Inhibition dehydrase

Beyond pharmaceutical screening activity developed on aminothiazoles derivatives, some studies at the molecular level were performed. Thus 2-aminothiazole was shown to inhibit thiamine biosynthesis (941). Nrridazole (419) affects iron metabohsm (850). The dehydrase for 5-aminolevulinic acid of mouse liver is inhibited by 2-amino-4-(iS-hydroxy-ethyl)thiazole (420) (942) (Scheme 239). l-Phenyl-3-(2-thiazolyl)thiourea (421) is a dopamine fS-hydroxylase inhibitor (943). Compound 422 inhibits the enzyme activity of 3, 5 -nucleotide phosphodiesterase (944). The oxalate salt of 423, an analog of levamisole 424 (945) (Scheme 240),... 

M. Morisaki, K. Bloch, Inhibition of (5 -hyd roxyd ecan-oyl thioester dehydrase by some allenic adds and their thioesters , Bioorg. Chem. 1971,1,188. 

Answer C. Lead inhibits both ferrochelatase (increasing the zinc protoporphyrin) and ALA dehydrase (increasing 5-ALA). 

Lead inhibits ALA dehydrase, which biocks the condensation of ALA moiecuies into porphobiiinogen. 

Although this may seem straightforward, in some cases, the response is only indirectly related and is therefore not a useful parameter of toxicity to use in a dose-response study. This may apply to situations where enzyme inhibition is a basic parameter but where it may not relate to the overall toxic effect. For example, inhibition by lead of aminolaevulinic acid dehydrase, an enzyme, which is involved in heme synthesis, can be readily demonstrated to be dose related, but is clearly not an appropriate indicator of lead-induced renal toxicity in vivo. 

Biochemical changes such as increased aminolaevulinate excretion and inhibition of amino-laevulinate dehydrase may be detected in urine and blood, respectively, at blood lead levels of 0.4 to 0.6 mg mL-1. Anemia is a late feature, however. Neurotoxicity may be detectable at blood lead levels of 0.8 to 1.0 mg mL-1. At blood lead levels greater than 1.2 mg mL-1, encephalopathy occurs. Peripheral nerve palsies are rare, and the foot and wrist drop, which were once characteristic of occupational lead poisoning, only occur after excessive exposure and are now rarely seen. Similarly, seizures and impaired consciousness may result from involvement of the CNS. Bone changes are usually seen in children and are detected as bands at the growing ends of the bones and a change in bone shape. 

Formation of porphobilinogen The dehydration of two molecules of ALA to form porphobilinogen by 8-aminolevulinic acid dehydrase is extremely sensitive to inhibition by heavy metal ions (see Figure 21.3, and p. 279). This inhibition is, in part, responsible for the elevation in ALA and the anemia seen in lead poisoning. 

Ferrochelatase and ALA dehydrase are particularly sensitive to inhibition by lead... 

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. 

A classic example of suicide inhibition is that of /3-hydroxy l-decanoyl-dehydrase by 3 -decenoy 1-N-acetylcy steamine.12 The enzyme catalyzes the reaction... 

Lead is widely destributed in the environment, especially in industrial and urban areas, and it is readily absorbed into the mammalian body where it exerts a number of undesirable physiological effects. Its most dramatic action is the inhibition of human red cell 5-aminolaevulinic acid dehydrase activity71), but it also depresses the activities of many enzymes having functionsl -SH groups. Attempts to remove lead from the body using agents such as dimercaptopropanol can result in the formation of lipid-soluble lead complexes that may be carried to the brain and exacerbate the effects of lead poisoning. 

Mechanism (1), first suggested almost three decades ago [150], continues to be the most invoked explanation of the role of the enzyme in promoting homolysis. Cleavage of the C—Co bond of sterically hindered alkylcobalamins (e.g., neopentylcobalamin) was markedly increased by diol dehydrase [72], Such cobal-amins do not function as coenzymes but convert to enzyme-bound hydroxocobal-amin in stoichiometric first-order reactions. The strong competitive inhibition by AdoB 12 indicates that labilization occurs at the active site of the enzyme and is suggested to be caused by a steric distortion of the corrin ring. 

During the last nine years, a number of workers have attempted to generalize this concept of enzyme inactivation to the inhibition of enzymes other than 3-hydroxydecanoylthioester dehydrase. The reader is referred to excellent reviews by Rando ( ),... 

There is a general requirement for pyridoxal-5-phosphate (24, 25, 27, 44) although not all of the activity lost on dialysis is restored by adding the cofactor. This requirement explains the inhibition by hydroxylamine and hydrazine (24, 25). The reaction is a typical pyridoxal-5-phosphate catalyzed a,/ -elimination with a mechanism similar to serine dehydrase and cysteine desulfhydrase (45). The coenzyme is probably bound as a Schiff base with an amino group of the enzyme since there is an absorption maximum at 415 nm in solutions of the purified garlic enzyme (40). The inhibition by L-cysteine is presumably caused by formation of a thiazolidine with the coenzyme (46). Added pyridoxal-5-phosphate also combines directly with the substrate. The dissociation constant for the complex is about 5 X lO M. When this is taken into account, the dissociation constant of the holoenzyme can be shown to be about 5 X 10 M (47). The higher enzyme activity in pyrophosphate buflFer than in Tris or phosphate may be explained by pyrophosphate chelation of metal ions which otherwise form tighter complexes with the substrate and coenzyme (47). This decreases the availability of added coenzyme. 

