S-methylation in Parkinson's disease

We report thiolmethyltransferase (TMT) activity in RBC membrane preparations from patients with Parkinson's disease (PD), normal controls, and patients with symptomatic parkinsonism. Unlike previous reports, our report found no significant decrease in TMT activity in PD patients compared with normal controls. Low S-methylation capacity does not appear to be a risk factor for PD in Japanese patients.

Monitoring of azathioprine-induced immunosuppression with thiopurine methyltransferase activity in kidney transplant recipients

Thiopurine methyltransferase (TPMT), one of the enzymes involved in azathioprine metabolism, exhibits a wide range of activity in the normal population. We prospectively evaluated the monitoring of erythrocyte TPMT activity (using a radiochemical method) in kidney transplant recipients with regard to the efficacy of azathioprine. Three patterns in TPMT activity variation were observed. In group 1 patients, TPMT activity rose as early as 8 days after transplantation and steadily until month 3. In group 3 patients, TPMT activity remained unchanged. In group 2 patients, TPMT activity rose at month 1 after transplantation. Interestingly, the incidence of acute rejection was significantly (P < 0.01) different among the 3 groups, with the lowest incidence in group 1 and the highest in group 3. We hypothesized that TPMT activity increase was induced by azathioprine in the patients with the lowest incidence of acute rejection. The inducibility of TPMT activity would then appear to be an interesting marker of azathioprine-induced immunosuppression.

Higher activity of polymorphic thiopurine S-methyltransferase in erythrocytes from neonates compared to adults

Thiopurine S-methyltransferase (TPMT) catalyses the S-methylation of aromatic and heterocyclic sulfhydryl compounds, including thiopurine antimetabolites (i.e. mercaptopurine and thioguanine). The activity of TPMT in erythrocytes and other tissues exhibits genetic polymorphism, which is inherited as an autosomal codominant trait. Although inheritance is the principal determinant of TPMT activity, other factors (e.g. renal function, race and thiopurine therapy) have been shown to influence erythrocyte TPMT activity. Because the TPMT polymorphism has not been established in early erythrocyte populations, and the activity of many enzymes differs in neonates, we determined the activity of TPMT in erythrocytes obtained from 60 full-term newborns. Median peripheral blood TPMT activity was 25.3 U per ml pRBC (range 9-52.8 U per ml pRBC), which was > 50% higher than race matched healthy adults (p < 0.001). Western blot analysis demonstrated higher TPMT protein content in erythrocytes from newborns compared to adults, and revealed a significant correlation between TPMT protein and TPMT activity in erythrocytes (rs = 0.63, p = 0.03). Although erythrocyte TPMT activity was significantly higher in newborns, the distribution of activity in newborns was consistent with the genetic polymorphism previously observed in adults.

Erythrocyte fraction affects red blood cell thiopurine methyltransferase activity

Red blood cell (RBC) thiopurine methyltransferase (TPMT) metabolizes the cytotoxic drugs 6-mercaptopurine and azathioprine. RBC TPMT activity has been reported to predict clinical outcome in children with acute lymphoblastic leukaemia and in kidney transplant patients. We first suspected that the erythrocyte fraction affected the calculated TPMT activity when we examined intraindividual TPMT activities in kidney transplant recipients. We demonstrated that the erythrocyte fraction affected the calculated TPMT activity, thus causing a methodological inaccuracy. A low erythrocyte fraction gave an erroneously low TPMT activity. Mean variation of 7.0% was observed within the normal limits of the haematocrit level in healthy subjects. The slopes of the TPMT activity between erythrocyte fraction 0.1 and 0.5 were all significantly different from zero, and the activity displayed good linearity from erythrocyte fraction 0.2. There was a strong association between TPMT activity and erythrocyte fraction in a population sample of children, but not in two other population samples. We propose that the TPMT assay should be performed in lysates at a standardized erythrocyte fraction to avoid variation in activity due to the range of the haematocrit in a population.

Monitoring of TPMT in heart transplant recipients under immunosuppressive therapy with azathioprine

Azathioprine (AZA) is routinely used in double and triple immunosuppressive therapy after cardiac transplantation. In some cases it causes severe myelosuppression. The interindividual differences in AZA toxicity is probably due to differences in the drug metabolism. Thiopurine methyltransferase (TPMT) is thought to be the most important enzyme in the catabolism of AZA. A deficiency in this enzyme will presumably increase the availability of 6-mercaptopurine for the anabolic pathway thereby leading to increased cytotoxicity. A 65-year-old male underwent heart transplantation at our institution with an uncomplicated course. Immunosuppression consisted of cyclosporine, prednisolone, and AZA. Several weeks after the administration of AZA, the patient developed severe leukopenia. TPMT activity was then measured in this patient and found to be below the detection limit. Subsequently the patient died from multiorgan failure due to septicaemia. As a result of this experience, we started to screen all patients for TPMT deficiency. In 58 healthy controls, the mean activity was found to be 11.8 nmol/h/ml of red blood cells (RBC) while in 13 patients on our waiting list, the mean activity was found to be 11.97 nmol/h/ml of RBC. In 15 patients after heart transplantation and azathioprine treatment, the mean activity was found to be 17.2 nmol/h/ml of RBC. We suggest screening for TPMT activity in transplant patients with leukopenia under AZA therapy. If TPMT deficiency is present, the AZA dosage should be adjusted or alternative immunosuppressive regimens should be considered.

Thiol methyltransferase activity in colonocytes and erythrocyte membranes

AIMS

To verify the improved thiol methyltransferase (TMT) assay and measure activity in isolated colonocytes and erythrocyte membranes of the same subjects.

METHODS

High performance liquid chromatography with radioactivity detection was used to measure 14C-methylmercaptoethanol formation, the reaction product of cell extracts incubated with mercaptoethanol and 14C-S-adenosylmethionine.

RESULTS

Verification of radiolabelled 14C-methylmercaptoethanol was by exogenous addition of methylmercaptoethanol and simultaneous ultraviolet detection at 214 nm. Using a substrate concentration of 10 mM mercaptoethanol, the Km for S-adenosylmethionine was 25 microM. The sensitivity of the radioactive method was 2 pmol, with coefficients of variation of 7% within assay and 6.4% between assay. TMT activities (mean +/- SE; n = 17) were 471 +/- 64 pmol/hour/mg protein for colonocytes and 73 +/- 7 pmol/hour/mg protein for erythrocyte membranes.

CONCLUSIONS

The direct assay of TMT activity is sensitive, specific and eliminates concern over non-enzymatic methylation of thiol compounds. High activities in colonic epithelial cells deserve evaluation in disease states.

Defective repair of O6-methylguanine-DNA in primary Sjögren's syndrome patients predisposed to lymphoma

OBJECTIVE

To investigate a role for mutation in the aetiogenesis of autoimmune disease by examining levels of repairing enzyme for the promutagenic DNA base lesion, O6-methylguanine, in lymphocyte extracts from patients with autoimmune diseases. We included primary Sjögrens syndrome (PSS) patients because of the additional relevance of their being at increased risk (> 40-fold) of developing lymphoma.

