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Current Enzyme Inhibition
ISSN: 1573-4080
Current Enzyme Inhibition
Volume 2, Number 1, February 2006
Contents
Regular Papers
Interaction of Nitrite with Hydrogen Peroxide-Destroying
Enzymes as an Important Element of Nitrite Toxicity Pp.
1-17
V. Yu. Titov
[Abstract]
Coil-to-Helix Transition within Phos-pholamban Underlies
Release of Ca-ATPase Inhibition in Response to β-Adrenergic
Signaling Pp. 19-27
Diana J. Bigelow and Thomas C. Squier
[Abstract]
Proteinaceous Xylanase Inhibitors: Structure,
Function and Evolution Pp. 29-35
Nathalie Juge and Jan A. Delcour
[Abstract]
Oxygen- and Redox-Induced Regulation of the Na/K
ATPase Pp. 37-59
A. Bogdanova, I. Petrushanko, A. Boldyrev and M. Gassmann
[Abstract]
Targeting Enzymes with Phosphonate-Based Inhibitors:
Mimics of Tetrahedral Transition States and Stable Isosteric
Analogues of Phosphates Pp. 61-72
L. Azéma, R. Baron and S. Ladame
[Abstract]
Eicosanoid Inhibitors as Therapeutic Targets for
Metabolic Syndrome Related Kidney Disease Pp.
73-77
John D. Imig and Xueying Zhao
[Abstract]
Roles of Calcium and Tyrosine Kinases in the Pathogenesis
of Type 2 Diabetes Mellitus Pp. 79-89
J.A. Rosado, P.C. Redondo, J.A. Pariente and
G.M. Salido
[Abstract]
Histone Deacetylase Inhibition: A Differentiation
Therapy for Cultured Primary Hepatocytes? Pp. 91-104
P. Papeleu, T. Vanhaecke and V. Rogiers
[Abstract
Abstracts
[Back to top]
Regular Papers
Interaction of Nitrite with Hydrogen Peroxide-Destroying
Enzymes as an Important Element of Nitrite Toxicity
V. Yu. Titov
It is known that peroxidase oxidation of nitrite to toxic
products is one of the basic mechanisms of its toxicity. Therefore,
the ratio between activities of hydrogen peroxide metabolized
enzymes and the impact of nitrite on these enzymes is an important
factor determining its toxicity. By use of calorimetric method
developed by us that makes possible to control H2O2
destroying kinetics and distinguish different pathways of
its destruction, we established that nitrite even in micromolar
concentrations significantly decreases the ability of catalase
to destroy hydrogen peroxide in the presence of chloride,
bromide and thiocyanate, but not without them. Also it was
established that this decrease is the result of reversible
inhibition of the enzyme without any change in the essence
of H2O2 destroying pathway. Mechanisms
of the inhibition are discussed. Heme-containing peroxidases,
such as horseradish peroxidase, lactoperoxidase and methemoglobin,
manifested multiple higher stability towards nitrite inhibition
than catalase does independently without halides and thiocyanate.
Therefore, the presence of Cl-, Br-
and SCN- in nitrite, catalase- and peroxidase-containing
systems should lead to increase the peroxidase pathway of
H2O2 destroying and, consequently, the
peroxidase oxidation of nitrite to a toxic product (probably
NO2•). This finding was demonstrated
in the example of nitrite-induced hemoglobin oxidation. Low
anion concentration in intracellular media and also the presence
of effective nitrite competitors for peroxidases appear to
be the most important mechanism of cell protection from nitrite
toxicity.
[Back to top]
Coil-to-Helix Transition within Phos-pholamban
Underlies Release of Ca-ATPase Inhibition in Response to β-Adrenergic
Signaling
Diana J. Bigelow and Thomas C. Squier
Phospholamban (PLB) is a major target of β-adrenergic
signaling, whose phosphorylation results in enhanced rates
of relaxation in the heart. Prior to phosphorylation, PLB
functions to reduce the calcium sensitivity of the Ca-ATPase,
resulting in slower rates of calcium resequestration into
the sarcoplasmic reticulum after each contractile event. Recent
structures indicate that the inhibitory interaction between
PLB and the Ca-ATPase requires PLB to assume an extended structure,
where the transmembrane and cytosolic portions of PLB undergo
specific binding interactions with distant sites on the Ca-ATPase.
