Identification of N-acetylcysteine as a new substrate for rat liver microsomal glutathione transferase. A study of thiol ligands

N-Acetyl-L-cysteine serves as an efficient substrate for the rat liver microsomal glutathione transferase with 1-chloro-2,4-dinitrobenzene as second substrate (8.8 +/- 0.37 mumol/min mg). The activity is actually higher than that obtained with glutathione (2-4 mumol/min mg). In examining the activity of liver subcellular fractions, no activity with N-acetyl-L-Cys could be detected in dialyzed or N-ethylmaleimide-treated (in order to remove endogenous glutathione) cytosol. The activity in rat liver microsomes was 0.11 +/- 0.007 mumol/min mg, which is accounted for by the content of microsomal glutathione transferase. Thus, N-acetyl-L-Cys can be used as a specific substrate for determining the conjugating activity of microsomal glutathione transferase. N-Acetyl-L-Cys was also shown to function as a substrate for the enzyme when other second substrates than 1-chloro-2,4-dinitrobenzene (with varying electrophilicity) are used. The pH dependence of microsomal glutathione transferase was studied. The kcat/Km(1-chloro-2,4-dinitrobenzene) was dependent on pH with an apparent pKa of 6, > or = 9, and > or = 8 with saturating glutathione, gamma-L-Glu-L-Cys, and N-acetyl-L- cysteine, respectively. Apparently the enzyme has the ability to lower the pKa of glutathione by 3 orders of magnitude. The kcat/Km(thiol) did not vary appreciably with pH (except for N-acetyl-L-cysteine), indicating that no rate-determining deprotonation occurs on the enzyme itself between pH 5.5 and 9. The abilities of histidine-, lysine-, and arginine-selective reagents to inactivate the enzyme when N-acetyl-L-cysteine and gamma-L-Glu-L-Cys were used as substrates were investigated. The activity toward N-acetyl-L-cysteine was decreased considerably less after treatment with the arginine-selective reagent phenylglyoxal, as compared to the activity toward GSH and gamma-L-Glu-L-Cys. This indicates that an arginine makes contact with gamma-L-Glu residue in GSH. With the other reagent/substrate combinations tested the enzyme was inactivated almost completely. The ability of microsomal glutathione transferase to stabilize the Meisenheimer complex formation between 1,3,5-trinitrobenzene and various glutathione analogues, including non-substrate thiols, has been examined. It is shown that, in general, substrates exhibited higher formation constants (approaching 50 mM-1) than non-substrates (4.5 +/- 1.7 mM-1, n = 7), whereas simpler thiols did not yield enzyme-bound complexes. The fact that the enzyme can stabilize Meisenheimer complexes from non-substrate thiol analogues of glutathione offers new possibilities for examining the substrate interactions of glutathione transferases.

Baseline measures and outcome predictions

This paper reviews the methodological issues in the epidemiological study of the outcome from stroke. Data are presented from an unselected series of patients in whom the underlying stroke pathology is clearly defined. Although the natural history varies among the different pathological subtypes of stroke, simple clinical baseline measures of the severity of the neurological deficit (incontinence, loss of consciousness and severity of paresis) and premorbid level of disability and social functioning independently predict disability-free survival by 1 year, and may help direct management and research.

MD candidates want better training in research

A survey of MD candidates in one teaching hospital and two postgraduate institutes was undertaken to determine their initial training needs, and their preferred methods of learning about the conduct of research. Fifty-six respondents (53% response rate) replied to a piloted questionnaire. Their replies indicated a need for intensive initial research training, offered at local level by the supervisor. The supervisor's subsequent role throughout the research period should be to provide ongoing specific critical help and support. It is concluded that teaching materials and designed teaching sessions might be provided for supervisors to assist them in their new role.

A loading-dose strategy for converting from oral to depot haloperidol

OBJECTIVE

The author's aim was to evaluate the safety and efficacy of a loading-dose regimen for initiating use of a depot medication, haloperidol decanoate, with patients who had been maintained on oral haloperidol. Patients were given a loading dose of about 20 times their oral maintenance dose in divided injections during the first two weeks of conversion to depot medication. The dose of haloperidol decanoate was gradually reduced, dropping to about ten times the oral dose in the third and fourth months. No supplemental oral medication was used.

METHODS

Haloperidol decanoate was initiated using the loading-dose regimen in 16 chronically ill patients. Lower initial doses of haloperidol decanoate were used in two other groups of patients, one that received supplemental oral haloperidol and one that did not. Plasma levels of haloperidol, severity of illness, and side effects were monitored from baseline to 56 days after the beginning of depot therapy.

RESULTS

Patients who received the loading-dose regimen showed statistically significant clinical improvement and reduced side effects over baseline by the 28th day. The second group of patients also maintained therapeutic response but improved no further. The third group relapsed during the first month and were returned to a regimen of oral haloperidol by the second month.

CONCLUSIONS

A loading-dose regimen for initiating treatment with haloperidol decanoate is safe and effective and can be useful in a clinical setting.

Teaching self-feeding skills to patients with Rett syndrome

Few studies have examined the extent to which functional hand-use may be taught to girls with Rett syndrome after those skills have been lost. In this investigation, five females with classic Rett syndrome were taught to feed themselves using a method of prompting and reinforcement. All patients demonstrated improvements in their self-feeding skills. The results are discussed in terms of the factors that may influence the acquisition of skills in girls with Rett syndrome, and directions for future research and proposed.