METHODS

Lymphocytes were prepared from patients with PSS (n = 22) (12 with parotid gland enlargement, an indicator of extensive lymphoproliferation), rheumatoid arthritis (n = 12), primary biliary cirrhosis (n = 11), osteoarthritis (n = 12), and healthy individuals (n = 11). MGMT amounts were determined in lymphocyte extracts by direct enzyme assay and expressed in relation to total extract DNA, protein, or cell number.

RESULTS

We found no defect in the repairing methyltransferase enzyme between any of the groups, except in PSS patients at increased risk of developing lymphoma (those with enlarged parotid glands): p < 0.0001 and p = 0.0056, compared with healthy controls and PSS patients without parotid gland swelling, respectively.

CONCLUSIONS

Our findings implicate persistence of O6-methylguanine-DNA in the aetiology of lymphoma associated with PSS, and raise the possibility that an alternative repair process for O6-methylguanine-DNA, nucleotide excision repair, might be defective in autoimmune disease.

High-performance liquid chromatographic assay of human red blood cell thiopurine methyltransferase activity

An assay is described for the determination of red blood cell (RBC) thiopurine methyltransferase (TPMT) activity. TPMT S-methylates the antileukaemic drugs 6-mercaptopurine (6-MP) and 6-thioguanine and enzyme activity is inherited as a genetic trait. The assay is based on the TPMT-catalysed conversion of 6-MP to 6-methylmercaptopurine (methyl-MP) with non-radioactive S-adenosyl-L-methionine as the methyl donor. The methyl-MP metabolite is extracted into toluene as a phenyl-mercury adduct and back-extracted into 0.1 M HCl. Methyl-MP is quantitated by reversed-phase high-performance liquid chromatography (HPLC) with ultraviolet detection using a C18 Resolve cartridge and a mobile phase of methanol-water (20:80, v/v) with 100 mM triethylamine adjusted to pH 3.2 with orthophosphoric acid. There was a strong correlation between the HPLC assay and the established radiochemical assay (P < 0.0001). TPMT activities were measured by both methods in a population study of 111 children. There was no significant difference between the two frequency distribution histograms (P > 0.6).

An assay for O6-alkylguanine-DNA alkyltransferase based on restriction endonuclease inhibition and magnetic bead separation of products

We describe an improved, rapid and cost effective assay for measuring O6-alkylguanine-DNA alkyltransferase (AGT) levels in large numbers of small biological samples. The assay is based on the use of a synthetic O6-methylguanine oligonucleotide bound to magnetic beads via a biotin-streptavidin linkage and bound to its complement. A 35S label is incorporated into the free end of the duplex. The basis of the assay lies in the observation that the restriction enzyme PvuII will not cleave the bead-bound duplex oligonucleotide having an O6-methylguanine within the restriction sequence. However, on removal of the methyl group by AGT present in cell extracts prior to incubation with PvuII, the 35S-labelled oligonucleotide is cleaved by the restriction enzyme, releasing a 35S-labelled 8 bp fragment. Due to the stoichiometric nature of the AGT reaction, quantitation of AGT is easily achieved by counting an aliquot of the supernatant after pelleting the unrestricted bead complexes with a magnet. AGT levels measured in certain cell lines and human lymphocytes by the reported assay are comparable to other methods. The assay can be performed in basic laboratories and allows for the rapid processing of many samples simultaneously, which could prove useful in clinical and epidemiological studies.

D-penicillamine metabolism in neurodegenerative diseases: an in vivo/in vitro sulphydryl methylation study

1. D-Penicillamine (125 mg) was administered orally to control (healthy volunteers), Parkinson's and Motor Neurone Disease patients following an overnight fast. 2. Blood was collected at 08:00 h for the preparation of red blood cell membranes used in the in vitro S-methylation studies. Urine was collected from 08:00 to 16:00 h and analysed for D-penicillamine and its metabolites. 3. Metabolism occurred via S-methylation, N-acetylation and disulphide formation. Both the Parkinson's and Motor Neurone Disease patients excreted significantly higher median levels of S-methyl-D-penicillamine in the urine than the controls (177 and 209% more for the Parkinson's and Motor Neurone Disease patients respectively). 4. The in vitro and in vivo production of S-methyl-D-pencillamine was highly correlated in the control (rs = 0.936), Parkinson's (0.986) and Motor Neurone Disease (0.752) populations.

Purine enzymes in rheumatoid arthritis: possible association with response to azathioprine. A pilot study

OBJECTIVE

To study the possible association of purine enzyme activities with response to azathioprine (AZA) treatment in rheumatoid arthritis (RA) and their correlation with parameters of disease activity.

PATIENTS AND METHODS

Lymphocyte activities of hypoxanthine-guanine phosphoribosyl-transferase (HGPRT), adenine phosphoribosyltransferase (APRT), purine nucleoside phosphorylase (PNP) and 5'-nucleotidase (5NT), and erythrocyte activities of thiopurine methyltransferase (TPMT) were measured in 14 healthy controls and 36 patients with RA. Eight patients had not previously been treated with AZA. Response to AZA therapy in 28 patients, determined in a prospective trial, was considered good in nine (group 1), insufficient in seven (group 2). In 12 patients AZA was withdrawn because of adverse reactions (group 3). Disease activity parameters were obtained simultaneously with purine enzyme measurements. Purine enzyme levels in the different groups were compared.

RESULTS

Levels of 5NT activity were significantly lower in patients with RA than in healthy controls. PNP activity was higher in patients with RA not using prednisone compared with those who did and healthy controls. No clear correlation between purine enzyme levels and disease activity parameters was found. 5NT activities were significantly higher in group one than in group three (p = 0.012; alpha = 0.017), and almost significantly higher than in group two (p = 0.03; alpha = 0.017).

CONCLUSIONS

The results indicate that purine enzyme activities in patients with RA differ from healthy controls, are associated with the outcome of AZA treatment and seem not to be associated with disease activity. Our findings may offer a clue to predict the response to AZA therapy in RA.

