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Background The antipsychotic aripiprazole is often used in the treatment of first-episode psychosis. Measuring aripiprazole blood levels provides an objective measure of treatment adherence, but this currently involves taking a venous blood sample and sending to a laboratory for analysis. Aims To detail the development, validation and utility of a new point of care (POC) test for finger-stick capillary blood concentrations of aripiprazole. Method Analytical performance (sensitivity, precision, recovery and linearity) of the assay were established using spiked whole blood and control samples of varying aripiprazole concentration. Assay validation was performed over a 14-month period starting in July 2021. Eligible patients were asked to provide a finger-stick capillary sample in addition to their usual venous blood sample. Capillary blood samples were tested by the MyCare™ Insite POC analyser, which provided measurement of aripiprazole concentration in 6 min, and the venous blood sample was tested by the standard laboratory method. Results A total of 101 patients agreed to measurements by the two methods. Venous blood aripiprazole concentrations as assessed by the laboratory method ranged from 17 to 909 ng/mL, and from 1 to 791 ng/mL using POC testing. The correlation coefficient between the two methods (r) was 0.96 and there was minimal bias (slope 0.91, intercept 4 ng/ml). Conclusions The MyCare Insite POC analyser is sufficiently accurate and reliable for clinical use. The availability of this technology will improve the assessment of adherence to aripiprazole and the optimising of aripiprazole dosing.

Supplemental Digital Content is Available in the Text., Background: Busulfan is an alkylating agent used in allogeneic hematopoietic stem cell transplantation for various malignant and nonmalignant disorders. Therapeutic drug monitoring of busulfan is common because busulfan exposure has been linked to veno-occlusive disease, disease relapse, and failed engraftment. The authors developed an automated immunoassay, along with stable calibrators and controls, and quantified busulfan in sodium heparin plasma. Methods: The authors evaluated a homogenous nanoparticle immunoassay, the MyCare Oncology Busulfan Assay Kit (Saladax Biomedical, Inc), for precision, sensitivity, accuracy, and linearity on an open channel clinical chemistry analyzer; they compared the method with 2 mass spectrometry methods (liquid chromatography–tandem mass spectrometry and gas chromatography/mass spectrometry), using anonymized, remnant patient samples. Results: The coefficients of variation for repeatability and within-laboratory precision were ≤9.0%. The linear range was 150–2000 ng/mL; samples up to 6000 ng/mL can be measured with sample dilution. Measured values deviated by ≤14% from assigned values. Comparison between validated mass spectrometry methods resulted in a correlation coefficient R ≥ 0.995. Conclusions: The MyCare Busulfan Assay Kit shows the precision, accuracy, linearity, and test range for performing busulfan concentration measurements in sodium heparin plasma on routine clinical chemistry analyzers.

Background: The use of clozapine demands regular monitoring of clozapine plasma concentrations and of white blood cell parameters. The delay between sending blood samples for analysis and receiving the results hinders clinical care. Point-of-care testing (POCT) can provide drug assay results within a few minutes. Aim: This study aimed to investigate the utility of a novel point-of-care device that can measure clozapine concentrations using capillary blood samples collected via a finger stick. Method: During a five-week period starting in June 2019 eligible patients were asked to provide a finger-stick capillary sample in addition to their usual venous blood sample. Samples were analysed by the novel point-of-care device and by the standard laboratory method. Capillary blood samples were tested by the MyCare™ Insite POCT analyser, and a quantitative measurement of clozapine concentration was provided within six minutes. Results: A total of 309 patients agreed to measurements by the two methods. Analysis revealed clozapine concentrations in venous blood as determined by the laboratory method ranged from 20 to 1310 ng/mL and by POCT from 7 to 1425 ng/mL. There was a strong positive correlation ( R = 0.89) between the results from the venous and the capillary sample methods. The slope of the association between standard assay and MyCare™ Insite was 1.0 with an intercept of –21 ng/mL, indicating minimal bias. Conclusion: Clozapine concentrations can be accurately measured at the point of care using capillary blood samples collected via a finger stick. This approach may be more acceptable than venous sampling to patients and, with almost instant results available, more useful to clinicians.

