Urethan enzyme inhibition

It was at one time generally assumed chat physostigmlne, neostigmine, and related Inhibitors that possess a carbamyl ester linkage or urethane structure, in addition to a tertiary amino or quaternary ammonium group, inhibit Che enzyme in the same reversible fashion. However, careful kinetic studies showed chat physostigmlne and neostigmine are hydrolyzed by cholinesterase (11 14). 

Based upon studies involving the interaction of 6-acetylmethylenepenicil-lanic acid (27) with TEM-1 yff-lactamase, as well as its chemical reactivity with sodium methoxide and hydroxylamine, Arisawa and Adam [45] proposed a mechanism for the inhibition of TEM-1 )8-lactamase by (27) Scheme 6.10). They proposed the initial formation of a rather stable inactivated enzyme (55). This was followed by either slow hydrolysis of the acyl bond, releasing active enzyme, or rearrangement to (56). The stability of this intermediate was explained on the grounds of its pyrrolic structure the conjugated ester behaves as a vinylogous urethane, which is known to be resistant to hydrolysis [72]. 

Von Euler and Adler (1935) reported that 5 X 10 M monobromoacetate inhibited 50%. IX 10 M iodoacetate reduced the activity by 92% the degree of inhibition was a function of time of contact between inhibitor and enzyme (Dixon, 1937). Lutwak-Mann (1938) found that the yeast ADH was inhibited by cyanide, whereas the mammalian and bacterial enzymes were not. Urethane, oxalate, maleate, and pyrophosphate similarly inhibit yeast ADH quinine, morphine, and nicotine did not. Von Euler and Adler (1935) found no effect with sodium fluoride. 

Example compounds affecting the adrenal cortex include acrylonitrile, amino-gluthemide, amytriptyline, aniline, carbon tetrachloride (Colby et al. 1994), chloroform, cimetidine, etomidate, domperidone, fluphenazine, glycyrrhizin, ketoconazole (Loose et al. 1983), methanol, parathion, pentabarbitone, phencyclidine, pyra-zole, spironolactone, tamoxifen, and urethane (Colby and Longhurst 1992 Szabo and Sandoz 1997). Amytriptyline and cimetidine reduce corticosterone secretion, whereas pentobarbitone and phencyclidine increase its secretion. Etomidate inhibits 11(3- and 17a-hydroxylating reactions. Spironolactone affects cytochrome P450 enzymes (Kossor et al. 1991), and domperidone blocks cortisol secretion. Some ACAT inhibitors cause adrenal cytotoxicity (Wolfgang et al. 1995). 

Cytochrome b. A fundamental difference between soluble succinic dehydrogenase and all of the particles capable of reducing oxidants other than phenazines is the presence in the latter preparations of heme. Reduction of the enzyme by succinate or hydrosulfite causes the appearance of bands near 560 m/i, 525 m i, and 428 mju. These are approximately the bands assigned to cytochrome b. When succinoxidase is treated with urethane, the over-all reaction is inhibited and on the addition of substrate the bands of reduced cytochrome b appear, but the other cytochromes remain oxidized. This observation can be duplicated by using... 

While the studies of Boyland and Roller and Elion and co-workers, which were conducted in vivo, do suggest that urethane has a specificity for pyrimidine biosynthesis, Kaye could not demonstrate in vitro any significant inhibition by urethane of several enzymes involved in nucleic acid metabolism. Both urethane and its A -hydroxy metabolite bear a structural resemblance to carbamyl phosphate and carbamyl-L-aspartate. The enzyme aspartate transcarbamylase begins pyrimidine biosynthesis by catalyzing the formation of carbamyl-L-aspartate from carbamyl phosphate and l-aspartate. Giri and Bhide have reported that in vivo administration of urethane decreased aspartate transcarbamylase activity of lung tissue of adult male and (to a lesser extent) female mice no in vitro inhibition could be demonstrated. 

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