Thiopurine methyltransferase activity in a French population: h.p.l.c. assay conditions and effects of drugs and inhibitors

1. Thiopurine methyltransferase (TPMT) is a cytosolic enzyme involved in the catabolism of thiopurine drugs, which are used to treat cancer patients and organ transplant recipients. Because TPMT activity is polymorphic and under genetic control, large interindividual variations in the immunosuppressive activity and toxicity of these drugs may, at least in part, be inherited. 2. We have developed a specific h.p.l.c. method for measuring 6-methyl mercaptopurine formed from 6-mercaptopurine (6-MP) in red blood cell lysates during the TPMT assay procedure. In blinded assays of 55 samples from adult blood donors, the results of the h.p.l.c. method correlated with those of the radiochemical reference method (r = 0.83, P < 0.001). 3. Using this h.p.l.c. assay, we tested the effect of known inhibitors of TPMT activity (syringic acid, p-anisic acid and tropolone) in vitro and showed that they were highly inhibitory. We also found that drugs often administered concomitantly with 6-MP (prednisone, prednisolone, 6-methylprednisolone, cyclophosphamide, methotrexate, and trimethoprim-sulphamethoxazole) had little or no effect on TPMT activity in vitro. 4. In a group of 300 French individuals, TMPT activity was highly variable, ranging from 4.7 to 35.3 nmol h-1 ml-1 of packed red blood cells (nmol h-1 ml-1 PRBC) with a mean value of 19.3 +/- 4.9. TMPT activity was not influenced by sex. 5. This sensitive and reproducible h.p.l.c. assay for TPMT activity in red blood cells may prove useful for prospective clinical studies designed to optimise dosage regimens of thiopurine drugs (detection limit for 6-methyl mercaptopurine is 5 ng ml-1, intra- and inter-assay variations are 6.8 and 8.2%, respectively).

Inactivation of O6-alkylguanine-DNA alkyltransferase in human peripheral blood mononuclear cells by temozolomide

O6-alkylguanine-DNA-alkyltransferase (ATase) activity was measured in extracts of peripheral blood mononuclear cells (PMCs) taken from eight patients at various times during 5 days of oral treatment with temozolomide (150 mg m-2, days 1-5). Pretreatment ATase levels ranged from approximately 70 to 600 fmol per mg of protein. Depletion of PMC ATase was seen within 4 h of the first dose of temozolomide and had a median nadir of 52.9% and values ranging from 44.4% to 71.0% of pretreatment levels. There was a correlation between the extent of ATase depletion (pretreatment minus nadir level) and the pretreatment ATase level (r = 0.97). A progressive depletion of ATase was observed during the 5 days of continuous temozolomide therapy with median ATase activities of 66.3%, 52.5%, 39.5%, 30.5% and 28.9% of the pretreatment values at days 2, 3, 4, 5 and 6 respectively. This suggests that the schedule-dependent anti-tumour activity of temozolomide seen in experimental models and clinics may be related to a cumulative depletion of ATase.

Thiopurine methyltransferase activity in American white subjects and black subjects

BACKGROUND

Thiopurine methyltransferase (TPMT) is a cytoplasmic enzyme that preferentially catalyzes the S-methylation of aromatic and heterocyclic sulfhydryl compounds, including 6-mercaptopurine. TPMT exhibits genetic polymorphism in white populations, with 89% of individuals having high TPMT activity, 11% having intermediate activity, and one in 300 having extremely low or absent activity. TPMT activity is inversely correlated with formation of active 6-mercaptopurine metabolites (thioguanine nucleotides), thereby influencing 6-mercaptopurine toxicity and efficacy.

METHODS

To investigate ethnic and gender differences in TPMT, we measured erythrocyte TPMT activity in 209 white healthy subjects and 196 black healthy subjects (202 women and 303 men).

RESULTS

The black population had lower TPMT activity than the white population (median, 14.4 versus 16.8 units/ml packed erythrocytes; p < 0.001). Maximum likelihood estimation of TPMT activity distribution identified 91.9% and 93.9% with high activity and 7.7% and 6.1% with intermediate activity in the white and black groups, respectively.

CONCLUSIONS

These data indicate that TPMT activity is similarly polymorphic in American black subjects and white subjects, although median TPMT activity is approximately 17% lower in black subjects.

Gender difference in red blood cell thiopurine methyltransferase activity

High red blood cell (RBC) thiopurine methyltransferase (TPMT) activity is associated with a higher relapse rate in children with acute lymphoblastic leukaemia on 6-mercaptopurine therapy. RBC TPMT activity is subject to genetic polymorphism and inter-ethnic variation. Higher TPMT activity in male subjects has previously been reported in RBC and liver tissue, but only in non-healthy subjects. In this healthy, drug-free study group the gender difference in the RBC TPMT high activity subgroup was confirmed with 8.3% higher TPMT activity in male subjects (n = 105).

Thiopurine S-methyltransferase activity in a Chinese population

Thiopurine S-methyltransferase is a cytosolic enzyme that catalyzes the S-methylation of thiopurine drugs. Although a genetic polymorphism has been recognized for this enzyme in populations of Caucasian descent, there has been scanty information about this polymorphism among Asians. In this study, we measured the erythrocyte thiopurine methyltransferase activity in 119 healthy Chinese subjects by a radiochemical assay. Methyltransferase activity was lower than what might have been expected for a white population. A bimodal frequency distribution was obtained that allowed the identification of four individuals with relatively low methyltransferase activity who may be heterozygotes for thiopurine S-methyltransferase deficiency; if so, the frequency of the mutant allele would be lower in this Chinese population than that observed in a white population (chi 2, p < 0.02). No gender-based differences were observed.

Thiopurine methyltransferase activity in a sample population of black subjects in Florida

Thiopurine methyltransferase (TPMT), is an enzyme detected in the human red blood cell that catalyzes the S-methylation of thiopurine drugs and is known to exist as a genetic polymorphism in white subjects. Investigations in this laboratory of red blood cell TPMT showed interethnic differences also existed in North American black subjects. A sample group of black subjects in Florida had a mean activity of 8.64 +/- 3.47 U/ml red blood cells and an antimode of 6.5 units, which represented values significantly lower than those obtained for both the mean activity and the antimode in other populations. The findings of this study suggest the possibility that TPMT activity may be under genetic control in North American black subjects and that this ethnic group may be at greater risk of experiencing thiopurine-induced toxicity caused by the lower overall mean activity of the enzyme.

Dosage and cycle effects of dacarbazine (DTIC) and fotemustine on O6-alkylguanine-DNA alkyltransferase in human peripheral blood mononuclear cells

There is increasing experimental evidence to suggest that endogenous expression of O6-alkylguanine-DNA-alkyltransferase (ATase) is a major factor in cellular resistance to certain chemotherapeutic agents including dacarbazine (DTIC). We have recently shown wide interindividual variation in the depletion and subsequent regeneration of ATase in peripheral blood mononuclear cells (PMCs) following DTIC and this has now been extended to ascertain whether or not depletion is related to dosage of DTIC used and repeated treatment cycles of chemotherapy. ATase levels were measured in three groups of 25 patients (pts) up to 24 h after receiving DTIC at 400 mg m-2, 500 mg m-2 or 800 mg m-2. Each group also received fotemustine (100 mg m-2), 4 h after DTIC. The lowest extent of ATase depletion (highest nadir ATase) was seen in patients receiving 400 mg m-2. The mean nadir ATase, expressed as a percentage of pre-treatment ATase, was respectively 56.3%, 26.4% and 23.9% for 400 mg m-2, 500 mg m-2 and 800 mg m-2. The median nadir of ATase activity for pts receiving 800 mg m-2 pts was at 4-6 h and for pts given lower doses it was at 2-3 h. In addition, repeated measures analysis of variance of observations before chemotherapy, then at 2, 3, 4, 6 and 18 h after chemotherapy provides some evidence that ATase was depleted to a lesser extent after cycle 1 than after subsequent cycles (P = 0.025). It also provides evidence that the change in ATase activity over time varied with dose and cycle. The findings can be interpreted on the basis of a dosage-dependent metabolism of DTIC to an agent capable of methylation of DNA and subsequent depletion of PMC ATase: with higher DTIC doses, the extent of ATase depletion may be limited by the pharmacokinetics of DTIC metabolism. PMC ATase was measured in another group of 8 pts at various times after receiving only fotemustine (100 mg m-2) and in contrast to DTIC, no ATase depletion was seen suggesting that insufficient concentrations of fotemustine and/or its metabolites were available to react with DNA to produce a depletion of PMC ATase activity.