Background The value of therapeutic drug monitoring (TDM) for paclitaxel (PTX) was recently demonstrated in the largest TDM trial ever conducted in oncology. The trial demonstrated significant reduction in neuropathy when using TDM. Dose adjustment for PTX was based on time above a threshold concentration (Tc>0.05). Tc>0.05 must be calculated with a pharmacokinetic model and complex nonlinear mixed-effects software. The use of the software and chromatographic methods to measure PTX requires specialized expertise. User-friendly methods to quantitate PTX and calculate Tc>0.05 could simplify the introduction of TDM into routine clinical practice. Methods The immunoassay (MyPaclitaxel) was used to quantitate PTX in samples from the clinical trial; the results were used to calculate Tc>0.05 using a stand-alone computer program with a simple, friendly graphical user interface for nonlinear mixed-effects pharmacokinetic calculations (MyCare Drug Exposure Calculator). The resulting dose recommendations from the calculated Tc>0.05 were compared with those using liquid chromatography-ultraviolet detection and NONMEM to examine the efficacy of the simpler tools for TDM. Results There was a good agreement between the immunoassay and liquid chromatography-ultraviolet detection: Passing-Bablok regression slope was 1.045 and intercept was -6.00, R was 0.9757, and mean bias was -1.77 ng/mL (-2.07 nmol/L). Dosing recommendations were identical for 70% of the cycles and within 10% for 89% of the samples. All Tc>0.05 values were at the same or adjacent medical decision points. Conclusions MyPaclitaxel assay and MyCare Drug Exposure Calculator are convenient, user-friendly tools that may be suitable for routine TDM of PTX in clinical care.

Background: Gemcitabine (20,20-difluoro-20-deoxycytidine) is a nucleoside analog used as a single agent and in combination regimens for the treatment of a variety of solid tumors. Several studies have shown a relationship between gemcitabine peak plasma concentration (Cmax) and hematological toxicity. An immunoassay for gemcitabine in plasma was developed and validated to facilitate therapeutic drug monitoring (TDM) by providing an economical, robust method for automated chemistry analyzers. Methods: A monoclonal antibody was coated on nanoparticles to develop a homogenous agglutination inhibition assay. To prevent ex vivo degradation of gemcitabine in blood, tetrahydrouridine was used as a sample stabilizer. Validation was conducted for precision, recovery, cross-reactivity, and linearity on a Beckman Coulter AU480. Verification was performed on an AU5800 in a hospital laboratory. A method comparison was performed with (LC-MS/MS) liquid chromatography tandem mass spectrometry using clinical samples. Selectivity was demonstrated by testing cross-reactivity of the major metabolite, 20,20-difluorodeoxyuridine. Results: Coefficients of variation for repeatability and withinlaboratory precision were ,8%. The deviation between measured and assigned values was ,3%. Linear range was from 0.40 to 33.02 m/mL (1.5-125.5 mM). Correlation with validated LC-MS/ MS methods was R2 = 0.977. The assay was specific for gemcitabine: there was no cross-reactivity to 20,20-difluorodeoxyuridine, chemotherapeutics, concomitant, or common medications tested. Tetrahydrouridine was packaged in single-use syringes. Gemcitabine stability in whole blood was extended to 8 hours (at room temperature) and in plasma to 8 days (2-88C). Conclusions: The assay demonstrated the selectivity, test range, precision, and linearity to perform reliable measurements of gemcitabine in plasma. The addition of stabilizer improved the sample handling. Using general clinical chemistry analyzers, gemcitabine could be measured for TDM.

