Intestinal cholesterol esterase: intracellular enzyme or contamination of cytosol by pancreatic enzymes?

The location of cholesterol esterase in rabbit intestine was re-evaluated. In three different experiments that were designed to eliminate contaminating mucus and pancreatic enzymes from the lumen of the small intestine, it was observed that the activities of cholesterol esterase and amylase in intestinal cytosol and whole homogenate decreased in parallel fashion. After the mucus was carefully wiped from the intestinal mucosa prior to the preparation of cytosol, amylase and cholesterol esterase activities decreased sevenfold. The recovery of the total activity of both enzymes in the cytosol was approximately 15%. When the lumen of the small intestine was filled with phosphate buffer and incubated at 37 degrees C for 20 min, cholesterol esterase and amylase activities in the cytosol prepared from this segment were further decreased. Moreover, the activities of amylase and cholesterol esterase were completely recovered from the lumen. Amylase and cholesterol esterase activities in the cytosol were eliminated if dithiothreitol was used as a mucolytic agent to prepare intestinal mucosa for the isolation of intestinal cells. In whole homogenates prepared from these intestinal segments, approximately 10-15% of the total cholesterol esterase activity remained. This activity, which could not be accounted for by pancreatic contamination, was associated with intestinal nuclei and cellular debris. Progesterone, ethinyl estradiol, and 25-hydroxycholesterol regulated microsomal acyl CoA:cholesterol acyltransferase activity and caused similar directional changes in the rate of cholesteryl ester synthesis in isolated intestinal cells. These same sterols, however, failed to affect cytosolic cholesterol esterase activity in vitro.     

Microbial genomics: rhetoric or reality?

The availability of complete genome sequences of many bacterial species is facilitating numerous computational approaches for understanding bacterial genomes. One of the major incentives behind the genome sequencing of many pathogenic bacteria is the desire to better understand their diversity and to develop new approaches for controlling human diseases caused by these microorganisms. This task has become even more urgent with the rapid evolution of antibiotic resistance among many bacterial pathogens. Novel drug targets are required in order to design new antimicrobials against antibiotic-resistant pathogens. The complete genome sequences of an ever increasing number of pathogenic microbes constitute an invaluable resource and provide lead information on potential drug targets. This review focuses on in silico analyses of microbial genomes, their host-specific adaptations, with specific reference to genome architecture, design, evolution, and trends in computational identification of microbial drug targets. These trends underscore the utility of genomic data for systematic in silico drug target identification in the post-genomic era.     

Immobilised enzymes: science or art?

Enzyme immobilisation is experiencing an important transition. Combinatorial approaches are increasingly applied in the design of robust immobilised enzymes by rational combination of fundamental immobilisation techniques (i.e. non-covalent adsorption, covalent binding, entrapment and encapsulation) or with other relevant technologies. The objective is to solve specific problems that cannot be solved by one of these basic immobilisation techniques.     

Immobilised enzymes: carrier-bound or carrier-free?

Recent advances have been made in the development of carrier-free immobilised enzymes and several criteria established for the selection of immobilised enzymes for biotransformations. The pros and cons of carrier-free versus carrier-bound immobilised enzymes and of each type of carrier-free enzyme are discussed.     

The enigma of the CLIC proteins: Ion channels, redox proteins, enzymes, scaffolding proteins?

Chloride intracellular channel proteins (CLICs) are distinct from most ion channels in that they have both soluble and integral membrane forms. CLICs are highly conserved in chordates, with six vertebrate paralogues. CLIC-like proteins are found in other metazoans. CLICs form channels in artificial bilayers in a process favoured by oxidising conditions and low pH. They are structurally plastic, with CLIC1 adopting two distinct soluble conformations. Phylogenetic and structural data indicate that CLICs are likely to have enzymatic function. The physiological role of CLICs appears to be maintenance of intracellular membranes, which is associated with tubulogenesis but may involve other substructures.     

Cytoplasmic oxysterol-binding proteins: sterol sensors or transporters?

Families of oxysterol-binding protein (ORP) homologues are present in eukaryotes from yeast to man. Their hallmark feature is a characteristic ligand binding domain that, for several family members, has been shown to accommodate different oxysterols and/or cholesterol. ORPs of the “long” subtype contain targeting determinants for the endoplasmic reticulum and to other organelle membranes, the most prominent of which are phosphoinositide-binding pleckstrin homology domains, while “short” ORPs comprise a ligand binding domain with little additional sequences. There is increasing evidence that both long and short ORPs can be enriched at membrane contact sites, junctions of the endoplasmic reticulum with other organelles, where they are suggested to execute regulatory or sterol transfer functions. In this review we discuss the current evidence for putative roles of ORPs as sterol sensors or transporters.     

Microbial hydantoinases – industrial enzymes from the origin of life?