Identification of factors regulating thiopurine methyltransferase activity in a Norwegian population

Red blood cell (RBC) thiopurine methyltransferase (TPMT), an inactivating pathway of 6-mercaptopurine, is controlled by genetic polymorphism and is subject to ethnic variation. RBC TPMT is a good predictor of clinical outcome in children with acute lymphoblastic leukemia. RBC TPMT activity was determined in 226 patients, 176 of them living in northern Norway (of which 123 were Saami (Lapps)). Demographic variables, use of drugs and presence of chronic diseases were evaluated as possible predictors of RBC TPMT activity by a multiple regression model. Men had higher RBC TPMT activity compared to women. Living in the northernmost county of Norway was associated with increased RBC TPMT activity irrespective of ethnicity. The use of diuretics was associated with increased RBC TPMT activity. The gender difference in RBC TPMT activity may indicate a need to treat male subjects more aggressively with thiopurine drugs compared to female subjects.

Cyclophosphamide decreases O6-alkylguanine-DNA alkyltransferase activity in peripheral lymphocytes of patients undergoing bone marrow transplantation

O6-alkylguanine-DNA-alkyltransferase (ATase) levels were measured in extracts of peripheral blood lymphocytes taken at various times during chemotherapy from 19 patients with various haematological malignancies. Seven patients with advanced Hodgkin's disease received preparative treatment consisting of cyclophosphamide (1.5 g m-2, daily) administered on days 1 to 4 and BCNU (600 mg m-2) on day 5 prior to autologous bone marrow rescue (ABMR) delivered on day 7. Treatment in the remaining 12 patients consisted of cyclophosphamide (1.8 g m-2, daily) given on days 1 and 2 followed at day 4 with total body irradiation (TBI) administered in six fractions over the subsequent 3 days to a total dose of 1200 cGy prior to bone marrow transplantation. In the Hodgkin's group, significant decreases in ATase activity were seen during the cyclophosphamide treatment, and the median ATase nadir was 32% (range 0% to 57%) of pretreatment levels following 4 days of cyclophosphamide. In one patient, no ATase activity was detectable following the 4th cyclophosphamide treatment. ATase activities decreased further after BCNU administration to a median of 19% (range 0% to 32%) of pretreatment levels. Extensive cyclophosphamide-induced reduction of lymphocyte ATase levels was also seen in the other group of 12 patients treated with cyclophosphamide/TBI: postcyclophosphamide median ATase nadir was 35% (range 12% to 78%) of the pretreatment levels. No ATase depletion was seen when cyclophosphamide (up to 10 mM) was incubated for 2 h with pure recombinant human ATase in vitro whereas ATase activity was reduced by 90% on preincubation with 100 microns acrolein or with greater than 1 mM phosphoramide mustard. This suggests that a cyclophosphamide-induced decrease in ATase levels in human peripheral lymphocytes in vivo may be due to depletion mediated by the production of intracellular acrolein. Since ATase appears to be a principal mechanism in cellular resistance to the cytotoxic effects of BCNU and related alkylating agents, these observations suggest that a cyclophosphamide-induced reduction in ATase activity may be an additional factor in the effectiveness of the combined sequential therapy.

The importance of thiopurine methyltransferase activity for the use of azathioprine in transplant recipients

The immunosuppressive efficacy of azathioprine is related to its rapid metabolism in vivo to 6-mercaptopurine (6MP), with subsequent conversion to thioguanine nucleotides by an anabolic route involving hypoxanthine-guanine phosphoribosyltransferase. Two alternative catabolic routes exist: oxidation to 6-thiouric acid via xanthine oxidase and methylation to 6-methylmercaptopurine via the enzyme thiopurine methyltransferase (TPMT). Catabolism via either route would restrict formation of the active metabolites. We analyzed TPMT activity in erythrocyte lysates of 25 controls, 25 uremic patients on dialysis, and 68 transplanted patients. Median activity was lower in controls (31.0 pmol/hr/mg Hb, range 16.2-43.0) and transplanted patients receiving only cyclosporine and prednisolone (31.7 pmol/hr/mg Hb, range 12.7-43.5) than in the azathioprine treated group, (36.1 pmol/hr/mg Hb, range 16.1-71.3), or the uremic group on dialysis, (35.5 pmol/hr/mg Hb, range 18.6-62.6) suggesting that both azathioprine and uremia induce the enzyme, but CsA does not. Only 3 patients demonstrated total intolerance to azathioprine, 2 of whom had very low TPMT activity (zero and 12.7 pmol/hr/mg Hb). The intolerance of the third patient, despite high TPMT activity, was attributed to concomitant cotrimoxazole therapy. Patients with intermediate activity (15-26 pmol/hr/mg Hb) could tolerate azathioprine well. Of 29 cadaver recipients given only azathioprine plus prednisolone, 24 with a better clinical outcome had a significantly lower activity (33.1 pmol/hr/mg Hb, range 16.1-46.1) than 5 with reduced allograft function (42.5 pmol/hr/mg Hb, range 33.8-51.5). TPMT activity in these 24 patients was also significantly lower than the general group of azathioprine-treated recipients. This inverse association between TPMT activity and allograft function was again found among 30 patients receiving triple therapy (azathioprine, CsA, prednisolone). Self-selection of the best recipients for azathioprine immunosuppression apparently occurred, based on low catabolism of the drug. We conclude that total intolerance to azathioprine is rare and usually appears in patients with very low TPMT activities. Our results also suggest that the wide range of TPMT activity may be an important factor in determining long-term graft survival in azathioprine-treated patients; those with high activity might benefit from doses near the upper limit generally recommended.

