markers, a reversed-phase high-performance liquid chromatographic (RP-HPLC) method and a factor analysis method have been used to study the excretion pattern of nucleosides of breast cancer patients. A clear cut differentiation of the breast cancer group and the healthy individuals in two clusters without overlapping was obtained.

Immunolocalization of heat shock protein 70 during reparative dentinogenesis

OBJECTIVE

To investigate the immunolocalization of heat shock protein 70 (hsp 70) during reparative formation and to discuss the role of heat shock response in dental pulp injury and repair.

METHODS

A single cavity was prepared in the mesial surface of the first molars of both maxilla and mandible in Wistar rat. The animals were sacrificed at 3, 15, and 30 days post-operation. After the histological process, the paraffin sections were reacted with monoclonal antibodies against rat hsp 70 using the strept-avidin-biotin-peroxidase complex method.

RESULTS

Immunolocalization demonstrated heavy staining for hsp 70 in normal pulp and at different stages of dental pulp repair. In normal pulp, immunoreactivity was visualized in the odontoblasts and the pulp fibroblast. In the group sacrificed at 3 days, heavy staining was located in the odontoblast process and cytoplasm. After 15 days, the newly formed odontoblast-like cells were strongly stained. At 30 days, the same staining intensity was observed in odontoblast-like cells and in pulp cells. No staining was seen in reparative dentin.

CONCLUSION

These results demonstrated that heat shock protein 70 might play an important role as a molecular chaperone during reparative dentin formation.

The research and development of noncontact 3-D laser dental model measuring and analyzing system

OBJECTIVE

To research and develop a new laser scanning 3-dimensional digitization system for collecting, showing, and measuring the data of dental casts.

MATERIAL AND METHODS

According to the 3-dimensional measuring method, by using 2 pulsate motors to make the synthetic movement of the dental cast, a special semiconductor laser, and 2 special line array CCD, the 3-D data could be found anywhere on the surface of the dental cast. The program, developed with Visual C++ language under the Windows (Windows tm) system, can control the scanner, rebuild the 3-D graphics and measure the data of the dental cast.

RESULTS

The system offers 70 x 70 x 70 mm measurement scope, 0.01 mm resoling power, < 0.1 mm mean deviation, and 25 mins for single cast scanning.

CONCLUSION

The advantages of this system are its precision, simplicity, high efficiency, and wide range of measurements with complete direct view. This system also supplied new information that cannot be found with traditional measurement methods.

Characterization and complementation of a Pichia stipitis mutant unable to grow on D-xylose or L-arabinose

Pichia stipitis CBS 6054 will grow on D-xylose, D-arabinose, and L-arabinose. D-Xylose and L-arabinose are abundant in seed hulls of maize, and their utilization is important in processing grain residues. To elucidate the degradation pathway for L-arabinose, we obtained a mutant, FPL-MY30, that was unable to grow on D-xylose and L-arabinose but that could grow on D-arabinitol. Activity assays of oxidoreductase and pentulokinase enzymes involved in D-xylose, D-arabinose, and L-arabinose pathways indicated that FPL-MY30 is deficient in D-xylitol dehydrogenase (D-XDH), D- and L-arabinitol dehydrogenases, and D-ribitol dehydrogenase. Transforming FPL-MY30 with a gene for xylitol dehydrogenase (PsXYL2), which was cloned from CBS 6054 (GenBank AF127801), restored the D-XDH activity and the capacity for FPL-MY30 to grow on L-arabinose. This suggested that FPL-MY30 is critically deficient in XYL2 and that the D-xylose and L-arabinose metabolic pathways have xylitol as a common intermediate. The capacity for FPL-MY30 to grow on D-arabinitol could proceed through D-ribulose.