Paclitaxel (PTX; Taxol, Abraxane) is used in many regimens for breast cancer, non-small cell lung cancer (NSCLC), and ovarian cancer. Multiple studies have demonstrated that PTX exhibits a greater than 10-fold interpatient variability of clearance rates when patients are dosed according to body surface area (BSA). Pharmacokinetic and pharmacodynamic relationships have been elucidated from BSA-based dosing. PTX is a candidate for dose management, and studies have shown that therapeutic dose management (TDM) is feasible and may provide improved outcomes for patients undergoing treatment.A PTX immunoassay (MyPaclitaxel) has been developed, which employs a novel PTX monoclonal antibody in a nanoparticle-based turbidimetric assay in a competitive format. Precision, accuracy, and linearity were evaluated by Clinical Laboratory Standards Institute protocols at 3 laboratories on the Olympus AU400 analyzer. Method comparison was done versus a validated high-performance liquid chromatography-tandem mass spectroscopy method using samples (n = 119) collected from patients on PTX therapy.The assay requires 8 μL of plasma sample and can produce 400 determinations per hour. The response curve is based on a 6-point nonlinear curve fit and has a range of 0-320 ng/mL, extended to 3200 ng/mL with 10-fold autodilution. Three controls and 4 patient pools were used in precision studies. For all samples across 3 sites, repeatability coefficient of variation percentages ranged 0.9%-4.9%, and within-laboratory coefficient of variation percentages were 1.0%-4.2% with standard curve stability up to 24 days. Linearity was demonstrated over the linear range. Lower limits of detection and quantitation were 11 and 19 ng/mL, respectively. Method comparison results were analyzed by Deming regression, demonstrating a slope = 1.002 and intercept = -3.029 and an R = 0.996. The PTX samples ranged from 24 to 3164 ng/mL with a mean of 745 ng/mL.The analytical performance of an automated immunoassay for PTX has been validated and may serve as a useful tool for TDM of this drug.

The detection of methamphetamine and other chemically related illicit drugs relies extensively on immunoassays. Here we report the cloning and affinity maturation of an anti-methamphetamine antibody which is being employed in the current commercial assays. An anti-methamphetamine scFv was cloned from hybridoma cells, expressed in bacteria and its affinity towards methamphetamine and N-ethylamphetamine (ethamphetamine) was determined by Surface Plasmon Resonance (SPR). The anti-methamphetamine scFv gene was subjected to random mutagenesis by error prone PCR and variants with improved affinity were isolated from the resulting library by a novel screening methodology termed Anchored Periplasmic Expression (APEx) [Harvey, B.R., Georgiou, G., Hayhurst, A., Jeong, K.J., Iverson, B.L., Rogers, G.K. (2004). Anchored periplasmic expression, a versatile technology for the isolation of high-affinity antibodies from Escherichia coli-expressed libraries. Proc. Natl. Acad. Sci. U. S. A. 101, 9193.]. The isolated clones exhibited improved affinity to these illicit drugs, yet maintained low cross-reactivity to over-the-counter drugs. In addition, all clones displayed improved expression characteristics in Escherichia coli. The affinity improved scFv antibodies are thus likely to be useful in methamphetamine class immunodiagnostics.

of the optical densities in ten repetitions of the standard curves was 5% for all standards. In the clinical part, at the cut off value 691 pg/mL, Ab 1-42 showed the sensitivity and the specificity of 69.3 %, and 88.9%, respectively, whereas at the cut off value 0.06, Ab 42/40 ratio showed even better sensitivity and specificity of 93.3%, and 100%, respectively. The area under the ROC curve for Ab 42/40 (0.974) was highly significantly larger compared to the AUC of the Ab 1-42 concentration ROC curve (0.827, p