Interethnic difference in thiopurine methyltransferase activity

A number of metabolic pathways are subject to both genetic polymorphism and interethnic differences. A catabolic pathway of 6-mercaptopurine, red blood cell (RBC) thiopurine methyltransferase (TPMT) activity showed genetic polymorphism in Caucasians, but variation according to ethnicity has not been studied. We investigated if red blood cell thiopurine methyltransferase was subject to interethnic variation in a Saami (Lappish; n = 36) and a Caucasian population (n = 50). The Saami population sample had 29% higher thiopurine methyltransferase activity, 17.0 +/- 3.3 U/ml red blood cell compared with the Caucasian population sample, 13.1 +/- 2.9 U/ml red blood cell (p much less than 0.001). Probit plots and frequency distribution histograms supported bimodality consistent with genetic polymorphism in both study populations. Differences in chronic diseases, drug consumption, age, or gender could not explain the interethnic difference in red blood cell thiopurine methyltransferase activity. The higher red blood cell thiopurine methyltransferase activity in the Saami population group indicates that these subjects may require higher dosages of thiopurine drugs than Caucasians.

Reduced thiol methyl transferase activity in red blood cell membranes from patients with rheumatoid arthritis

Patients with rheumatoid arthritis (RA) have a reduced capacity for S-oxidation and formation of drug sulfate conjugates. We investigated S-methylation catalyzed by thiol methyl transferase (TMT) (E.C. 2.1.1.9) as an alternative pathway for metabolism of aliphatic compounds. TMT activity was measured in vitro using red blood cell membrane preparations from 120 patients with RA and 35 controls. Mean values for controls were 10.1 +/- 3 units/mg protein and for RA 3.7 +/- 3 units/mg protein (p less than 0.05). TMT activity was not related to the acute phase response or to drug administration. However, patients with RA with higher TMT activity tended to have higher rheumatoid factor levels. This evidence is consistent with a generalized disturbance of sulfur metabolism in rheumatoid disease.

Metabolic biomarkers of Parkinson's disease

Enzymic systems involved with metabolism of foreign chemicals, xenobiotics, have been studied in Parkinson's disease. Enzymes involved with sulphur oxidation and methylation are under-active. Cysteine levels are high and sulphate levels low. Differences in the activity of the enzyme monoamine oxidase-B are evident. Pathways involved with N-methylation of pyridines are different from controls leading to a rise in potentially toxic N-methylated derivatives.

Mechanism of ketotifen action in hypersensitivity reactions. Its effect on cellular enzymatic activities

In this study we observed that asthmatics had less methyltransferase activity and greater phosphodiesterase activity than healthy individuals. These enzymatic activities were nearer to values obtained in healthy individuals when we preincubated cells with ketotifen. The modulator effect of this drug on these two enzymes permits, on the one hand, to re-establish the beta-receptor numbers expressed on the membrane, and on the other hand, to inhibit mediator secretion provoked by antigenic stimulus. With its action on adenylate cyclase and phosphodiesterase activities, it allows cAMP intracellular accumulation and hinders the secretory process. Through its action on methyltransferase activity, it is responsible for the normalization of beta-receptor expression observed in asthmatic patients treated with ketotifen.

In vivo formation and repair of O6-methylguanine in human leukocyte DNA after intravenous exposure to dacarbazine

Blood leukocyte DNA obtained from 11 Hodgkin's disease patients undergoing ABVD chemotherapy was analysed for the presence of the precarcinogenic adduct O6-methylguanine (O6-meG) at various times (1-2 h up to 49 h) after i.v. treatment with the methylating drug dacarbazine. Adduct formation was detected in all but one of the patients examined at levels ranging up to 0.45 fmol/micrograms DNA (7.2 x 10(-7) mol/mol guanine). The levels of the adduct decreased by approximately 30% over the 24 h following exposure and were usually not detectable 49 h after exposure. In five out of seven individuals examined after more than one treatment, consistent methylation responses were noted, while in the remaining two cases the responses were mixed. No correlation between the extent of adduct formation and lymphocyte levels of the repair enzyme O6-alkylguanine-DNA alkyltransferase was observed. The average extent of O6-meG formation 1 h after dacarbazine treatment was (4.3 +/- 3.1) x 10(-2) fmol/micrograms DNA per mg/kg dose [( 1.2 +/- 0.8) x 10(-3) fmol/micrograms DNA per mg/m2 dose)]. Following exposure of rats to similar doses of dacarbazine, the corresponding levels of adduct in blood leukocyte DNA were 1.1 x 10(-2) fmol/micrograms DNA per mg/kg dose (2.6 x 10(-3) fmol/micrograms DNA per mg/m2 dose).

Effect of ketotifen on the methyltransferase activity of asthmatic patients

The results of this study have revealed that asthmatic individuals have less methyltransferase activity, compared to healthy individuals. This is reflected in a less fluidity of the plasmatic membrane and thus, presents with less beta-receptors compared to healthy subjects. Ketotifen is capable of partially reversing this situation where it stimulates methyltransferase activity of asthmatic individuals approaching the enzyme activity to healthy individuals.

Genetic variation in response to 6-mercaptopurine for childhood acute lymphoblastic leukaemia

6-mercaptopurine (6-MP) can be inactivated by S-methylation, which is catalysed by thiopurine methyltransferase (TPMT). An alternative metabolic route leads to the formation of cytotoxic 6-thioguanine nucleotides (6-TGN). To investigate whether these two pathways compete with each other to affect the therapeutic response to 6-MP, 6-TGN concentrations and TPMT enzymatic activity were measured in erythrocytes (RBC) from 95 children on long-term 6-MP therapy for lymphoblastic leukaemia (ALL). RBC TPMT activities were also measured in 130 control children and 104 long-term survivors of ALL no longer on treatment. The 95 children on 6-MP showed wide interindividual differences in RBC 6-TGN concentrations at the full protocol dose of 75 mg/m2, and RBC 6-TGN concentrations correlated negatively with RBC TPMT activity. Children with 6-TGN concentrations below the group median had higher TPMT activities and a higher subsequent relapse rate. 50 of the 104 long-term survivors had been treated with "gentle" low-dose protocols, and this subgroup contained an excess of children with lower TPMT activities compared with normal controls. These results indicate that genetically determined TPMT activity may be a substantial regulator of the cytotoxic effect of 6-MP, an effect which in turn could be important in influencing the outcome of therapy for childhood ALL.

Cyclic nucleotide phosphodiesterases and methyltransferases in purified lymphocytes, monocytes and polymorphonuclear leucocytes from healthy donors and asthmatic patients

Cyclic nucleotide phosphodiesterase and phospholipid N-methyl-transferase activities were simultaneously measured in purified polymorphonuclear cell-, mononuclear cell-, lymphocyte- and monocyte-homogenates from control subjects, from patients with atopic asthma and from patients with non-atopic asthma. Whereas cyclic AMP and cyclic GMP phosphodiesterase activities were found to be about 10-fold lower in polymorphonuclear than in mononuclear cells, phospholipid N-methyltransferase proved to be rather similar in each cell type from control donors. Cyclic AMP phosphodiesterase and phospholipid N-methyltransferase were significantly decreased in polymorphonuclear cells and monocytes from asthmatic patients compared with the control group while cyclic GMP phosphodiesterase was significantly impaired only in the monocyte subpopulation.

