Heavy metals enzyme inhibition

Considerable DNase but no RNase activity results if Ca-+ is replaced by Sr-+, while Fe-+ and Cu J+ cause minimal activation (3, 40). A number of heavy metal cations inhibit DNase and RNase activities competitively with Ca-+ Hg-+, Zn2+, and Cd-+ are the most potent of these (3). Studies with synthetic substrates, to be discussed below, indicate that Ca2+ is not only required for the proper binding of substrates but also that it is required for the subsequent independent hydrolytic process. Although several divalent cations can substitute for Ca2t in the binding function, as evidenced by their competitive inhibition of enzymic activity (3) and their ability to promote nucleotide binding (62), the catalytic role of Ca2+ appears to be unique. 

Enzyme inhibition sensors are of interest in the environmental context, the most used being those involving acetylcholinesterase inhibited by pesticides e.g. these can involve rather complex architectures, and characterisation of such systems by EIS is becoming more widespread. Complex sensor architectures have been used for endocrine disrupters, with similar characterisation by electrochemical impedance. Recently, EIS was used for the first time to characterise the response of glucose biosensors in the presence of heavy metal ion inhibition. ... 

The principle of this test is as follows The liquid suspected of containing urea is treated with dilute acid or alkali until its pH is about 7. A solution of the enzyme is also made and its pH adjusted to 7. The two solutions are mixed and the resulting conversion of urea to ammonium carbonate causes the pH of the solution to rise to over 8 this change is noted by the use of a suitable indicator, phenol-red being the one usually employed. Proteins do not interfere with the test, but the reaction is inhibited by traces of heavy metals. 

High concentrations of trace elements and heavy metals inhibit soil microorganisms and certain soil enzymes. The effects of trace elements on... 

Mn2+ active transport system in Staphylococcus aureus. These metal-microbe interactions result in decrease microbial growth, abnormal morphological changes, and inhibition of biochemical processes in individual (Akmal et al. 2005a,b). The toxic effects of metals can be seen on a community level as well. In response to metal toxicity, overall community numbers and diversity decrease. Soil is a living system where all biochemical activities proceed through enzymatic processes. Heavy metals have also adverse effects on enzyme activities (Fig. 1). 

The renal tubular dysfunction of galactosemia is very similar to that caused by, e.g., heavy metal poisoning in both cases it seems likely that the inhibition of enzyme systems prevents the cells of the renal tubule... 

Mercury can influence ion, water, and nonelectrolyte transport in different cells [ 14, 77]. The cell membrane is believed to be the first point of attack by heavy metals however, intracellular enzymes and metabolic processes may also be inhibited [70, 78, 79]. The attachment of heavy metals to ligands in or on the plasma membrane may result in changes in passive permeability or selective blockage of specific transport processes. Many membrane transport systems are known to be sensitive to sulphydryl-group modification [ 14, 80, 81]. 

Mn2+, D.F.P.-ase is further activated by cysteine, histidine, thiolhistidine, and serine, histamine and 2 2 -dipyridyl. Reagents reacting with metal ions, SH groups and carbonyl groups inhibit D.F.P.-ase activity. Work is proceeding on the further elucidation of such mechanisms.1 In a somewhat similar connexion attention is called to the fact that the non-enzymic hydrolysis of D.F.P. is accelerated by heavy metals and their complexes, in particular by copper chelates of ethylene diamine, o-phenanthroline, 2 2 -dipyridyl and histidine.2... 

Analyses of enzyme reaction rates continued to support the formulations of Henri and Michaelis-Menten and the idea of an enzyme-substrate complex, although the kinetics would still be consistent with adsorption catalysis. Direct evidence for the participation of the enzyme in the catalyzed reaction came from a number of approaches. From the 1930s analysis of the mode of inhibition of thiol enzymes—especially glyceraldehyde-phosphate dehydrogenase—by iodoacetate and heavy metals established that cysteinyl groups within the enzyme were essential for its catalytic function. The mechanism by which the SH group participated in the reaction was finally shown when sufficient quantities of purified G-3-PDH became available (Chapter 4). 

Disinfectants come from various chemical classes, including oxidants, halogens or halogen-releasing agents, alcohols, aldehydes, organic acids, phenols, cationic surfactants (detergents) and formerly also heavy metals. The basic mechanisms of action involve de-naturation of proteins, inhibition of enzymes, or a dehydration. Effects are dependent on concentration and contact time. 