Docetaxel (Taxotere) (DTX) is a widely used chemotherapy agent used in many regimens for the treatment of solid tumors, for example breast cancer, non-small cell lung cancer, gastric, prostate, and head and neck cancers. This drug meets the criteria for therapeutic dose management, in that it is associated with high pharmacokinetic variability and dose-limiting toxicity; it has a narrow therapeutic window, and there is a significant pharmacokinetic-pharmacodynamic relationship. Measures of exposure and area under the time-concentration curve have been associated with both toxicity and outcomes, making therapeutic dose management for this drug an unmet clinical need. The current methodologies for measuring DTX are based on physical methods, making the analysis less available and costly. An automated immunoassay has been developed to provide greater access to DTX dose management.A DTX immunoassay (MyDocetaxel) has been developed using a generic nanoparticle turbidimetric method that can be used on a wide variety of automated clinical chemistry analyzers including the Beckman Coulter AU400 and AU640 instruments, which were used in this study. The assay is based on a competitive assay format using a selective DTX monoclonal antibody. Clinical Laboratory Standards Institute protocols for establishing manufacturer's claims were used to verify performance. Testing at 3 clinical laboratories was undertaken using the same protocols for laboratory validation of precision, accuracy, and linearity. Method comparison (n = 89) was done using samples collected from patients on DTX therapy. The comparative method was LC-MS/MS validated according to Food and Drug Administration guidance on bioanalytical methods. Institutional review board approval was obtained for prospective collection of samples from patients on DTX therapy.The assay on the AU400 uses 2 μL of sample, provides the first result in 9.0 minutes and can generate 400 determinations per hour. Internal studies established a lower limit of detection ≤25 ng/mL and a lower limit of quantitation ≤30 ng/mL. Additional studies demonstrated no interference from coadministered drugs, major metabolites, or related compounds. Linearity from 50 to 1000 ng/mL was validated. Method comparisons between laboratories and to the physical method gave slopes: 1 ± 0.5, intercepts:2.0 ng/mL, R0.99, with the range of DTX concentrations measured by the assay 31-9754 ng/mL, with a mean of 689 ng/mL. In all 3 laboratories, the coefficient of variation percentage for repeatability ranged from 0.8% to 6.2% and the within-laboratory precision ranged from 1.4% to 10.1%.This immunoassay is suitable for quantifying DTX in plasma with advantages of small sample size, no sample pretreatment, and the ability to be applied to a wide range of clinical analyzers. With the validation of this method, the application of DTX testing in clinical practice may gain wider acceptance for individualizing patient DTX dosing.

Phosphatidylcholine (PtdCho) is the major membrane phospholipid in mammalian cells, and its synthesis is controlled by the activity of CDP:phosphocholine cytidylyltransferase (CCT). Enforced CCT expression accelerated the rate of PtdCho synthesis. However, the amount of cellular PtdCho did not increase as a result of the turnover of both the choline and glycerol components of PtdCho. Metabolic labeling experiments demonstrated that cells compensated for elevated CCT activity by the degradation of PtdCho to glycerophosphocholine (GPC). Phospholipase D-mediated PtdCho hydrolysis and phosphocholine formation were unaffected. Most of the GPC produced in response to excess phospholipid production was secreted into the medium. Cells also degraded the excess membrane PtdCho to GPC when phospholipid formation was increased by exposure to exogenous lysophosphatidylcholine or lysophosphatidylethanolamine. The replacement of the acyl moiety at the 1-position of PtdCho with a non-hydrolyzable alkyl moiety prevented degradation to GPC. Accumulation of alkylacyl-PtdCho was associated with the inhibition of cell proliferation, demonstrating that alternative pathways of degradation will not substitute. GPC formation was blocked by bromoenol lactone, implicating the calcium-independent phospholipase A2 as a key participant in the response to excess phospholipid. Owing to the fact that PtdCho is biosynthetically converted to PtdEtn, excess PtdCho resulted in overproduction and exit of GPE as well as GPC. Thus, general membrane phospholipid homeostasis is achieved by a balance between the opposing activities of CCT and phospholipase A2.