[Computerized surgical simulation with 3D reconstruction of upper limbs]

This paper reports our study on how to realize computerized surgical simulation through reconstructing 3D anatomy of upper limbs. CT images and serial sections of upper limbs were matched to reconstruct 3D images. We compiled the computer programs of 3D reconstruction and surgical simulation with borland C++ computer language, and built 3D digital model of anatomic structure of upper limbs. The results showed that all structures reconstructed could be displayed alone, in any group or totally. While operating, we could choose one of three segments of the upper limbs model and operate on any parts and in any direction. The surgical simulation system could be used to design operative schemes, choose the best operative paths, and teach the processes of operations and anatomy. It could run in 586-personal computers.

Effects of delaying puberty on bone mineralization in female rats

The effect of delaying puberty on bone mineralization was studied using female rats as a model. Repeated injections of gonadotrophin-releasing hormone antagonist (GnRHa) were used to suppress the onset of puberty from the age of 6-10 weeks. A group of control female rats was given aqueous solution injections at the same age and for the same duration. The effect of delaying puberty on bone mineralization was examined using dual energy X-ray absorptiometry (DXA) and peripheral quantitative computerized tomography (QCT), both methods being adapted for small animals. Bone mineral parameters were measured at baseline and at the ages of 10, 17 and 24 weeks in total body, femur and spine. Compared to controls, bone mineral content (BMC) and bone mineral density (BMD), as measured by DXA, were significantly decreased in GnRHa-treated rats in total body and femur at 10 and 24 weeks of age (P < 0.05). The results were even more significant after adjusting for weight. After this adjustment, spine BMC and BMD at 10, 17 and 24 weeks were significantly lower in the treatment group (P < 0.05). Trabecular BMD at the distal femur in the GnRHa treated group as measured by peripheral QCT was significantly lower (P < 0.05). However, cortical bone in the mid-femur had higher BMD, concurrent with lower cortical thickness in the treatment group. In conclusion, a delay in the onset of sexual maturation may cause prolonged, possibly irreversible defect in bone mineralization.

[Epidemiological aspects of the genotype of hepatitis C virus]

OBJECTIVE

To research the epidemiological characteristics of genotype of hepatitis C virus.

METHODS

107 HCV isolates from 7 cities in southern, northern and northeast of China were analyzed using PCR and second generation line probe assay (INNO-LiPA).

RESULTS

The analysis of HCV genotypes showed that: 1. HCV l b was the common genotype (83 17%), however the frequency of genotype l b in northeast (72.22%) was lower than northern (87.65%, P <0.01), and the frequency of the genotype 2 was in low level (6.86%). The rate of coinfection with l b and other genotype was 10.29%. 2. The rate of infection with genotype l b in female patients was higher (93.61%) than that in male (75.0%) showing statistical significance (P <0.01). 3. In three age groups (< 30, 30 - 49 and > 50 years old) the rates of infection with genotype l b were 77.77%, 83.92% and 90.90%, respectively. 4. The detection rates of genotype l b in patients with infection period > 10 years and <10 years were 100% and 80.43% respectively (P <0.01).

CONCLUSIONS

There are some relationship between HCV genotype and HCV geographic distribution, sex, age and infection period of patients.

Multiple types of calcium signals are associated with cell division in zebrafish embryo

Recent studies suggested that a Ca(2+) signal is involved in the regulation of cell division. For example, using a confocal imaging technique, we have shown that a localized Ca(2+) elevation was clearly associated with the onset of cytokinesis in zebrafish embryo [Chang and Meng (1995) J. Cell Biol. 131:1539-1545]. This finding was later confirmed in studies using aequorin as a Ca(2+) probe. Here, we used a 4-D confocal measurement technique to further characterize the properties of the Ca(2+) signal associated with cell division. We found evidence that there were three types of Ca(2+) signals associated with different stages of cell cleavage in embryonic cell. The first type was repetitive Ca(2+) spikes that emerged several minutes before the first cell cleavage began. These Ca(2+) spikes were first distributed broadly over the central region of the blastodisc and then gradually localized in the equatorial region; they appeared to play the role of determining the position of the first cleavage plane. The second type was a calcium wave that propagated along the cleavage furrow and appeared to guide the furrow extension during the progression of cytokinesis. The third type was a group of post-cleavage calcium spikes that appeared to be responsible for furrow deepening and maintenance of the contractile band. When this type of Ca(2+) transient was blocked by injecting BAPTA or heparin, cell cleavage regressed and the structure of the contractile band could no longer be maintained.

