Blood esterases

Esterase-Catalyzed Production of Hyperpolarized 13C-Enriched Carbon Dioxide in Tissues for Measuring pH

¹³C Magnetic resonance imaging of hyperpolarized (HP) ¹³C-enriched bicarbonate (H¹³CO₃^-) and carbon dioxide (¹³CO₂) is a novel and sensitive technique for tissue pH mapping in vivo. Administration of the HP physiological buffer pair is attractive, but poor polarization and the short T₁ of ¹³C-enriched inorganic bicarbonate salts are major drawbacks for this approach. Here, we report a new class of mixed anhydrides for esterase-catalyzed production of highly polarized ¹³CO₂ and H¹³CO₃^- in tissue. A series of precursors with different alkoxy and acyl groups were synthesized and tested for chemical stability and T₁. ¹³C-enriched ethyl acetyl carbonate (¹³C-EAC) was found to be the most suitable candidate due to the relatively long T₁ and good chemical stability. Our results showed that ¹³C-EAC can be efficiently and rapidly polarized using BDPA. HP ¹³C-EAC was rapidly hydrolyzed by esterase to ¹³C-enriched monoacetyl carbonate (¹³C-MAC), which then decomposed to HP ¹³CO₂. Equilibrium between the newly produced ¹³CO₂ and H¹³CO₃^- was quickly established by carbonic anhydrase, producing a physiological buffer pair with ¹³C NMR signals that can be quantified for pH measurements. Finally, in vivo tissue pH measurements using HP ¹³C-EAC was successfully demonstrated in the liver of healthy rats. These results suggest that HP ¹³C-EAC is a novel imaging probe for in vivo pH measurements.

Biochemistry of the Nervous System

Biochemical research has continued in spite of the war and considerable advances have been reported during the period under review. While the task of applying this new biochemical knowledge in the field of psychiatry has hardly yet begun, the underlying chemical mechanisms concerned in nervous activity are becoming considerably clearer and a few beginnings in this direction have been made.

In vivo degeneration and the fate of inorganic nanoparticles

What happens to inorganic nanoparticles (NPs), such as plasmonic gold or silver, superparamagnetic iron oxide, or fluorescent quantum dot NPs after they have been administrated to a living being? This review discusses the integrity, biodistribution, and fate of NPs after in vivo administration. The hybrid nature of the NPs is described, conceptually divided into the inorganic core, the engineered surface coating comprising of the ligand shell and optionally also bio-conjugates, and the corona of adsorbed biological molecules. Empirical evidence shows that all of these three compounds may degrade individually in vivo and can drastically modify the life cycle and biodistribution of the whole heterostructure. Thus, the NPs may be decomposed into different parts, whose biodistribution and fate would need to be analyzed individually. Multiple labeling and quantification strategies for such a purpose will be discussed. All reviewed data indicate that NPs in vivo should no longer be considered as homogeneous entities, but should be seen as inorganic/organic/biological nano-hybrids with complex and intricately linked distribution and degradation pathways.

Cell-mediated assembly of phototherapeutics

Light-activatable drugs offer the promise of controlled release with exquisite temporal and spatial resolution. However, light-sensitive prodrugs are typically converted to their active forms using short-wavelength irradiation, which displays poor tissue penetrance. We report herein erythrocyte-mediated assembly of long-wavelength-sensitive phototherapeutics. The activating wavelength of the constructs is readily preassigned by using fluorophores with the desired excitation wavelength λ(ex). Drug release from the erythrocyte carrier was confirmed by standard analytical tools and by the expected biological consequences of the liberated drugs in cell culture: methotrexate, binding to intracellular dihydrofolate reductase; colchicine, inhibition of microtubule polymerization; dexamethasone, induced nuclear migration of the glucocorticoid receptor.

Anticholinesterase activity and charge delocalisation in “aliphatic” and “aromatic” quaternary ammonium compounds

The anticholinesterase activities of a homologous series of trimethyl(phenylalkyl)-ammonium bromides have been determined and compared with anti-acetylcholinesterase activities previously reported. The pattern of results obtained with the two enzymes differs widely. The surface activities of the compounds at a constant molar concentration was determined in order to investigate the influence of the “distribution effect” on the anticholinesterase activities of the compounds. The anticholinesterase activities of a series of polycyclic aromatic quaternary compounds have also been determined and compared with their previously reported anti-acetylcholinesterase activities. With these compounds, the pattern of results obtained with the two enzymes was similar.