The regulatory mechanism of substrate activation in yeast pyruvate decarboxylase is triggered by the interaction of pyruvic acid with C221 located on the beta domain at20 A from the thiamin diphosphate (ThDP). To trace the putative information transfer pathway, substitutions were made at H92 on the alpha domain, across the domain divide from C221, at E91, next to H92 and hydrogen bonded to W412, the latter being intimately involved in the coenzyme binding locus. Additional substitutions were made at D28, E51, H114, H115, I415 and E477, all near the active center. The pH-dependent steady-state kinetic parameters, including the Hill coefficient, provide useful insight to this effort. In addition to C221, the residues H92, E91, E51 and H114 and H115 together appear to have a critical impact on the Hill coefficient, providing a pathway for information transfer. To study the activation by ThDP.Mg(II), variants at G231 (of the conserved GDG triplet) and at N258 and C259 (all three being part of the putative ThDP fold) of the E1 component of the Escherichia coli pyruvate dehydrogenase multienzyme complex were studied. Kinetic and spectroscopic evidence suggests that the Mg(II) ligands are very important to activation of the enzymes by cofactors.

Oligonucleotide-directed site-specific mutagenesis was carried out on pyruvate decarboxylase (EC 4.1.1.1) from Saccharomyces cerevisiae at two cysteines on the beta domain (221 and 222) and at H92 on the alpha domain, across the domain divide from C221. While C221 has been shown to provide the trigger for substrate activation [Baburina, I., et al. (1994) Biochemistry 33, 5630-5635], the information must be transmitted from the substrate bound at this site [Arjunan, D., et al. (1996) J. Mol. Biol. 256, 590-600] to the active center thiamin diphosphate located at the interface of the alpha and gamma domains. Substitution at H92 with G, A, or C leads to great reduction of the Hill coefficient (from 2.0 in the wild-type enzyme to 1.2-1.3), while substitution for Lys affords an active enzyme with a Hill coefficient of 1.5-1.6. Iodoacetate at 10 mM reduced the Hill coefficient from 2.0 to 1.1, while also causing significant inactivation of the enzyme, presumably by carboxymethylation of C221. 1,3-Dibromoacetone, a potential cross-linker when added to the H92C/C222S variant at 0.1 mM, abolished substrate activation while reducing the activity only by 30%. Therefore, 1,3-dibromoacetone may cross-link C92 and C221. It was concluded that H92 is on the information transfer pathway during the substrate activation process and the interaction between C221 on the beta domain and H92 on the alpha domain is required for substrate activation. Extensive pH studies of the steady-state kinetic constants provide support for the interaction of C221 and H92 and the transmission of regulatory information to the active center via this pathway and pKaS for the two groups. This important interaction between the C221-bound pyruvate and His92 probably has both electrostatic and steric components.

Oligonucleotide-directed site-specific mutagenesis was carried out on pyruvate decarboxylase (EC 4.1.1.1) from Saccharomyces cerevisiae at three of the four cysteines (152, 221, and 222), the fourth (69) being buried according to X-ray crystallographic results [Arjunan et al. (1996) J. Mol. Biol. 256, 590-600]. All of the variants still retained significant activity, and all could be purified to homogeneity. FT-IR experiments were run on the C221S, C222S, C221S/C222S and C152A variants, as well as on the wild-type enzyme. There is a band present at 2557 cm-1 in the spectra of all variants and the wild-type enzyme, except in the spectrum of the C152A variant. This frequency is appropriate to a cysteine S-H stretching mode. It was therefore concluded that C152 is the only undissociated cysteine on the enzyme at pH 6.0, the pH optimum of this enzyme, whereas C221, C222, and C69 are all ionized. Isoelectric focusing experiments were carried out on all of these variants, as well as on the H92A variant (H92 is across the domain divide on the alpha domain, from C221 located on the beta domain). The variation in isoelectric points deduced from the data was consistent with removal of negative charges concomitant with the C221S, C222S, and C221S/C222S substitutions and removal of a positive charge with the H92A substitution when compared to that of the wild-type enzyme. The results of these two types of experiments are in good accord and suggest that the site of substrate activation at C221 [Baburina et al. (1994) Biochemistry 33, 5630-5635] is comprised of a Cys221S- +HHis92 ion pair, not unlike that found in papain and glyceraldehyde-3-phosphate dehydrogenase. This finding suggests that the regulatory site of this enzyme has been optimized for nucleophilic reactivity between the thiolate of C221 and the keto carbon of the 2-oxoacid.