Very toxic to most plants cumulative poison in mam- maJs. Inhibits 8-aminolevulinate dehydrase and thus hemogiobih synthesis in mammals (see Al). One of the symptoms of lead poisoning is anemia. Toxic to central nervous system. 

Amlno-l, 2,4-triazole NH, Inhibits imidazole glycerol phosphate dehydrase 9... 

Levels of delta-aminolevulinic acid (ALA) in the urine are also used as a measure of lead exposure. Increasing concentrations of ALA are believed to result from the inhibition of the enzyme delta-aminolevulinic acid dehydrase (ALA-D). Although the test is relatively easy to perform, inexpensive, and rapid, the disadvantages include variability in results, the necessity to collect a complete 24 hour urine sample which has a specific gravity greater than 1.010, and also the fact that ALA decomposes in the presence of light. 

FIGURE 1.6 In vitro inhibition of enzyme activity by metal ions is correlated with the absolute value of the logarithm of the first hydrolysis constant (i.e., K(,h for M"++ HjO —> MOH"" + H+) and the softness index (Op). Inhibition data for these three enzymes were produced by Christensen (1971/1972) and Christensen and Tncker (1976). White diamonds=catfish carbonic anhydrase ICjq, grey sqnares=white sucker lactic dehydrase ICjo and black triangles = white sucker glutamic oxaloacetic transaminase ICjo-... 

The results in Table 6 were obtained when both the overall rates of chain elongation and the condensation reaction were measured with 16 0 CoA, 6,9-18 2 CoA and 6,9,12-18 3 CoA as substrates The 8-hydroxy acyl-CoA dehydrase reaction was assayed only with the CoA derivatives of DL-8-hydroxy-stearic acid (8-OH-18 0 CoA) and DL-8-hydroxy-8,11-eicosadienoic acid (8-OH-8,11-20 2 CoA). The 2-trans-enoyl-CoA reductase reaction was assayed with the CoA derivatives of 2-trans-octadecenoic acid (2 trans-18 1 CoA) and 2-trans-8,ll-eicosatrienoic acid (2-trans-8,11-20 3 CoA) All reactions were assayed with the microsomes from rats raised on both a chow diet and a fat-free diet. Both the rates of the condensation reaction and the overall chain elongation reaction were depressed when rats were fed a chow diet These depressed rates of conversion were more pronounced when 16 0 CoA was used as substrate than when 6,9-18 2 CoA or 6,9,12-18 3 CoA were used as substrates. These results support our N-ethylmaleimide inhibition studies and are consistent with the concept that rat liver microsomes contain at least two different condensing enzymes. One condensing enzyme would preferentially utilize saturated substrates while another would act on unsaturated substrates. 

An absolute requirement for pyridoxal phosphate was found for the Neurospora enzyme, none for the E. coli enzyme. The failure to observe this in the latter may be connected with the difficulty of obtaining dehydrase preparations free of pyridoxal phosphate.Pyridoxamine phosphate could not substitute for the pyridoxal phosphate. That there is a requirement for pyridoxal phosphate is supported by the fact that the threonine dehydrase activity is strongly inhibited by 5 X 10 M hydroxylamine and cyanide. 

D-Serine dehydrase from both E. coli and Neurospora required pyridoxal phosphate as a coenzyme. In contrast to L-serine dehydrase, threonine is attacked very poorly by the D-enz3une. Magnesium ions had no activating affect on the E. coli enzyme, but it partially reversed the inhibition caused by the metal-binding reagents, phosphate, citrate, cysteine, cyanide, and 8-hydroxyquinoline of the Neurospora enzyme. ... 

Binkley and Okeson purified the enzyme system that cleaves cystathionine and found that neither phosphate nor ATP was required for activity, thus correcting the previous report that ATP was required. In addition to splitting cystathionine, this enzyme preparation also produced H2S from cysteine. The authors suggest that their enzyme may be identical with cysteine desulfhydrase. Binkley also reported that he had been able to synthesize cystathionine enzymatically from homocysteine and serine by a fractionated liver preparation which had been freed from the cystathionine cleavage enzyme, serine dehydrase and homoserine deaminase. The activity of the enzyme synthesizing cystathionine was either inhibited or unaffected by ATP, DPN, AMP, and various metal ions. 

FIGURE 17-5. Effects of lead on heme synthesis. Enzymes inhibited by lead are in italics. ALA aminolevulinic acid -ALAD = 5 amlnolevulinic acid dehydrase sucdnyl CoA = succinyl coenzyme A. 

---