Methods based on the inhibitory effect of the analyte and the use of an enzyme thermistor have primarily been applied to environmental samples and typically involve measuring the inhibitory effect of a pollutant on an enzyme or on the metabolism of appropriate cells [162]. The inhibiting effect of urease was used to develop methods for the determination of heavy metals such as Hg(II), Cu(II) and Ag(I) by use of the enzyme immobilized on CPG. For this purpose, the response obtained for a 0.5-mL standard pulse of urea in phosphate buffer at a flow-rate of 1 mL/min was recorded, after which 0.5 mL of sample was injected. A new 0.5-mL pulse of urea was injected 30 s after the sample pulse (accurate timing was essential) and the response compared with that of the non-inhibited peak. After a sample was run, the initial response could be restored by washing the column with 0.1-0.3 M Nal plus 50 mM EDTA for 3 min. Under these conditions, 50% inhibition (half the initial response) was obtained for a 0.5-mL pulse of 0.04-0.05 mM Hg(II) or Ag(I), or 0.3 mM Cu(II). In some cases, the enzyme was inhibited irreversibly. In this situation, a reversible enzyme immobilization technique... 

The role of GSH in cellular protection (see below) means that if depleted of GSH, the cell is more vulnerable to toxic compounds. However, GSH is compartmentalized, and this compartmentalization exerts an influence on the relationship between GSH depletion or oxidation and injury. The loss of reduced GSH from the cell leaves other thiol groups, such as those in critical proteins, vulnerable to attack with subsequent oxidation, cross-linking, and formation of mixed disulfides or covalent adducts. The sulfydryl groups of proteins seem to be the most susceptible nucleophilic targets for attack, as shown by studies with paracetamol (see chap. 7), and are often crucial to the function of enzymes. Consequently, modification of thiol groups of enzyme proteins, such as by mercury and other heavy metals, often leads to inhibition of the enzyme function. Such enzymes may have critical endogenous roles such as the regulation of ion concentrations, active transport, or mitochondrial metabolism. There is... 

The enzyme catalyzing the transfer of D-apiose from UDP-apiose to 4, 5-dihydroxyflavon-7-yl /3-D-glucopyranoside is commonly called apiin synthetase.7 Activity is measured by the formation of [14C]apiin from UDP-[U-14C]apiose. Apiin synthetase can be measured by the rapid separation and isolation, by poly(ethylenimine)-paper chromatography, of a product of the reaction, namely, [14C] apiin, from UDP-D-[U-14C]xylose and degradation products of UDP-[U-14C]apiose.31 There are reports of the isolation and purification of apiin synthetase from parsley leaves,31 from cell-suspension cultures of parsley,121 and from foxglove (Digitalis purpurea).31 Apiin synthetase isolated from parsley does not require metal ions, NAD+, or other cofactors, and is soluble. It is inhibited by several heavy metals, but not by tetra-N-... 

Irreversible Inhibition of an Enzyme Many enzymes are inhibited irreversibly by heavy metal ions such as Hg2+, Cu2+, or Ag+, which can react with essential sulfhydryl groups to form mercaptides ... 

Heavy metals stimulate or inhibit a wide variety of enzyme systems (16, 71, 72), sometimes for protracted periods (71, 73). These effects may be so sensitive as to precede overt toxicity as in the case of lead-induced inhibition of 8 ALA dehydrase activity with consequential interference of heme and porphyrin synthesis (15, 16). Urinary excretion of 8 ALA is also a sensitive indicator of lead absorption (74). Another erythrocytic enzyme, glucose-6-phosphatase, when present in abnormally low amounts, may increase susceptibility to lead intoxication (75), and for this reason, screens to detect such affected persons in lead-related injuries have been suggested (76). Biochemical bases for trace element toxicity have been described for the heavy metals (16), selenium (77), fluoride (78), and cobalt (79). Heavy metal metabolic injury, in addition to producing primary toxicity, can adversely alter drug detoxification mechanisms (80, 81), with possible secondary consequences for that portion of the population on medication. 

NADH as an end product. This implicates oxidized malic acid, either pyruvic or oxaloacetic acid, as another end product. By adding commercial preparations of L-lactic dehydrogenase or malic dehydrogenase to the reaction mixture, Morenzoni (90) concluded that the end product was pyruvic acid. Attempts were then made to show whether two enzymes—malate carboxy lyase and the classic malic enzyme, malate oxidoreductase (decarboxylating), were involved or if the two activities were on the same enzyme. The preponderance of evidence indicated that only one enzyme is involved. This evidence came from temperature inactivation studies, heavy-metal inhibition studies, and ratio measurements of the two activities of partially purified preparations of Schiitz and Radlers malo-lactic enzyme (76, 90). This is not the first case of a single enzyme having two different activities (91). 

Ion Concentration. Heavy metals, particularly calcium, inhibit enzyme activity. The only feasible method of removing them is with ion exchange resins. 

UREASE. Enzyme present in low-percentages in jackbean and soybean water soluble, its action is inhibited by heavy-metal ions. Its principal use is in the determination of urea in urine, blood, and other body fluids it splits urea into ammonia and carbon dioxide or ammonium carbonate. 

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