Improving the ex vivo stability of drug ester compounds in rat and dog serum: inhibition of the specific esterases and implications on their identity

In drug development, it has been noticed that some drug compounds, especially esters, are unstable in serum samples ex vivo. This can lead to a substantial underestimation of the actual drug concentration. The rat and the dog, representing a rodent and non-rodent species, respectively, are widely used in preclinical studies. We studied the degradation of three structurally different drug esters in rat and dog serum. Moreover, the efficiency of selected enzyme inhibitors to prevent these degradations was investigated. Furthermore, we found indications of the identity of the drug-specific esterases by means of their inhibitor sensitivity as well as by protein purification and identification. The studied drugs were sagopilone, drospirenone, and methylprednisolone aceponate (MPA) all of which are used in (pre-)clinical drug development. The sagopilone-cleaving esterases in rat serum were inhibited by serine hydrolase inhibitors. We partly purified these esterases resulting in an activity yield of 5% and a purification factor of 472. Using matrix-assisted laser desorption ionization (MALDI)-time of flight (TOF)-mass spectrometry (MS), the rat carboxylesterase isoenzyme ES-1 was identified in these fractions, thus pointing to its involvement in sagopilone cleavage. Drospirenone cleavage in rat serum was effected by butyrylcholinesterase (BChE) and paraoxonase 1 (PON1) as we deduced from the high efficacy of certain serine hydrolase and metallohydrolase inhibitors, respectively. Likewise, some inhibition characteristics implied that MPA was cleaved in rat serum by BChE and serine proteases. Partial purification of the MPA-specific esterases resulted in activity yields of 1-2%, exhibiting up to 10,000-fold purification. In dog serum, we found that sagopilone was not degraded which was in contrast to MPA and drospirenone. MPA degradation was mainly prevented by serine hydrolase inhibitors. We used a three-step purification to isolate the esterases cleaving MPA. This procedure resulted in an activity yield of 12% and 645-fold purification. By protein identification using liquid chromatography (LC)-electrospray ionization (ESI)-MS, we identified alpha(2)-macroglobulin (alpha(2)M) in the active fractions. We therefore assumed that serine hydrolases, probably butyrylcholinesterase, known to form esteratically active complexes with alpha(2)M, were responsible for MPA cleavage. In contrast, PON1 was assumed to be involved in drospirenone cleavage due to the high efficiency of metallohydrolase inhibitors. This indication was supported by the presence of PON1 in drospirenone-cleaving fractions as we found by affinity chromatography and Western immunoblotting for isolation and detection of PON1, respectively. The identity of the assumed cleaving enzymes remains, however, to be further studied. The inhibitors we found can serve as a tool for stabilizing drug ester compounds in biological samples ex vivo.

Inhibitor studies of purified haemopoietic (myeloid) cell esterases. Evidence for the existence of distinct enzyme species

Human myeloid cells synthesize and express two major species of esterase, defined by isoelectric focusing (IEF). The first of these (MonEst) is specifically associated with haemopoietic cells of monocytic lineage, whereas the other species (ComEst) is common to all myeloid cells (granulocytes and monocytes) irrespective of lineage affiliation. Having recently purified these two species of human myeloid cell esterase, this present study extensively investigated the effects of 17 different inhibitors on their ability to hydrolyse the synthetic substrate alpha-naphthyl acetate (alpha NA). Significant inhibition of both ComEst and MonEst was exerted by 1% sodium dodecyl sulphate (SDS) and 1.0 mM diethyl pyrocarbonate (DEPC), but the patterns of inhibition for the two esterase species with the remaining compounds studied differed considerably; for example, 0.2 mM phenylmethylsulphonyl fluoride (PMSF), 5.0 x 10(-3) M dichloroisocoumarin (DCIC) and 0.1 mM N-tosyl-L-phenylalanine chloromethyl ketone (TPCK) all inhibited MonEst but not ComEst. Mechanisms of inhibition were also examined and these studies established that SDS, PMSF, DCIC and TPCK irreversibly inactivated MonEst whilst the inhibition of ComEst by SDS was reversible. Analysis of inhibition kinetics further showed that (a) the reversible inhibition of both ComEst and MonEst by sodium fluoride (NaF) was noncompetitive (with Ki values of 1.28 and 0.01 mM, respectively, indicating a marked difference in sensitivity); (b) the inhibition of MonEst by PMSF was of 'mixed' noncompetitive-competitive type; and (c) that DEPC exerted noncompetitive inhibition with similar Ki values (0.05 mM) for both esterase species. These observations unequivocably demonstrate that ComEst and MonEst are unrelated enzyme species, with a common ability to hydrolyse alpha NA, and that these esterase show marked differences with respect to their active sites as adjudged by inhibitor sensitivities. These observations are particularly relevant to the histochemical analysis of these enzymes and to the elucidation of their in vivo functions.