Identification of isoaspartyl-containing sequences in peptides and proteins that are usually poor substrates for the class II protein carboxyl methyltransferase

We have found that a chicken egg lysozyme derivative (beta-101-lysozyme) containing an L-isoaspartyl residue at position 101 has a Km for methylation by the human erythrocyte L-isoaspartyl/D-aspartyl protein methyltransferase (EC 2.1.1.77) of 183 microM, about 30 times higher than that expected from previous studies with isoaspartyl-containing peptides. In the course of investigating the reasons for this poor enzyme recognition, we found that charged residues on the carboxyl side of isoaspartyl residues had a large effect on the affinity of the enzyme for synthetic peptides. This is best illustrated by the lysozyme-related peptide YVSisoDGDG, which has a Km for methylation of 469 microM. When the penultimate aspartyl residue is replaced by a cysteinyl residue, the Km drops to 4.6 microM, comparable to other peptides of similar size. Furthermore, replacing it with a cysteic acid residue results in a Km of 104 microM, suggesting that a negative charge at this position may lead to a weaker affinity of the peptide substrate for the methyltransferase. Assays with additional synthetic peptides indicate that moving the negative charge to the first or third residue on the carboxyl side of the isoaspartyl residue has a similar but less severe effect in reducing its affinity for the methyltransferase. Enzymatic methylation has recently been proposed to be the first step in the conversion of abnormal isoaspartyl residues to aspartyl residues. The results reported here, however, along with previous evidence that protein tertiary structure can inhibit isoaspartyl methylation, suggest that only a subclass of damaged sites are capable of efficiently entering a putative repair pathway; the sites not recognized by the methyltransferase may accumulate in vivo.

Distribution of methylated histones and histone methyltransferases in chicken erythrocyte chromatin

The relationship between histone methylation and the transcriptionally active chromatin state was investigated. Immature chicken erythrocytes, which were obtained from the peripheral blood of anemic birds, were incubated with L-[methyl-3H]methionine and cycloheximide. Under these conditions histones H3 and H4 are methylated. The erythrocyte nuclei were incubated with micrococcal nuclease, and the chromatin fragments were fractionated according to their solubility in EDTA and 0.15 M NaCl. Chromatin fractions, which were enriched in transcriptionally active genes, were enriched in methylated histones. Moreover, the acetylated species of histones H3 and H4, which are complexed with active genes (Hebbes, T. R., Thorne, A. W., and Crane-Robinson, C. (1988) EMBO J. 7, 1395-1402), were preferentially methylated. The methylation of these histones was not dependent on ongoing transcription. The distribution of histone H3 methyltransferase activity among the various chromatin regions was also studied. This enzyme activity was greatest for the chromatin fragments that were enriched in active/competent genes. However, our results suggest that histone H3 methyltransferase is bound to the nucleosome. The enzyme, which may be localized in the active gene chromatin domains, may ensure that the histones associated with active genes are methylated. Histone methylation, which has a slow turnover rate, may contribute to the maintenance of the transcriptionally active chromatin state.

O6-methylguanine-DNA methyltransferase in lymphocytes of the elderly with and without Alzheimer's disease

Deficiency in DNA repair has been linked to aging, mutagenesis, carcinogenesis and several types of primary neuronal degeneration. O6-Methylguanine-DNA methyltransferase is a key enzyme in the repair of DNA alkylation damage that removes a methyl group from the O6 position of methylguanine. This study was carried out to determine whether there were any changes in the activity of this enzyme in lymphocytes of patients with Alzheimer's disease (AD) as compared to lymphocytes of age-matched non-demented elderly. The transferase activity in lymphocytes from 19 elderly patients with AD (mean 87.7 fmole/100 micrograms protein +/- SD 44.7) was not statistically different from that in 19 age/sex-matched controls (mean 91.3 fmole/100 micrograms protein +/- SD 40.0). There was no significant trend with age in transferase activity and the activity levels in the elderly subjects studied were the same as those reported previously for younger individuals by this laboratory. It is concluded that a reduction in O6-methylguanine-DNA methyltransferase activity is unlikely to be involved in the etiology or the pathogenesis of AD.

Specificity of endoproteinase Asp-N (Pseudomonas fragi): cleavage at glutamyl residues in two proteins

Endoproteinase Asp-N, a metalloprotease from a mutant strain of Pseudomonas fragi, has been reported to specifically cleave on the N-terminal side of aspartyl and cysteic acid residues. We utilized this enzyme to generate fragments for determining the amino acid sequence of the D-aspartyl/L-isoaspartyl methyltransferase isozyme I from human erythrocytes. Surprisingly, we identified cleavage sites for this enzyme at the N-terminal side of several glutamyl residues in addition to the expected cleavage sites at aspartyl residues. The ability of this enzyme to cleave polypeptides at both glutamyl and aspartyl residues was confirmed by mapping additional sites on erythrocyte carbonic anhydrase I. These results indicate that a more appropriate name for this enzyme may be Endoproteinase Asp/Glu-N.

S-methylation in motorneuron disease and Parkinson's disease

Thiolmethyltransferase activity has been measured in newly diagnosed, untreated patients with idiopathic Parkinson's disease and motorneuron disease, and in normal volunteers. In Parkinsonian patients, mean thiolmethyltransferase activity was low (300 U/mg protein [SD 96]) compared with that in controls (947 [409]), whereas activity was high in patients with motorneuron disease (2077 [825]).

Pharmacogenetics of acute azathioprine toxicity: relationship to thiopurine methyltransferase genetic polymorphism

Azathioprine therapy can cause acute myelosuppression. Toxicity is in part caused by the incorporation of azathioprine-derived 6-thioguanine nucleotides (6-TGN) into deoxyribonucleic acid (DNA). The enzyme thiopurine methyltransferase (TPMT) plays an important role in azathioprine catabolism. TPMT activity is controlled by a common genetic polymorphism, and one in 300 subjects has very low enzyme activity. Azathioprine was withdrawn in five study patients because of acute myelosuppression. The duration of azathioprine treatment was 21 to 70 days (median, 28), and the daily oral dose was 1.0 to 2.5 mg/kg. Sixteen control patients who had been taking oral azathioprine (1.1 to 2.0 mg/kg daily for more than 6 months) with no history of myelosuppression were studied. All subjects had normal liver and kidney function. When compared with the control group, the five patients with myelosuppression had very low TPMT activities and abnormally high 6-TGN concentrations. Inherited low TPMT activity appears to be a major risk factor for acute azathioprine-induced myelosuppression.

