Metabolic blocking

FIGURE 18.12 The use of inhibitors to reveal the sequence of reactions in a metabolic pathway, (a) Control Under normal conditions, the steady-state concentrations of a series of intermediates will be determined by the relative activities of the enzymes in the pathway, (b) Plus inhibitor In the presence of an inhibitor (in this case, an inhibitor of enzyme 4), intermediates upstream of the metabolic block (B, C, and D) accumulate, revealing themselves as intermediates in the pathway. The concentration of intermediates lying downstream (E and F) will fall. 

As an inhibitor of xanthine oxidase, allopurinol also markedly decreases oxidation of both hypoxanthine and xanthine itself to the sole source of uric acid (19) in man. This metabolic block thus removes the source of uric acid that in gout causes the painful crystalline deposits in the joints. It is of interest that allopurinol itself is oxidized to the somewhat less effective drug, oxypurinol (21), by xanthine oxidase. 

The second type of fatty liver is usually due to a metabolic block in the production of plasma lipoproteins, thus allowing triacylglycerol to accumulate. Theoretically, the lesion may be due to (1) a block in apolipoprotein synthesis, (2) a block in the synthesis of the lipoprotein from lipid and apolipoprotein, (3) a failure in provision of phospholipids that are found in lipoproteins, or (4) a failure in the secretory mechanism itself. 

Metabolic disorders of urea biosynthesis, while extremely rare, illustrate four important principles (1) Defects in any of several enzymes of a metabolic pathway enzyme can result in similar clinical signs and symptoms. (2) The identification of intermediates and of ancillary products that accumulate prior to a metabolic block provides insight into the reaction that is impaired. (3) Precise diagnosis requires quantitative assay of the activity of the enzyme thought to be defective. (4) Rational therapy must be based on an understanding of the underlying biochemical reactions in normal and impaired individuals. 

All defects in urea synthesis result in ammonia intoxication. Intoxication is more severe when the metabolic block occurs at reactions 1 or 2 since some covalent linking of ammonia to carbon has already occurred if citrulline can be synthesized. Clinical symptoms common to all urea cycle disorders include vomiting, avoidance of high-protein foods, intermittent ataxia, irritability, lethargy, and mental retardation. The clinical features and treatment of all five disorders discussed below are similar. Significant improvement and minimization of brain damage accompany a low-protein diet ingested as frequent small meals to avoid sudden increases in blood ammonia levels. 

There is no associated impairment of hydroxyprohne catabolism. The metabolic block in type II hyperpro-linemia is at glutamate-7-semiaIdeliyde dehydrogenase, which also functions in hydroxyprohne catabolism. Both proline and hydroxyprohne catabohsm thus are affected and A -pyrroline-3-hydroxy-5-carboxylate (see Figure 30-10) is excreted. 

Patel, J. B., J. Mehta et al. (2007). Novel retinoic acid metabolism blocking agents have potent inhibitory activities on human breast cancer cells and tumour growth. Br. J. Cancer 96(8) 1204—1215. 

Schwarz et al. (S10) showed that the erythrocytes of galactosemic subjects accumulated large amounts of galactose-l-phosphate, both in vivo and in vitro. This indicated that the metabolic block in galactosemia occurred at reaction (2) or reaction (3). Kalckar et al. (K7) and Isselbacher et al. (19) later showed that normal erythrocytes contained all 4 enzymes needed for reactions (1), (2), (3), and (4), and that in galactosemia there was virtually complete absence of galactose-l-phos-... 

Figure 21. Secondary metabolism blocks and amino acid derivation. Note that shikimic acid can be derived directly from photosynthesis and glycolysis through the pentose phosphate cycle, or alternatively as a pyruvic acid postcursor.

As a challenge to the hypothesis that hydroxylation at the 6-position of the N,N-dialkyltryptamines might play some role in the expression of the activity, this position was metabolically blocked by the insertion of a fluorine atom there, giving 6-F-DET. This compound, with DET as the control, was studied in some twelve hospitalized alcoholics at doses of about... 

Figure 20-11 Biosynthesis and catabolism of glycosphingolipids. The heavy bars indicate metabolic blocks in known diseases.

Figure 14.1 Triglyceride cycle in the pathogenesis of fatty liver. = are metabolic blocks. (From S. A. Meyer, Introduction to Biochemical Toxicology, 3rd ed., Wiley, 2001.)...