Brewers' yeast pyruvate decarboxylase (EC 4.1.1.1), a thiamin diphosphate and Mg(II)-dependent enzyme, isolated from Saccharomyces cerevisiae possesses four cysteines/subunit at positions 69, 152, 221, and 222. Earlier studies conducted on a variant of the enzyme with a single Cys at position 221 (derived from a gene that was the product of spontaneous fusion) showed that this enzyme is still subject to substrate activation [Zeng, X., Farrenkopf, B., Hohmann, S., Jordan, F., Dyda, F., & Furey, W. (1993) Biochemistry 32, 2704-2709], indicating that if Cys was responsible for this activation, it had to be C221. To further test the hypothesis, the C221S and C222S single and the C221S-C222S double mutants were constructed. It is clearly shown that the mutation at C221, but not at C222, leads to abolished substrate activation according to a number of kinetic criteria, both steady state and pre steady state. On the basis of the three-dimensional structure of the enzyme [Dyda, F., Furey, W., Swaminathan, S., Sax, M., Farrenkopf, B., Jordan, F. (1993) Biochemistry 32, 6165-6170], it is obvious that while C221 is located on the beta domain, whereas thiamin diphosphate is wedged at the interface of the alpha and gamma domains, addition of pyruvate or pyruvamide as a hemiketal adduct to the sulfur of C221 can easily bridge the gap between the beta and alpha domains. In fact, residues in one or both domains must be dislocated by this adduct formation. It is very likely that regulation as expressed in substrate activation is transmitted via this direct contact made between the two domains in the presence of the activator.(ABSTRACT TRUNCATED AT 250 WORDS)

PtdCho accumulation is a periodic, S phase-specific event that is modulated in part by cell cycle-dependent fluctuations in CTP:phosphocholine cytidylyltransferase (CCT) activity. A supply of fatty acids is essential to generate the diacylglycerol (DG) precursors for phosphatidylcholine (PtdCho) biosynthesis but it is not known whether the DG supply is also coupled to the cell cycle. Although the rate of fatty acid synthesis in a macrophage cell line was dramatically stimulated in response to the growth factor, CSF-1, it was not regulated by the cell cycle. Increased fatty acid synthesis correlated with elevated acetyl-CoA carboxylase (ACC) and fatty acid synthase (FAS) steady-state mRNA levels. Cellular fatty acid synthesis was essential for membrane PL synthesis. Cerulenin inhibition of endogenous fatty acid synthesis also inhibited PtdCho synthesis, which was not relieved by exogenous fatty acids. Inhibition of CCT activity by the addition of lysophosphatidylcholine (lysoPtdCho) or temperature-shift of a conditionally defective CCT diverted newly synthesized DG to the TG pool where it accumulated. Enforced expression of CCT stimulated PtdCho biosynthesis and reduced TG synthesis. Thus, the cellular DG supply did not regulate PtdCho biosynthesis and CCT activity governs the partitioning of DG into either the PL or TG pools, thereby controlling both PtdCho and TG biosynthesis.