Phosphatidylethanolamine N-methyltransferase in human red blood cell membrane preparations. Kinetic mechanism

The successive methylations of phosphatidylethanolamine to form phosphatidylcholine were measured using exogenously added intermediates and membrane preparations from human red blood cells. The addition of phosphatidylethanolamine resulted in no increase in methylation rate over that with endogenous substrate; however, the addition of monomethylphosphatidylethanolamine (PME) and dimethylphosphatidylethanolamine (PDE) markedly increased the reaction rate and allowed studies into the kinetic mechanism for the second and third methylation reactions. The data are consistent with catalysis of the last two methylations being by a single enzyme with a random Bi-Bi sequential mechanism. Analysis of PDE:phosphatidylcholine product ratios indicates that the enzyme can conduct multiple methylations of enzyme-bound phospholipid. The nature of the acyl chain (16:0 versus 18:1) of the phospholipid had only a small effect on the value of the kinetic constants. The maximal velocities obtained with the 18:1 substrate were less than 5% lower than those obtained with the 16:0 substrate. The Km values for the two phospholipids were 20-45 and 10-14 microM for the methylation of PME and PDE, respectively. The Km for S-adenosylmethionine (AdoMet) was 5-9 microM with PME and 4 microM with PDE as substrates. Depending on the acyl chain and the phospholipid, the Ki(AdoMet) varied from 8 to 19 microM, the Ki(PME) from 41 to 82 microM, and the Ki(PDE) from 35 to 61 microM. The Ki for S-adenosylhomocysteine (AdoHcy) was between 1.0 and 1.4 microM depending upon the variable substrate. The endogenous concentrations of PME and PDE in red blood cell membranes were estimated to be 0.49 and 0.24 mumol/liter packed cells, respectively. The product from the utilization of AdoMet, S-adenosylhomocysteine (AdoHcy), was shown to be a competitive inhibitor of its precursor, AdoMet, and a noncompetitive inhibitor of the two phospholipid substrates.

Chromosomal sensitivity of lymphocytes from individuals with therapy-related acute nonlymphocytic leukemia

A small fraction of those individuals exposed to cytotoxic chemotherapy or radiation for the treatment of a primary malignant disease will develop a second malignancy some time later. Although exposure to the cytotoxic agents is believed to be the causative factor, the reason only certain individuals develop the second malignancy is unknown. Some studies have suggested that these individuals might be predisposed to cancer because of an inherent sensitivity to the alkylating agents used in cancer therapy. We have reported that these individuals with therapy-related acute nonlymphocytic leukemia (t-ANLL) have reduced endogenous levels of the repair protein O6-alkylguanine alkyltransferase (AGT). To further investigate the etiology of this disease, alkylation-induced sister-chromatid exchange (SCE) formation in individuals who developed second malignancies, was compared to other patient groups and normal controls. Peripheral blood lymphocytes from patients with (1) t-ANLL, (2) primary forms of acute nonlymphocytic leukemia (ANLL de novo), (3) patients with primary malignancies at risk of developing secondary disease, and (4) unexposed, healthy controls were treated in vitro with N-methyl-N'-nitro-nitrosoguanidine or mitomycin C. Baseline and mutagen-induced frequencies of SCEs were determined. These studies failed to detect any increased sensitivity in those patients who developed second malignancies as compared to controls or patients with de novo forms of the same disease. Also, no correlation between sensitivity to the alkylating agent N-methyl-N'-nitro-nitrosoguanidine and endogenous levels of the AGT repair protein was found. These results suggest that t-ANLL patients are not sensitive to SCE induction by either MNNG or MMC.(ABSTRACT TRUNCATED AT 250 WORDS)

Low O6-alkylguanine DNA alkyltransferase activity in the peripheral blood lymphocytes of patients with therapy-related acute nonlymphocytic leukemia

Chemotherapeutic agents such as procarbazine, which produce methylated bases in DNA, are used to treat many Hodgkin's disease (HD) and non-Hodgkin's lymphoma (NHL) patients. A small proportion of such patients develop secondary malignancy. We examined the possibility that those patients who develop secondary malignancy have low endogenous levels of O6-alkylguanine DNA alkyltransferase (AGT) activity and are therefore more sensitive to the mutagenic and carcinogenic effects of their treatment. We assayed AGT activity in peripheral blood lymphocytes from patients with HD, NHL, acute nonlymphocytic leukemia (ANLL) de novo, and therapy-related ANLL, as well as a group of normal control subjects. Studies in normal controls showed that at least over a short term of 1 week, individuals have characteristic AGT levels, although some individuals sampled repeatedly over several months showed high variation. Mean AGT activities +/- SE for the various groups studied were (fmol/micrograms of DNA): normal control group, 7.05 +/- 0.36; HD and NHL patients (prior to treatment), 4.97 +/- 0.42; HD-NHL patients receiving procarbazine, 3.88 +/- 0.44; ANLL de novo, 7.78 +/- 1.72; and therapy-related ANLL, 4.30 +/- 0.58. AGT activity decreased in the peripheral blood lymphocytes of some individuals taking procarbazine. The mean AGT activity in the procarbazine-treated patients was low, as was the activity for the therapy-related ANLL patients.

Methyltransferase and phospholipase A2 activity in the cell membrane of neutrophils and lymphocytes from patients with Behçet's disease, systemic lupus erythematosus, and rheumatoid arthritis

Phospholipid methylation and phospholipase A2 activation in the membrane of neutrophils and lymphocytes, which participate in the induction of cell activation, were assessed in patients with Behçet's disease, systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA). [3H-methyl] incorporation and phospholipase A2 activity of neutrophils from active cases of Behçet's disease and RA were significantly increased compared with normal controls. In lymphocytes from the patients with active Behçet's disease and RA, a significant increase in methyltransferase activity and a marked enhancement of phospholipase activity were found. A modest increase in these two membrane phospholipid enzyme activities was observed in lymphocytes of patients with active SLE. In addition, these enzyme activities were significantly enhanced in normal leukocytes preincubated with serum from patients with active SLE and malignant RA. The potentiated functions of neutrophils and lymphocyte abnormalities in the patients tested thus seem to be at least partly due to an increase in these enzymatic activities in the cell membrane.

Human lymphocyte thiopurine methyltransferase pharmacogenetics: effect of phenotype on 6-mercaptopurine-induced inhibition of mitogen stimulation

Thiopurine methyltransferase (TPMT) catalyzes the S-methylation of 6-mercaptopurine (6-MP) and other thiopurine drugs. The level of TPMT activity in human tissue is controlled by a common genetic polymorphism. Our experiments were performed to study the relationship between TPMT phenotype and the effect of 6-MP on [3H]thymidine ([3H]TdR) incorporation into mitogen-stimulated human peripheral blood lymphocytes. Lymphocytes were isolated from blood samples obtained from 12 subjects, four each with each of the three classes of TPMT phenotype. The effect of 6-MP concentration on [3H]TdR incorporation was determined in the presence of two mitogens, phytohemagglutinin at concentrations of 1 and 10 micrograms/ml and concanavalin A at concentrations of 1 and 5 micrograms/ml. ED50 values for 6-MP inhibition of [3H]TdR incorporation into lymphocytes from subjects who genetically lacked TPMT activity were uniformly higher than were ED50 values for lymphocytes from subjects with intermediate or high enzyme activities. However, in the presence of either mitogen, ED50 values decreased as the level of stimulation increased. Therefore, to make it possible to account for degree of mitogen stimulation, the effect of 6-MP on [3H]TdR incorporation was studied in the presence of a series of phytohemagglutinin concentrations from 1 to 10 micrograms/ml. Lymphocytes from subjects who genetically lacked TPMT activity had significantly higher Ki values (1.37 +/- 0.340 microM, mean +/- S.E.M., n = 3) for inhibition of [3H]TdR incorporation by 6-MP than did lymphocytes from subjects with intermediate or high enzyme activities (0.529 +/- 0.068 and 0.327 +/- 0.064 microM, respectively, P less than .05 for both comparisons, n = 3 in both cases).(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphatidylethanolamine methyltransferase and cAMP, cGMP phosphodiesterases in lymphocytes and monocytes in sarcoidosis