In the extended conformation, PLB binding to the Ca-ATPase
functions to inhibit the Ca-ATPase through a reduction in
the rates of catalytically important motions involving the
nucleotide binding domain. Phosphorylation of PLB at either
Ser16 or Thr17 releases the inhibitory
interaction between PLB and the Ca-ATPase. These sites of
phosphorylation are within a hinge region in PLB that separates
the highly structured transmembrane and cytosolic portions
that associate with the Ca-ATPase. The helical content of
the hinge region increases following the phosphorylation of
PLB, which induces a shortening of the maximal dimensions
of PLB and a release of the inhibitory interaction with the
Ca-ATPase. Following phosphorylation, PLB remains associated
with the Ca-ATPase in a more compact form that has no inhibitory
capability. Thus, the conformational switch involving PLB
regulation of the Ca-ATPase relies upon a physical mechanism,
whereby the phosphorylation-dependent stabilization of the
structure of PLB functions to destabilize the inhibitory interaction
between PLB and the Ca-ATPase. Upon hydrolysis of the phosphoester
linkages by endogenous phosphatases, PLB is poised to reassume
the inhibited state through re-association with inhibitory
sites on the nucleotide binding domain of the Ca-ATPase.
[Back to top]
Proteinaceous Xylanase Inhibitors: Structure,
Function and Evolution
Nathalie Juge and Jan A. Delcour
Endo-(1,4)-β-D-xylanases
of plant and microbial origin play an important role in the
degradation of arabinoxylan from plant cell wall. To date,
two distinct types of xylanase inhibitors, the TAXI (Triticum
aestivum xylanase inhibitor) and
XIP (Xylanase inhibitor protein)
types have been identified in cereals (rye, barley, maize,
rice, durum and bread wheat). TAXI inhibits fungal and bacterial
xylanases from glycoside hydrolase (GH) family 11 (GH11) whereas
XIP inhibitors display species selectivity for both GH10 and
GH11 xylanases. The evolution and biological role of the xylanase
inhibitors are discussed in the light of the features which
have recently become available by resolution of the crystal
structures of the inhibitors isolated from wheat, XIP-I and
TAXI-I, free and in complex with target xylanases.
[Back to top]
Oxygen- and Redox-Induced Regulation of the
Na/K ATPase
A. Bogdanova, I. Petrushanko, A. Boldyrev and M. Gassmann
The Na/K ATPase, or Na/K pump, is an enzyme converting chemical
energy of ATP hydrolysis into the energy of electrochemical
sodium and potassium gradient that is used to maintain cellular
volume, pH and Ca2+ levels. Apart from maintaining
the cellular Na+ and K+ concentrations,
Na/K ATPase is involved in signal transduction including activation
of mitogenic and proliferative signalling cascades. Changes
in the Na/K ATPase activity have an impact on cellular metabolic
and redox state, since it controls mitochondrial reactive
oxygen species (ROS) levels and ATP utilisation rates. Na/K
ATPase is known to be oxygen- and redox-sensitive. Alterations
in the Na/K pump activity in response to hypoxia-reoxygenation
or shifts in cellular and environmental redox state play an
important role in pathological and adaptive responses of cells
to hypoxia or ischemia. Despite intensive studies, the mechanisms
involved in redox- and oxygen-induced regulation of the Na/K
ATPase remain largely unknown. The present review focuses
on the possible targets of action of oxygen, oxidants and
reductants on the Na/K ATPase and mechanisms of oxygen- and
redox-sensitivity of the enzyme at the molecular and cellular
levels. Finally, we also consider factors involved in systemic
responses to hypoxia that influence activity of the transporter
as well as potential physiological and pathological outcome
of such changes.
[Back to top]
Targeting Enzymes with Phosphonate-Based
Inhibitors: Mimics of Tetrahedral Transition States and Stable
Isosteric Analogues of Phosphates
L. Azéma, R. Baron and S. Ladame
Phosphorus plays a fundamental role in cell since it is a
part of many biomolecules and biological metabolites which
include phospholipids, nucleic acids, proteins, polysaccharides
or nucleotide cofactors. Phosphorus is most commonly found
under its highest oxidized state as in orthophosphate or phosphate
esters. However, there are also examples of naturally occurring
molecules bearing phosphorus atoms at lower oxidized state
that contain one carbon to phosphorus (P-C) bond. These compounds,
so-called phosphonates, are much more resistant to chemical
and enzymatic hydrolysis, thermal decomposition and photolysis
than phosphates. The first example of a natural phosphonate,
2-aminoethylphosphonic acid (2-AEP), was reported in 1959
and shown to be a constituent of lipids, proteins and polysaccharides.
Since the beginning of the 20th century, with the discovery
of the Arbuzov reaction, synthetic phosphonates have been
developed which have applications as therapeutic agents (e.g.
antibiotics, anti-viral, or trypanocidal drugs) as well as
insecticides and herbicides.