The residue C221 on pyruvate decarboxylase (EC. 4.1.1.1) from Saccharomyces cerevisiae has been shown to be the site where the substrate activation cascade is triggered [Baburina et al. (1994) Biochemistry 33, 5630-5635] and is located on the beta domain [Arjunan et al. (1996) J. Mol. Biol. 256, 590], while the active-center thiamin diphosphate is located20 A away, at the interface of the alpha and gamma domains. The reactivity of all three exposed cysteines (152, 221, and 222) was examined under the influence of known activators and inhibitors. Protein chemical methods, in conjunction with [1-14C] and [3-3H] analogues of the mechanism-based inhibitor p-ClC6H4CH=CHCOCOOH, demonstrated that the holoenzyme bound approximately 2-3 atoms of tritium/atom of C-14. However, when the labeled enzyme was subjected to trypsinization, followed by sequencing of the labeled peptide, only the tritium label was in evidence at C221, with a stoichiometry of 2 atoms of tritium/tetrameric holoenzyme. Apparently, the product of decarboxylation bonded to the enzyme survived the limited proteolysis and sequencing, but the bound 2-oxoacid was released during the protocol. Surprisingly, the C221S or C222A variants, although they still possess 20-30% specific activity compared to the wild-type enzyme, could still be inhibited by the XC6H4CH=CHCOCOOH class of inhibitors/substrate analogues, as well as by the product of decarboxylation from such compounds, cinnamaldehydes. Other potential nucleophilic sites for the inhibitor [C152 (the third exposed cysteine), residues D28, H114, H115, and E477 at the active center and H92 at the regulatory site] were also substituted by a nonnucleophilic side chain. All variants were still subject to inhibition by p-ClC6H4CH=CHCOCOOH, the active-center variants being inactivated even faster than the wild-type enzyme, suggesting that the active center is involved in the inactivation process. It appears that C221 is one of only two sites of interaction with such compounds (perhaps the result of a Michael addition across the C=C bond), yet the bound [1-14C]-labeled inhibitor could no longer be detected after peptide mapping at this site or at the catalytic site. Upon combining the tritiated inhibitor with [2-14C]-thiamin diphosphate, no evidence could be found for a thiamin-inhibitor-protein ternary complex, suggesting that the thiamin-bound enamine intermediate did not react further with the protein. It is likely that the second form of inhibition is at the active center, with the inhibitor cofactor-bound, which would have been released during the proteolytic protocol. Among other known activators, ketomalonate was found to react at C221 only. Glyoxalic acid, a mechanism-based inhibitor, on the other hand, could react at both the regulatory and the catalytic center. The high reactivity of C221 is consistent with it being in the thiolate form at the optimal pH of the enzyme [forming a Cys221S(-) + HHis92 ion pair; see Baburina et al. (1996) Biochemistry 35, 10249-10255, and Baburina et al. (1998) Biochemistry 37, 1235-1244]. Several additional compounds were tested as potential regulatory site-directed reagents: iodoacetate, 1,3-dibromoacetone, and 1-bromo-2-butanone. All three compounds reduced the Hill coefficient and hence appear to react at C221. It was concluded that either substitution of C221 by a nonnucleophilic residue or large groups attached to C221 in the wild-type enzyme lead to a distortion of domain interactions, interactions which are required for both optimal activity and substrate activation.

A HeLa cell line was constructed for the regulation of CTP:phosphocholine cytidylyltransferase (CCT) expression via a tetracycline-responsive promoter to test the role of CCT in apoptosis triggered by exposure of cells to the antineoplastic phospholipid 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (ET-18-OCH3). Basal CCT expression in the engineered HeLa cell line was the same as in control HeLa cells lines, and CCT activity and protein were elevated 25-fold following 48 h of induction with doxycycline. Increased CCT expression prevented ET-18-OCH3-induced apoptosis. Acylation of exogenous lysophosphatidylcholine circumvented the requirement for CCT activity by providing an alternate route to phosphatidylcholine, and heightened CCT expression and lysophosphatidylcholine supplementation were equally effective in reversing the cytotoxic effect of ET-18-OCH3. Neither CCT overexpression nor lysophosphatidylcholine supplementation allowed the HeLa cells to proliferate in the presence of ET-18-OCH3, indicating that the cytostatic property of ET-18-OCH3 was independent of its effect on membrane phospholipid synthesis. These data provide compelling genetic evidence to support the conclusion that the interruption of phosphatidylcholine synthesis at the CCT step by ET-18-OCH3 is the primary physiological imbalance that accounts for the cytotoxic action of the drug.