Among the various hypotheses proposed to explain immune cell defect in sarcoidosis, we examined thoroughly that of Faguet who described abnormalities of signal transmission at lymphocyte membrane level. Phosphatidylethanolamine methyltransferase and cAMP cGMP phosphodiesterases were studied in blood lymphocytes and monocytes from 8 subjects with sarcoidosis disease. Phosphatidylethanolamine methyltransferase (PMT1) plays an important regulatory role in membrane signal transmission. cAMP and cGMP phosphodiesterases (PDE) regulate cytoplasmic cyclic nucleotide levels and so participate in the modulation of the cell cycle. We observed a decreased PMT1 activity in lymphocytes and monocytes and a decreased cAMP and cGMP PDE activities in monocytes. It is not now possible to say if these abnormalities are primary or secondary. Whatever the origin of this dysfunctioning, these results evoke simultaneous disturbances of membrane signal transmission and cell cycle in monocytes and membrane abnormalities in lymphocytes. These abnormalities could explain some immune cell defects in sarcoidosis disease.

Multiple species of Bacillus subtilis DNA alkyltransferase involved in the adaptive response to simple alkylating agents

Three molecular species of methyl-accepting proteins exist in Bacillus subtilis cells, which collect methyl groups from methylated DNA. A 20-kilodalton (kDa) protein was constitutively present in the cells of the ada+ (proficient in adaptive response) strain as well as in those of six ada (deficient in adaptive response) mutant strains and was assigned to the O6-methylguanine:DNA methyltransferase. Another species of O6-methylguanine:DNA methyltransferase, which had a molecular size of 22 kDa, emerged after adaptive treatment of the ada+ but not any of the ada mutant cells. A 27-kDa methyl-accepting protein, which preferred methylated poly(dT) to methylated calf thymus DNA as a substrate, was assigned to the methylphosphotriester:DNA methyltransferase. It was produced, after adaptive treatment, in the cells of ada+, ada-3, ada-4, and ada-6 strains but not in the cells of ada-1, ada-2, or ada-5 strains. These results support and extend our proposition that ada mutants can be classified into two groups; one (the ada-4 group) is defective only in the inducible synthesis of O6-methylguanine:DNA methyltransferase (22-kDa protein), and the other (the ada-1 group) is deficient in the adaptive response in toto. The finding that inducible and constitutive methyltransferases reside in different molecular species of methyl-accepting proteins is intriguing compared with the regulatory mechanisms of the adaptive response to simple alkylating agents in other organisms.

Lymphocyte and granulocyte phosphatidylethanolamine N-methyltransferase: properties and activity in cystic fibrosis

Human lymphocyte and granulocyte membranes contain an enzyme, phosphatidylethanolamine N-methyltransferase (PEMT), which catalyzes the transfer of a methyl group from S-adenosylmethionine to the polar head group of phosphatidylethanolamine to form phosphatidylmonomethylethanolamine. This enzyme, in lymphocyte membranes, has Km for S-adenosylmethionine of 7.01 +/- 2.9 (SD) microM, and specific activity 0.57 +/- 0.31 pmol/mg protein/15 min, is inhibited by S-adenosylhomocysteine, displays optimal activity at pH 8.0-9.0, and is stimulated by isoproterenol in dose-dependent, propranolol-inhibitable fashion, to a lesser extent by epinephrine, but not by norepinephrine, prostaglandin E1, concanavalin A, or adenosine 3':5' cyclic monophosphate, with or without phosphodiesterase inhibitors. Granulocyte membrane PEMT has Km for S-adenosylmethionine of 4.4 microM and specific activity 0.54 +/- 0.51 pmol/mg protein/15 min, is inhibited by S-adenosylhomocysteine, displays optimal activity at pH 8.0-9.5, and is stimulated by isoproterenol greater than epinephrine greater than norepinephrine, but not by prostaglandin E1, serum-treated zymosan, formyl-methionyl-leucyl-phenylalanine, or adenosine 3':5' cyclic monophosphate. Because activation of PEMT reportedly contributes to several processes known to be abnormal in cystic fibrosis, including coupling of the beta-adrenergic receptor to adenylate cyclase, activity of PEMT was compared in lymphocyte and granulocyte membrane preparations from cystic fibrosis patients and healthy controls, in which abnormal coupling of beta-adrenergic receptor to adenylate cyclase had been demonstrated. For both cell types, the Km and specific activity of PEMT were comparable in normal and cystic fibrosis samples. Therefore, the hypothesis that reduced PEMT activity accounts for the impaired coupling of beta-adrenergic receptor to adenylate cyclase in lymphocytes and granulocytes in cystic fibrosis is rejected.

Methionine synthetase activity of human lymphocytes both replete in and depleted of vitamin B12

The activity of the enzyme methionine synthetase (MS) (methyltetrahydrofolate:homocysteine methyltransferase) (EC 2.1.1.13) was measured in human lymphocytes of various types and cobalamin (vitamin B12) status. Total and holo MS activity was low in unstimulated peripheral blood lymphocytes from persons with tissue deficiency of cobalamin, but not in cells from those with low serum cobalamin levels for other reasons. The MS activity of the lymphocyte was increased by treatment of the patients with vitamin B12. The number of lymphocytes was often low or low normal in the circulation of those deficient in cobalamin. Holo MS activity was low in an established line of human B cells, RPMI 6410 cells, depleted of cobalamin. The total and holo MS activity of both RPMI 6410 cells, replete or depleted, and lymphocytes stimulated in culture was increased by cobalamin in vitro; 222 nmol/L free cobalamin was roughly the equivalent of 0.22 nmol/L cobalamin bound to transcobalamin II. Both lymphocytes and RPMI 6410 cells required folate for growth and could meet these needs via methylfolate, homocysteine, and the cobalamin-dependent MS reaction. Depleted RPMI 6410 cells, however, used cobalamin in some way in addition to the provision of available folate from methylfolate. The consequences of the reduced MS activity in deficient cells could include a reduction in available folate with diminished capacity for clonal expansion of lymphocytes in reaction to infection and impairment of essential methylations including those of protein synthesis. The prompt induction of MS activity by cobalamin, especially in the in vitro model, suggests an effect of therapeutic vitamin B12 well in advance of the numerical increase in cells of the blood.