Hydantoinases are valuable enzymes for the production of optically pure d- and l-amino acids. They catalyse the reversible hydrolytic ring cleavage of hydantoin or 5′-monosubstituted hydantoins and are therefore classified in the EC nomenclature as cyclic amidases (EC 3.5.2.). In the EC nomenclature, four different hydantoin-cleaving enzymes are described: dihydropyrimidinase (3.5.2.2), allantoinase (EC 3.5.2.5), carboxymethylhydantoinase (EC 3.5.2.4), and N-methylhydantoinase (EC 3.5.2.14). Beside these, other hydantoinases with known metabolic functions, such as imidase and carboxyethylhydantoinase and enzymes with unknown metabolic function, are described in the literature and have not yet been classified. An important question is whether the distinct hydantoinases, which are frequently classified as l-, d-, and non-selective hydantoinases depending on their substrate specificity and stereoselectivity, are related to each other. In order to investigate the evolutionary relationship, amino acid sequence data can be used for a phylogenetic analysis. Although most of these enzymes only share limited sequence homology (identity<15%) and therefore are only distantly related, it can be shown (i) that most of them are members of a broad set of amidases with similarities to ureases and build a protein superfamily, whereas ATP-dependent hydantoinases are not related, (ii) that the urease-related amidases have evolved divergently from a common ancestor and (iii) that they share a metal-binding motif consisting of conserved histidine residues. The difference in enantioselectivity used for the classification of hydantoinases on the basis of their biotechnological value does not reflect their evolutionary relationship, which is to a more diverse group of enzymes than was assumed earlier. This protein superfamily probably has its origin in the prebiotic conditions of the primitive earth. Received: 24 August 1998 / Received revision: 9 November 1998 / Accepted: 21 November 1998     

Lecithin:cholesterol acyltransferase: old friend or foe in atherosclerosis?

Lecithin:cholesterol acyltransferase (LCAT) is a key enzyme that catalyzes the esterification of free cholesterol in plasma lipoproteins and plays a critical role in high-density lipoprotein (HDL) metabolism. Deficiency leads to accumulation of nascent preβ-HDL due to impaired maturation of HDL particles, whereas enhanced expression is associated with the formation of large, apoE-rich HDL(1) particles. In addition to its function in HDL metabolism, LCAT was believed to be an important driving force behind macrophage reverse cholesterol transport (RCT) and, therefore, has been a subject of great interest in cardiovascular research since its discovery in 1962. Although half a century has passed, the importance of LCAT for atheroprotection is still under intense debate. This review provides a comprehensive overview of the insights that have been gained in the past 50 years on the biochemistry of LCAT, the role of LCAT in lipoprotein metabolism and the pathogenesis of atherosclerosis in animal models, and its impact on cardiovascular disease in humans.     

Neuronal variability: noise or part of the signal?

Sensory, motor and cortical neurons fire impulses or spikes at a regular, but slowly declining, rate in response to a constant current stimulus. Yet, the intervals between spikes often vary randomly during behaviour. Is this variation an unavoidable effect of generating spikes by sensory or synaptic processes ('neural noise') or is it an important part of the 'signal' that is transmitted to other neurons? Here, we mainly discuss this question in relation to sensory and motor processes, as the signals are best identified in such systems, although we also touch on central processes.     

O-GlcNAc glycosylation: a signal for the nuclear transport of cytosolic proteins?

Year 2004 marks the 20th anniversary of the discovery of O-linked N-acetylglucosamine (O-GlcNAc) by Gerald W. Hart. Despite interest for O-GlcNAc, the functions played by this single monosaccharide remain poorly understood, though numerous roles have been suggested, among which is the involvement of O-GlcNAc in the nuclear transport of cytosolic proteins. This idea was first sustained by studies on bovine serum albumin that showed that the protein could be actively carried to the nucleus when it was modified with sugars. In this paper, we will review data on this puzzling problem. We will first describe the well-established nuclear localisation signal (NLS)-dependent nuclear transport by presenting the different factors involved, and then, we will examine where and how O-GlcNAc could be involved in nuclear transport. Whereas it has been suggested that O-GlcNAc could interfere at two levels in the nuclear transport both by modifying proteins to be translocated to the nucleus and by modifying the nucleoporins of the nuclear pore complex, according to us, this second idea seems unlikely. Part of this study will also be dedicated to a relatively new concept in the nuclear transport: the role of the 70-kDa heat shock proteins (HSP70). The action of the chaperone in nuclear translocation was put forward 10 years ago, but new findings suggest that this mechanism could be linked to O-GlcNAc glycosylation.     

Nucleocytoplasmic trafficking of proteins: With or without Ran?

Proteins and RNAs move between the nucleus and cytoplasm by translocation through nuclear pore complexes in the nuclear envelope. To do this, they require specific targeting signals, energy, and a cellular apparatus that catalyzes their transport. Several of the factors involved in nucleocytoplasmic trafficking of proteins have been identified and characterized in some detail. The emerging picture for nuclear transport proposes a central role for the small GTPase Ran and proteins with which it interacts. In particular, asymmetric distribution of these proteins between nucleus and cytoplasm appears to be responsible for the vectorial nature of nucleocytoplasmic transport. Here, we summarize the role of Ran and Ran-binding proteins in nuclear trafficking of proteins with classical nuclear localisation signals. We also discuss examples of the growing number of alternative pathways that are involved in transport of proteins across the nuclear envelope. BioEssays 21:579–589, 1999. © 1999 John Wiley & Sons, Inc.     