[Study on human leucocyte antigen-DQ region gene polymorphism in cases of habitual abortion with anticardiolipin antibody]

OBJECTIVE

To investigate the relationship between human leucocyte antigen(HLA)-DQ region gene polymorphism and habitual abortion with anticardiolipin antibody(ACL).

METHODS

Polymerase chain reaction-restrictive fragment length polymorphism (PCR-RFLP) was used to type HLA-DQA1 and HLA-DQB1 alleles in 30 cases of habitual abortion with anticardiolipin antibody and 90 women with normal pregnancy history.

RESULTS

The frequency of DQB1 * 0303 was found to be significantly higher in ACL(+) habitual abortion group (33.3%) than those in normal control group (P < 0.05). Whereas, there was no significant difference found not only in frequency of other DQB1 alleles, but also in frequency of DQA1 alleles and DQA1-DQB1 haplotypes between two groups.

CONCLUSION

The results show an association between ACL(+) habitual abortion and HLA-DQB1 * 0303, which suggest DQB1 * 0303 may be susceptible gene to ACL(+) habitual abortion.

Validation of a new technique to detect malingering of cognitive symptoms: the b Test

We administered the b Test, a new measure to identify malingering requiring recognition of overlearned information, to 34 suspected malingerers and to 161 subjects in various clinical groups (moderate to severe head injury, elderly depressed, learning disability, schizophrenia, right and left CVA, and elderly normals). Comparisons of groups revealed more commission and omission errors in the suspected malingerers relative to all groups except the right stroke patients. In addition, suspected malingerers took longer to complete the task than all groups except right and left stroke patients and normal elderly. A cutoff of >2 commission errors produced a sensitivity of 76.5% and specificity for all comparison groups combined of 82.6%. Lower sensitivity rates were documented for omissions (58.8 using cutoff of >40) and time (57.6% using cutoff of >12 minutes), but specificity remained high at 85.1% and 83.9%, respectively. Thus, the b Test shows considerable potential as a malingering detection tool.

Application of a mid-infrared fiber bundle in remote measurement of gas concentrations in a chemical vapor deposition chamber

A fiber bundle has been designed for measurement of gas concentrations in a chemical vapor deposition (CVD) chamber. The bundle is broadband, covering the spectrum range from 500 to 6000 cm(-1). Using this bundle, we have designed Fourier Transform infrared (FTIR) sampling configurations with reduced beam divergence (from 0.4 to 0.1 rad) and focal spot diameters (from 12 to 4 mm). The FTIR spectrum of tetraethoxysilane vapors in a CVD chamber was determined with the bundle and modified SAM connectors. An ellipsoid reflector was designed that yielded improved signal-to-noise ratios (SNR's) in the fiber-based spectra by a factor of 2. The incorporation of this reflector in spectrometers by use of the fiber bundle has the potential to reduce the sampling times for spectra while retaining acceptable SNR's.

[The research of a multimedia consultative system for prosthodontics]

OBJECTIVE

To study how to develop the consultative multimedia system for prosthodontics facilitating the communication between the dentists and patients, using multimedia techniques.

METHODS

The Founder Author Tools V2.1 was used to develop the consultative system in the Pwin95/98 environment. Many forms (such as the articles, pictures, sound and video) of the prosthodontic knowledge were edited in this system.

RESULTS

The consultative multimedia system for prosthodontics was developed, and there were three main parts of the system: (1) the background of medical departments, (2) introductions of the medical specialists (including their photos and working time), (3) introducing the common acknowledge of prosthodontics.

CONCLUSION

The visualization of the prosthodontic problems consulted by patients frequently can facilitate the communication between the dentists and patients and spare visit time. This system are welcome by the patients.