38 research outputs found
Microsomal glutathione transferase : catalysis, in vitro mutagenesis and heterologous expression
Microsomal glutathione transferase is a membrane-bound member of the
glutathione transferases, a
family of multifunctional enzymes involved in the cellular detoxification
of xenobiotics and reactive
endogenous compounds formed during oxidative stress. Rat liver microsomal
glutathione transferase is a
trimeric protein with a molecular mass of 17.3 kDa per subunit which
displays unique features regarding
DNA and amino acid sequence, molecular weight, enzymatic properties and
ability of activation by
various agents as compared to its cytosolic counterparts. At the outset
of this work we were interested in
obtaining: specific substrates and expression systems for the enzyme.
These experimental tools combined
with in vitro mutagenesis were used to obtain information on the
functional role of individual amino
acids as well as membrane topology.
N-acetyl-L-cysteine was found to serve as a substrate for the microsomal
glutathione transferase
(with l-chloro-2,4-dinitrobenzene (CDNB) as second substrate). In
examining the activity of liver
subcellular fractions, no activity with N-acetyl-L-cysteine could be
detected in cytosols devoid of
endogenous glutathione. Thus, N-acetyl-L-cysteine is a specific substrate
for microsomal glutathione
transferase. The pH dependence of kCat/Km(CDNB) for the microsomal
glutathione transferase with
different thiol substrates indicates that the enzyme has the ability to
lower the pKa of bound glutathione
by 3 orders of magnitude. The microsomal glutathione transferase
stabilizes Meisenheimer complex
formation between 1,3,5-trinitrobenzene and various glutathione
analogues, including some non-substrate
thiols, thus offering new possibilities for examining ligand interactions
of glutathione transferases.
Rat liver microsomal glutathione transferase was successfully expressed
both in mammalian COS-
cells and in E. Coli BL 21 (DE3). Significant amounts of enzymatically
active protein was expressed in
the inner membrane of this E. Coli strain. Recombinant rat microsomal
glutathione transferase was
purified from bacterial membranes and was found to be indistinguishable
from the liver enzyme with
regard to enzymatic activity, molecular mass, immunoreactivity and
N-terminal amino acid sequence.
Chemical modification of rat liver microsomal glutathione transferase
indicated that arginine 107
and Iysine 67 are essential for enzyme activity and may thus reside in
the active site. A set of mutant
forms of the rat enzyme were constructed by site-directed mutagenesis and
heterologously expressed in
E. coli BL21(DE3). Arginine 107 was exchanged for alanine and Iysine
residues. The alanine mutant
(R107A) displayed a decreased thermostability and an important structural
role is suggested for this
residue. Neither mutation of Iysine 67 to alanine and arginine nor
replacement of the three histidines by
glutamines yielded any drastic changes of activity in contrast to the
chemical modification experiments.
All tyrosine to phenylalanine substitutions resulted in mutants with
activities similar to the wild type.
Thus, the microsomal glutathione transferase must perform an alternate
stabilization of the thiolate anion
of glutathione than through interaction with a phenolic hydroxyl group of
a tyrosine. Substitution of
cysteine 49 with alanine resulted in a semi-activated mutant enzyme which
was not affected by N-
ethylmaleimide. Cysteine 49 is therefore unambiguously demonstrated as
the site of modification that
results in activation of microsomal glutathione transferase.
The membrane topology of rat liver microsomal glutathione transferase was
investigated by in situ
proteolysis of intact and permeabilized rat liver microsomes. Lysine 4 is
accessible at the luminal surface
of the endoplasmic reticulum, whereas Iysine 41 faces the cytosol. These
positions are separated by a
hydrophobic stretch of 25 amino acids which comprises a likely
membrane-spanning region. The
cytosolic location of the active site was demonstrated using radiolabeled
glutathione. Additional
membrane anchoring(s) are indicated since the C-terminal part of the
trypsin-cleaved protein was not
separated from the membrane fraction by intensive salt washing or phase
separation in Triton X-l 14.
Cleavage of the purified protein at Iysine 41 and subsequent separation
of the fragments yielded a
catalytically competent C-terminal polypeptide.
The topology of recombinant rat microsomal glutathione transferase
expressed in E. coli was
investigated by comparing the proteolytic cleavage products from intact
and permeabilized spheroplasts.
Tryptic cleavage at Iysine 4 in intact spheroplasts shows that this
residue is directed towards the
periplasmic side, whereas Iysine 41 faces the inside of the E. Coli inner
membrane. Intact spheroplasts
treated with pronase yielded a cleavage pattern consistent with two
additional C-terminal sites exposed to
the periplasmic side indicating a polytopic membrane association of
microsomal glutathione transferase .