Reactivity of penicillin-binding proteins with peptidoglycan-mimetic beta-lactams: what's wrong with these enzymes?

Beta-lactams exert their antibiotic action through their inhibition of bacterial DD-peptidases (penicillin-binding proteins). Bacteria, in general, carry several such enzymes localized on the outside of their cell membrane to catalyze the final step in cell wall (peptidoglycan) synthesis. They have been classified into two major groups, one of high molecular weight, the other of low. Members of the former group act as transpeptidases in vivo, and their inhibition by beta-lactams leads to cessation of bacterial growth. The latter group consists of DD-carboxypeptidases, and their inhibition by beta-lactams is generally not fatal to bacteria. We have previously shown that representatives of the former group are ineffective at catalyzing the hydrolysis/aminolysis of peptidoglycan-mimetic peptides in vitro [Anderson et al. (2003) Biochem. J. 373, 949-955]. The theme of these experiments is expanded in the present paper where we describe the synthesis of a series of beta-lactams (penicillins and cephalosporins) containing peptidoglycan-mimetic side chains and the kinetics of their inhibition of a panel of penicillin-binding proteins spanning the major classes (Escherichia coli PBP 2 and PBP 5, Streptococcus pneumoniae PBP 1b, PBP 2x and PBP 3, the Actinomadura R39 DD-peptidase, and the Streptomyces R61 DD-peptidase). The results of these experiments mirror and expand the previous results with peptides. Neither peptides nor beta-lactams with appropriate peptidoglycan-mimetic side chains react with the solubilized constructs of membrane-bound penicillin binding proteins (the first five enzymes above) at rates exceeding those of generic analogues. Such peptides and beta-lactams do react at greatly enhanced rates with certain soluble low molecular weight enzymes (R61 and R39 DD-peptidases). The former result is unexpected and interesting. Why do the majority of penicillin-binding proteins not recognize elements of local peptidoglycan structure? Possible answers are discussed. That this question needs to be asked casts fascinating shadows on current studies of penicillin-binding proteins for new drug design.     

Plasmodium falciparum signal sequences: simply sequences or special signals?

The malaria parasite, Plasmodium falciparum, synthesises and exports several proteins inducing morphological and biochemical modifications of erythrocytes during the erythrocytic cycle. The protein trafficking machinery of the parasite is similar to that of other eukaryotic cells in several ways. However, some unusual features are also observed. The secretion of various polypeptides was inhibited when P. falciparum-infected erythrocytes were incubated with Brefeldin A. Immunoelectron microscopy studies revealed substantial morphological changes in the endoplasmic reticulum following exposure of parasitised erythrocytes to the drug. Immunofluorescence studies of Brefeldin A-treated parasites suggest that polypeptide sorting to different intracellular destinations begins at the endoplasmic reticulum. The parasite also secretes polypeptides by a Brefeldin A-insensitive route that bypasses the classical endoplasmic reticulum-Golgi complex pathway.     

Receptors coupling to G proteins: is there a signal behind the sequence?

Upon the binding of their ligands, G protein-coupled receptors couple to the heterotrimeric G proteins to transduce a signal. One receptor family may couple to a single G protein subtype and another family to several ones. Is there a signal in the receptor sequence that can give an indication of the G protein subtype selectivity? We used a sequence analysis method on biogenic amine and adenosine receptors and concluded that a weak signal can be detected in receptor families where specialization for coupling to a given G protein occurred during a recent divergent evolutionary process. Proteins 2000;41:448-459.     

Dipeptidyl peptidase IV-like molecules: homologous proteins or homologous activities?

Membrane-bound proteases are widely distributed among various cell systems. Their expression in a particular cell type is finely regulated, reflecting the specific functional cell implications and engagement in defined physiological pathways. Protein turnover, ontogeny, inflammation, tissue remodeling, cell migration and tumor invasion are among the many physiological and pathological events in which membrane proteases play a crucial role, both as effector as well as regulatory molecules. The presence of proline residues gives unique structural features to peptide chains, substantially influencing the susceptibility of proximal peptide bond to protease cleavage. Among the rare group of proline-specific proteases, dipeptidyl peptidase IV (DPP-IV, EC 3.4.14.5) was originally believed to be the only membrane-bound enzyme specific for proline as the penultimate residue at the amino-terminus of the polypeptide chain. However, other molecules, even structurally non-homologous with the DPP-IV but bearing corresponding enzyme activity, have been identified recently. This review summarizes the present knowledge of "DPP-IV activity- and/or structure-homologues" (DASH) and provides some insight into their multifunctional roles.