Bearder, J.R., MacMillan, J., Weis, C.M., Chaffey, M.B., Phinney, B.O. "Position of the metabolic block for gibberellin biosynthesis in mutant Bl-Ula of Gibberella fujikuroi." Phytochemistry, 19lh, 13, 911-917. 

Guo L., Latli B., Prestwich G. D. and Blomquist G. J. (1991) Metabolically-blocked analogs of the housefly sex pheromone II. Metabolism studies. J. Chem Ecol. 17, 1769-1782. 

Methylmalonyl-CoA mutase is a cobalamin-linked enzyme of mitochondria that catalyzes the isomerization of methylmalonyl-CoA to succinyl-CoA. A reduction of this enzyme due to vitamin B12 deficiency will result in a metabolic block with the urinary excretion of methylmalonic acid, and the measurement of this metabolite has been used to confirm a deficiency of vitamin B12. The test has also been useful in investigating rare abnormalities of this enzyme that result in the excretion of methylmalonic acid in the presence of adequate vitamin B12. Given an oral loading dose of valine or isoleucine will increase the urinary excretion of methylmalonic acid in patients with a vitamin B12 deficiency (G4). However, Chanarin and his colleagues (CIO) found that one-quarter of their patients with pernicious anemia excreted a normal concentration of methylmalonic acid even after a loading dose of valine. Normal subjects excrete up to 15 mg of methylmalonic acid in their urine over a 24-hour period (Cll). 

El-Kazzaz, W., Morita, T., Tagami, H., Inada, T., Aiba, H. Metabolic block at early stages of the glycolytic pathway activates the Res phosphorelay system via increased synthesis of dTDP-glucose in Escherichia coli. Mol Microbiol 51 (2004) 1117-1128. 

Wittman MD, Altstadt TJ, Fairchild C et al. (2001) Synthesis of metabolically blocked Paclitaxel analogues. Bioorg Med Chem Lett 11 809-810... 

Fig. 5 Metabolic blocks caused by biotin deficiencies. There are a small number of proteins that are biotinylated in vivo. Note that the naturally occurring biotin and biotinylated proteins may interfere with applications that use strept(avidin) on biological samples...

Effect of Spore Dormancy. Spore dormancy may contribute to a decline in bioherbicide efficacy. Spore dormancy in fungal spores may be maintained by chemicals as well as metabolic blocks (3A) The presence of inhibitors may be indicated by an Increase in efficacy upon addition of an adsorbent such as activated, charcoal to the bioherbicide at the time of inoculation. This was the case when adsorbent was added to F. lateritium spores (10 spores/ml) and the occurrence of dew (20 h) was delayed for 8 h after inoculation (Figure 6). The agent causing spore dormancy may be synthesized by either the spore or host. 

Josefsson, E. (1973) Studies on the biochemical background to differences in glucosi-nolate content in Brassica napus (L.). 111. Further studies to localize metabolic blocks. Plant Physiol, 29, 28-32. 

Puri A, Rawat SS, Lin HM, Finnegan CM, Mikovits J, Ruscetti FW, Blumenthal R. An inhibitor of glycosphingolipid metabolism blocks HIV-1 infection of primary T-cells. Aids 2004 18 849-858. 

Acclimation. The magnitude of the toxic effect generated by a substance can often be reduced significantly if the concentration is increased slowly. This is the phenomenon of acclimation which represents an adjustment of the biological population to the adverse effects of the toxin. In a waste treatment system this does not usually represent either a mutation or a selection of the fittest because rarely is all of the toxic effect eliminated. Rather acclimation represents a rearrangement of the metabolic resources of the organism to overcome the metabolic block produced by the toxic substance. Significant reductions in toxicity may be obtained if the concentration of the toxic substance is increased slowly rather than increased all at once. 

In hereditary orotic aciduria, orotic acid is excreted in the urine because the enzymes that convert it to uridine monophosphate, orotate phosphoribosyl transferase and orotidine 5 -phosphate decarboxylase, are defective (see Figure 7-20). Pyrimidines cannot be synthesized, and therefore, normal growth does not occur. Oral administration of uridine bypasses the metabolic block and provides the body with a source of pyrimidines. 

---