Doctoral Thesis
c 1996 Rolf Weinander
ISBN 91-628-2217-
SprÄkutvecklande arbetssÀtt i SO- och NO-undervisning i grundskolans mellanÄr
I mÄnga av skolans Àmnen stÀlls stora krav pÄ elevernas sprÄkliga förmÄgor gÀllande lÀsning, skrivande och begreppsförstÄelse. Elever i sprÄklig sÄrbarhet kan möta stora utmaningar i SO- och NO-Àmnena dÀr explicit undervisning och scaffolding behövs för att stödja eleverna i Àmnesspecifika lÀs- och skrivaktiviteter. Om undervisningen inte kan möta elevernas sprÄkliga behov och erbjuda adekvat stöttning kan det fÄ stora konsekvenser för elevernas kunskapsutveckling och mÄluppfyllelse. DÀrför Àr ett sprÄkutvecklande förhÄllningssÀtt en nyckel till en mer tillgÀnglig undervisning. Syftet med arbetet Àr att undersöka hur SO- och NO-lÀrare pÄ mellanstadiet beskriver att de arbetar sprÄkutvecklande i sina Àmnen. I arbetet har en kvalitativ metod anvÀnts dÀr datainsamling skett genom fokusgruppssamtal och enskilda intervjuer med 12 mellanstadielÀrare, varav hÀlften undervisar i SO och hÀlften i NO. Resultatet visar att lÀrarna överlag har en medvetenhet kring sprÄkets betydelse i sina Àmnen och att de anser sig ha en roll bÄde som ÀmneslÀrare och sprÄklÀrare. NÄgra slutsatser som kan dras Àr att det, trots en samstÀmmig syn, finns stor variation i lÀrarnas sÀtt att stötta elever i sprÄklig sÄrbarhet samt att begrepp som stöttning kan ha olika innebörd för olika lÀrare. En mÀngd faktorer kan ocksÄ pÄverka hur den sprÄkutvecklande undervisningen gestaltar sig, och de sprÄkliga utmaningar som finns i elevgruppen fÄr konsekvenser för vad man prioriterar att arbeta med. Generellt lÀggs dock stort fokus pÄ begreppsinlÀrning och ÀmnessprÄk samt gemensam textlÀsning
Glutathione transferase mimics: micellar catalysis of an enzymic reaction
Substances that mimic the enzyme action of glutathione transferases
(which serve in detoxification) are described. These micellar catalysts enhance the reaction rate between thiols and activated halogenated nitroarenes as well as α,ÎČ-unsaturated carbonyls. The nucleophilic aromatic substitution reaction is
enhanced by the following surfactants in descending order: poly(dimethyldiallylammonium-co-dodecylmethyldiallylammonium) bromide (86/14) » cetyltrimethylammonium bromide > zwittergent 3-16 (n-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulphonate) > zwittergent 3-14 (n-tetradecyl-N,N-dimethyl-
3-ammonio-1-propanesulphonate) â N,N-dimethyl-laurylamine N-oxide > N,N-dimethyloctylamine N-oxide. The most efficient catalyst studied is a polymeric material that incorporates surfactant properties (n-dodecylmethyldiallylammonium bromide) and opens up possibilities for engineering sequences of
reactions on a polymeric support. Michael addition to α,ÎČ-unsaturated carbonyls is exemplified by a model substance, trans-4-phenylbut-3-en-2-one, and a toxic compound that is formed during oxidative stress, 4-hydroxy-2-undecenal. The
latter compound is conjugated with the highest efficiency of those tested. Micellar catalysts can thus be viewed as simple models for the glutathione transferases highlighting the influence of a positive electrostatic field and a non-specific hydrophobic binding site, pertaining to two catalytic aspects, namely thiolate anion
stabilization and solvent shielding.
Human microsomal prostaglandin E synthase-1: purification, functional characterization, and projection structure determination.
Human, microsomal, and glutathione-dependent prostaglandin (PG) E synthase-1 (mPGES-1) was expressed with a histidine tag in Escherichia coli. mPGES-1 was purified to apparent homogeneity from Triton X-100-solubilized bacterial extracts by a combination of hydroxyapatite and immobilized metal affinity chromatography. The purified enzyme displayed rapid glutathione-dependent conversion of PGH2 to PGE2 (Vmax; 170 ”mol minâ1 mgâ1) and high kcat/Km (310 mMâ1 sâ1). Purified mPGES-1 also catalyzed glutathione-dependent conversion of PGG2 to 15-hydroperoxy-PGE2 (Vmax; 250 ”mol minâ1 mgâ1). The formation of 15-hydroperoxy-PGE2 represents an alternative pathway for the synthesis of PGE2, which requires further investigation. Purified mPGES-1 also catalyzed glutathione-dependent peroxidase activity toward cumene hydroperoxide (0.17 ”mol minâ1 mgâ1), 5-hydroperoxyeicosatetraenoic acid (0.043 ”mol minâ1 mgâ1), and 15-hydroperoxy-PGE2 (0.04 ”mol minâ1 mgâ1). In addition, purified mPGES-1 catalyzed slow but significant conjugation of 1-chloro-2,4-dinitrobenzene to glutathione (0.8 ”mol minâ1 mgâ1). These activities likely represent the evolutionary relationship to microsomal glutathione transferases. Two-dimensional crystals of purified mPGES-1 were prepared, and the projection map determined by electron crystallography demonstrated that microsomal PGES-1 constitutes a trimer in the crystal, i.e. an organization similar to the microsomal glutathione transferase 1. Hydrodynamic studies of the mPGES-1-Triton X-100 complex demonstrated a sedimentation coefficient of 4.1 S, a partial specific volume of 0.891 cm3/g, and a Stokes radius of 5.09 nm corresponding to a calculated molecular weight of 215,000. This molecular weight, including bound Triton X-100 (2.8 g/g protein), is fully consistent with a trimeric organization of mPGES-1