Current Enzyme Inhibition - Volume 6, Issue 3, 2010

Improvements in surgical technique, chemotherapy, and radiation have improved the prognosis of sarcoma patients, but it has since plateaued in recent years. Novel approaches are desperately needed for improved prognosis in the remaining patients. Targeting underlying molecular events have provided striking effects in some sarcomas. Signal transducers and activators of transcription 3 (STAT3) is a member of transcription factors that are activated by phosphorylation in the cytoplasm which are then translocated to the nucleus to regulate various gene expressions. STAT3 has been reported to be constitutively phosphorylated in various human cancers and has been cited as one of the instigators of neoplastic transformation. STAT3 and its associated signaling systems have been linked with resistance to chemotherapy and apoptosis suppression. These findings implicate to the possibility of targeting STAT3 for therapeutic intervention. This review provides an overview of STAT3 as a potential therapeutic target in the treatment of sarcoma.

Tartrate-resistant acid phosphatases (TRAcPs) belong to the family of binuclear metallohydrolases and catalyse the hydrolysis of a wide range of phosphomonoester and amide substrates. These enzymes have been characterised from numerous animal, plant and fungal sources. They are distinguished from other phosphatases by their resistance to inhibition by tartrate, their optimal catalytic efficiency at low pH and their characteristic purple colour, the latter due to an interaction between a conserved tyrosine residue in the active site and an Fe(III). Consequently, these enzymes are often referred to as purple acid phosphatases (PAPs). TRAcPs catalyse the hydrolysis of a range of substrates (including ATP) and are inhibited by the reaction product phosphate and related tetraoxo anions (i.e. molybdate, arsenate, vanadate), as well as fluoride. The interactions between these inhibitors and TRAcPs have been investigated with structural, kinetic and spectroscopic techniques, resulting in the proposal of a comprehensive eight-step reaction mechanism. In animals, TRAcP activity is directly linked to bone resorption; serum levels of the enzyme are therefore a main marker for the diagnosis of osteoporosis. Furthermore, high expression levels in macrophages during pathogen invasion indicate a pivotal role for TRAcPs in the immune response. TRAcP has thus emerged as a major target to develop chemotherapeutics to combat osteoporosis and other bone-related disorders. In this minireview, we will revisit recent developments in TRAcP inhibitor design and synthesis, and discuss how inhibitors have been pivotal in developing an increased understanding of the structure, function and mechanism of this enzyme.

Adverse drug reaction is a frequent cause of drug withdrawals from the market. The drug-drug interaction (DDI) potential of new drug candidates is an increasing safety concern of pharmaceutical companies. DDIs frequently occur between coadministered drugs on a pharmacokinetic ground and sometimes create life-threatening conditions. The unexpected increase of the exposure of a drug is most frequently evoked by the inhibition of metabolic enzymes. While the inhibition of CYPs by new drug candidates is unwanted, one has to recognize that several currently marketed successful drugs with relatively clean record of drug-drug interactions are time-dependent inhibitors of drug metabolic enzymes. Therefore, the correct and high throughput prediction of drug-drug interaction propensity of new chemical entities (NCEs) at an affordable cost is a major interest of pharmaceutical research. False negatives and positives, also the misinterpretation of otherwise flawlessly generated data are equally costly or hazardous. While screening methods for the detection of CYP inhibition are available now, the quantitative estimation of DDI potential and its PK effect leaves an ample room for improvement. This is reflected in the great research activity focused on the establishment of correct in vitro-in vivo correlations. The impressing number of recent publications and methods recommended in this field provide an increasingly solid ground for the selection of lead compounds and development candidates.

The Raf-MEK-ERK signaling pathway promotes cell cycle progression and cell proliferation, and it has been shown to play a key role in tumorigenesis and cancer cell survival. Targeting MEK to inhibit the activity of this survival pathway has been a logical strategy for treating cancers. Several MEK inhibitors have been developed and widely used in laboratories as tools to study the signaling pathway. Despite their promising anti-tumor activity in preclinical studies, MEK inhibitors have failed to generate satisfactory responses in clinical trials. Here we review the history of two categories of highly selective MEK inhibitors: non-ATP-competitive small-molecule inhibitors (PD 098059, U0126, PD 184352 and its derivatives) and biological inhibitors (anthrax lethal toxin and Yersinia outer protein J). This review presents a discussion of the mechanisms of these inhibitors and is intended to provide insights into the potential applications of these inhibitors to cancer studies and treatments.

Enzyme inhibitors are important tools of nature for regulating the activity of enzymes in case of emergency. Proteinaceous inhibitors are the best studied in this group. Comparatively less is known about the inhibitors of α-amylase, which might be an attractive candidate having potentials in various fields, from the treatment of diabetes to crop protection. The area has benefited from the recent determination of structures of α-amylases, inhibitors and complexes. Besides, discussing various types of α-amylase inhibitors and their structural basis of inhibition, the biotechnological approaches for exploitation of its inhibitory function have been outlined in this minireview.

The double-reciprocal, or Lineweaver-Burke plot appears in virtually every treatment of enzyme kinetics and enzyme inhibition from the textbook level to research papers and monographs. It is also widely used in research to extract kinetic parameters, such as the Vmax, Km, and Ki values. It has long been recognized that in the process of taking reciprocals, the very worst data are emphasized; the smallest numbers have the most influence. Various graphical and later analytical (non-linear regression) solutions have been proposed over the years, but the double reciprocal plot remains the standard for pattern recognition of the three principal forms of reversible enzyme inhibition: competitive, uncompetitive, and non-competitive. However, a separate problem coexists with double reciprocal plots that is seldom mentioned: they are strongly anti-intuitive. Few people are adept at thinking in double-reciprocal space, that is, how an inverse substrate concentration might affect an inverse velocity. This is compounded by the fact that the double reciprocal plots, reduced to pattern recognition, allow no interpretation at all. Corresponding cartoon diagrams used to indicate how inhibition might occur are misleading. Combined with the fact that few investigators use the correct kinetic constants in analyzing enzyme inhibition, it is not surprising that very few scientists have a sound understanding of any inhibition form other than competitive - and that one is only evident because of common experience with the idea. In this contribution, I demonstrate the meanings of the intercepts of the double reciprocal plot by showing the mapping to direct velocity-substrate plots. In addition, I present different model drawings to extend understanding of modes of reversible inhibition. Reliance on double reciprocal plots to extract kinetic constants themselves is a mathematical error; reliance on the patterns themselves is an intuitive error. Overcoming these provides a simple, yet fundamental understanding of all forms of reversible enzyme inhibition.