Recently, phosphonate derivatives have been extensively used
as enzyme inhibitors due to their specific structural and
electronic properties. First, since they differ from phosphates
by the single substitution of one oxygen atom with a carbon
atom, phosphonates have been widely used as isosteric mimics
of phosphates in the design of analogues of enzyme substrates
or cofactors. They offer the advantage of a much greater stability
to hydrolysis and resistance to proteases than phosphates
and therefore can be used to mimic highly chemically unstable
phosphates like carboxy-phosphates. Such a strategy was shown
successful for generating competitive inhibitors of glycolytic
enzymes (triose/hexose phosphate analogues), phosphatases
or viral DNA polymerases (phosphonate nucleotides), with applications
as chemotherapeutic agents against trypanosomiasis, hepatitis
B or human immunodeficiency virus. Moreover, introduction
of one or two fluorine atoms on the α-methylene
group was also used to provide a better mimic of phosphate
geometric and electronic properties by lowering the pKa of
the phosphonate and increasing its P-Cα-Cβ
dihedral angle. Phosphonic acids have also been extensively
studied as enzyme inhibitors mimicking tetrahedral transition
states. They proved to be potent competitive inhibitors of
peptidases as mimics of the tetrahedral gem-diolate
transition state of peptide bond hydrolysis, and of lipases,
as mimics of configuration and charge distribution of the
first transition state in triglyceride hydrolysis. This chapter
summarizes the most recent studies on phosphonate-based enzyme
inhibitors with special emphasis on isosteric analogues of
phosphorylated substrates and mimics of tetrahedral transition
states for peptide bond hydrolysis.
[Back to top]
Eicosanoid Inhibitors as Therapeutic Targets
for Metabolic Syndrome Related Kidney Disease
John D. Imig and Xueying Zhao
Obesity has reached epidemic proportions worldwide and is
now a major healthcare problem. Likewise, there are a number
of cardiovascular risk factors and metabolic factors associated
with obesity and this clustering contributes to the disease
known as metabolic syndrome or syndrome X. Metabolic syndrome
over a number of years can cause end organ damage resulting
in morbidity and mortality. Metabolic syndrome and obesity
are major contributing factors to the increase in nephropathy
and end stage renal disease. Interestingly, an imbalance between
cyclooxygenase-2 (COX-2) and cytochrome P450 (CYP450) enzymes
in the kidney may contribute to the nephropathy associated
with metabolic syndrome. Recent studies have demonstrated
that COX-2 inhibition decreases renal cytokine levels and
glomerular injury in obese rats. Therefore, COX-2 and CYP450
metabolites are therapeutic targets for the treatment of renal
disease related to metabolic syndrome.
[Back to top]
Roles of Calcium and Tyrosine Kinases in the Pathogenesis
of Type 2 Diabetes Mellitus
J.A. Rosado, P.C. Redondo, J.A. Pariente and
G.M. Salido
Diabetes mellitus type 2 is a metabolic disease associated
with chronic hyperglycaemia, which leads to a wide range of
complications, including microvascular and macrovascular alterations,
retinopathy, nephropathy and renal disease or peripheral neuropathy.
Several intracellular pathways have been shown to be associated
to type 2 diabetes mellitus, ranging from an altered insulin
receptor-associated signalling to an abnormal intracellular
calcium homeostasis or disturbances in Na+ handling.
A number of diabetes-associated complications have been reported
to be associated with hyperactivity of certain protein tyrosine
kinases, such as those cytosolic kinases of the Src family,
involved in the altered intracellular calcium mobilisation
and platelet-derived cardiovascular problems and in glomerular
injury, or the JAK family of tyrosine kinases involved in
hyperglycaemia-induced renal failure. There has been a considerable
effort in several laboratories to identify suitable targets
for the design of drugs against this disease. The development
of tyrosine kinase inhibitors suitable for medical purposes
might represent a significant advance in the therapy of complications
associated to type 2 diabetes mellitus.
[Back to top]
Histone Deacetylase Inhibition: A Differentiation
Therapy for Cultured Primary Hepatocytes?
P. Papeleu, T. Vanhaecke and V. Rogiers
Histone deacetylase (HDAC) inhibitors are now widely recognized
as powerful anticancer agents. Unlike conventional chemotherapeutics,
they not only inhibit tumor growth and survival but also reinitiate
differentiation processes. Surprisingly, little attention
has been paid so far to their capacity as differentiating
agents for primary non-malignant cells. Dedifferentiation,
however, is a common pernicious event occurring in almost
all primary epithelial cell cultures and limits their use
in fundamental and applied research. In this review, we elaborate
on the innovative concept of HDAC inhibition as a key tool
in the development of functional long-term cultures of primary
hepatocytes. The need for long-term hepatocyte-based in
vitro models is high and underscored by the fact that
(sub)-chronic liver toxicity testing still consumes an important
number of animals and by recent changes in EU legislation
(e.g. chemicals policy REACH and cosmetics Directive 2003/15/EC).
In addition, many cell-based therapies would benefit from
the availability of highly differentiated primary hepatocytes
as well. We believe that the HDAC inhibition approach could
be applicable to create stable, differentiated culture systems
of other primary cell types as well. In addition, HDAC inhibitors,
in combination with tissue-specific growth factors, can be
used to generate tissue-specific cell types from stem cells.
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