Nonspecific esterases of Mus musculus

Seventeen genes controlling the expression of carboxylic ester hydrolases, commonly known as esterases, have been identified in the mouse Mus musculus. Seven esterase loci are found on chromosome 8, where two clusters of esterase loci occur. It seems probable that the genes within these clusters have arisen from a common ancestral gene by tandem duplication. Close linkage of esterase genes is also found in the rat, rabbit, and prairie vole. Some mouse esterases appear to be homologous with certain human esterases. The function of these nonspecific enzymes is still unknown.

Snake venom action: are enzymes involved in it?

Enzymes were the first clearly recognized components of snake venoms. When several more were discovered, attempts were made to correlate venom action with enzymic functions. The last few years have seen most successful efforts in the identification, isolation and structrual elucidation of highly toxic polypeptides present in snake venoms, in particular of 'neurotoxins' and membrane-active toxins. Following this development the polypeptides were called the true toxic components and the enzymes lost their previous central position in venom pharmacology. The time, therefore, has come re-evaluate the role of enzymes in the complex interaction between snake and prey. While highly active polypeptides indeed dominate the actionof hydrophiid venoms, they appear to play a lesser role in crotalid venom action as compared with enzyme components. Enzymes are involved in many levels of venom action, e.g. by serving as spreading factors, of by producing very active agents, such as bradykinin and lysolecithins in tissues of preys or predators. Some toxins, e.g. the membrane-active polypeptides appear to participate in the interaction between membrane phospholipids and venom phospholipases. The classical neurotoxin, beta-bungarotoxin, has been recognized as a powerful phospholipase. Several instances are known which indicate that some enzymes potentiate the toxic action of others; the analysis of a single enzyme may, therefore, not fully reveal its biofunction. For 3 enzymes,ophidian L-amino acid oxicase, ATPpyrophosphatase, and acetylcholinesterase, some of the problems pertaining to venom toxicity are discussed.

Cholinesterases from plant tissues. VI. Preliminary characterization of enzymes from Solanum melongena L. and Zea mays L

Enzymes capable of hydrolyzing esters of thiocholine have been assayed in extracts of Solanum melongena L. (eggplant) and Zea Mays L. (corn). The enzymes from both species are inhibited by the anti-cholinesterases neostigmine, physostigmine, and 284c51 and by AMO-1618, a plant growth retardant and they both have pH optima near pH 8.0. The enzyme from eggplant is maximally active at a substrate concentration of 0.15 mM acetylthiocholine and is inhibited at higher substrate concentrations. On the basis of this last property, the magnitude of inhibition by the various inhibitors, and the substrate specificity, we conclude that the enzyme from eggplant, but not that from corn, is a cholinesterase.

The appearance of acetylcholinesterase in the myotome of the embryonic rabbit. An electron microscope cytochemical and biochemical study

Acetylcholinesterase (AChE) activity has been studied in the myoblast of skeletal muscle of the 9-13 day fetal rabbit. Cytochemical activity is present in the nuclear envelope and the endoplasmic reticulum, including its derivatives the subsurface reticulum and the sarcoplasmic reticulum. End product is also found in the Golgi complex of the more differentiated myoblasts. The formation of reticulum-bound acetylcholinesterase in the myoblast appears to be independent of nerve-muscle contact, since the enzyme is present before the outgrowth of the spinal nerve. The nerve lacks cytochemical end product until the myoblast is well differentiated. Possible mechanisms of spontaneous muscle contraction have been discussed. A second type of myotomal cell, which exhibits a poorly localized end product of AChE activity, has been described. The ready solubility of the enzyme or diffusibility of its end product suggests that the enzyme may be a lyoesterase. This cell may be the precursor of the morphologically undifferentiated cell which is closely apposed to the myotubes in later stages of skeletal muscle development. Biochemical studies show a significant increase in AChE activity in the dermomyotome by day 12, when many of the myoblasts are well differentiated and the second type of myotomal cell is prominent. Cytochemical studies have indicated that many of the cells in the sample lack reaction product of enzymic activity, whereas others are very active. Biochemical values, therefore, reflect the amount of enzyme in the dermomyotome as a whole, but give little information on the enzymic content of individual cells.