Hydrolytic Pathways

Many drugs have functional groups that can be metabolized by the addition of water. The major functional groups involved are esters, amides, and epoxides. Several phase II metabolites such as sulfates and glucuronides, which will be discussed in Chapter 7, can also be hydrolyzed back to the parent drug. 

A good example to illustrate the difference in the rates of hydrolysis of esters and amides is to compare the metabolism of procaine and procainamide because the only difference between the two drugs is that one is an ester and the other is an amide (Fig. 6.2). Procaine has a half-life of about 1 minute due to the rapid hydrolysis of the ester, whereas 

FIGURE 6.2 Examples of the relative rates of hydrolysis of an ester, the amide of an aromatic amine, and the amide of an aliphatic amine. 

There are few drugs that are thioesters, but you may recall that one of the intermediates in the oxidation of aldehydes by aldehyde dehydrogenase is a thioester involving the thiol of the enzyme (Fig. 30 in Chapter 4), which is readily hydrolyzed back to the native form of the enzyme, a thiol, and the carboxylic acid product. Some drugs that are carboxylic acids, such as enaloprilate, are administered as ester prodrugs (enalopril), which are more readily absorbed from the intestine than the carboxylic acid and are then readily hydrolyzed to the active drug by esterases as mentioned in Chapter 1 (Fig. 1 in Chapter 1). 

Although hydrolytic enzymes, esterases and amidases, are named after their major substrates, the same enzyme can often hydrolyze esters, thioesters, and amides therefore, the differentiation between esterases and amidases is sometimes artificial. The highest hydrolytic activity is in the liver, but the enzyme pseudocholinesterase is found in the serum. Gut bacteria also contain hydrolytic enzymes. 

Most chemical reactions involving phosphates in the biological system are either the addition (phosphorylation) or removal (hydrolysis) of phosphate. In the case of biological phosphorylations, the phosphate source or donor is the energy store ATP (more shall be said about this in Section 3.4.1) and the reaction is catalyzed by an enzyme often referred to as a kinase. A simple example is the phosphorylation of the sugar glucose  

Acid and base-catalyzed hydrolysis of carboxylate esters usually proceeds OH 

Acyl fission merely regenerates starting materials. The acid-catalyzed hydrolysis of tcrt-butyl acetate proceeds by an SnI process (stable carbonium intermediate) with alkyl fission. 

Neutral phosphate triesters will readily undergo acid or base-catalyzed hydrolysis with alkyl and/or phosphoryl fission. For example, trimethyl and triethyl phosphate will undergo hydrolysis in neutral water via an Sn2 

The base-catalyzed hydrolysis of trimethyl phosphate proceeds by phos-phoryl fission and allows a preparative synthesis of dimethyl phosphate. 

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Castro CE, SK O Shea, W Wang, EW Bartnicki (1996) Biodehalogenation oxidative and hydrolytic pathways in the transformations of acetonitrile, chloroacetonitrile, chloroacetic acid, and chloroacetamide by Methylosinus trichosporium OB-3b. Environ Sci Technol 30 1180-1184. 

The metabolism of fluorobenzoates has been examined over many years. Early studies using Nocardia erythropoUs (Cain et al. 1968) and Pseudomonas fluorescens (Hughes 1965) showed that although the rates of whole-cell oxidation of fluorobenzoates were less than for benzoate, they were comparable to, and greater than for, the chlorinated analogs. As for their chlorinated analogs, both dioxygenation and hydrolytic pathways may be involved, and studies have revealed that the different pathways depended on the positions of the fluorine substituents. 

When the drug is nonionizable in water, three hydrolytic pathways are available [Eq. (33)] it can degrade by specific acid catalysis represented by the first kinetic term in Eq. (33), water hydrolysis (second term), and specific base catalysis (third term) ... 

Fig. 8.1. Hydrolytic pathways in the activation of glycolic acid esters as prodrugs of active carboxylic acids. Reaction a is the desirable, direct activation, whereas Reaction b must be seen as parasitic in view of the relative slowness of Reaction c.

Fig. 8.2. Hydrolytic pathways in the activation of prodrugs of carboxylic acids that contain a) an (acyloxy)methyl or b) an [(alkoxycarbonyl)oxy]methyl group. Liberation of the active acid occurs with comparable rates whatever the initial site of hydrolytic attack is. Formaldehyde or acetaldehyde is liberated when R = H or Me, respectively.

For a given pesticide which undergoes hydrolysis, any or all of these hydrolytic pathways may be relevant at various pH s. Organophosphorothioates, for example, have measurable neutral and alkaline hydrolysis rate constants (7). Esters of 2,4-dichlorophenoxyacetic acid (2,4-D), on the other hand, hydrolyze by acid and alkaline catalyzed reactions, but have extremely small neutral hydrolysis rate constants ( ). Thus, any study of the hydrolysis of sorbed pesticides must be prefaced by an understanding of the hydrolytic behavior of individual pesticides in aqueous solution. 

The phosphoryl group of the intermediate can enter two different reaction pathways leading to its decomposition. The phosphoryl group can either be transferred to water or to ADP. The hydrolytic pathway leading to the liberation of phosphate must be coupled to calcium translocation as it infers from the fixed coupling between calcium accumulation and phosphate liberation. 

Genetic factors influence the rate of not only synthesis of proteins but also their breakdown, i.e., the rate of turnover. As we have seen in Chapter 10, some enzymes are synthesized as inactive proenzymes which are later modified to active forms, and active enzymes are destroyed, both by accident and via deliberate hydrolytic pathways. Protein antienzymes may not only inhibit enzymes but may promote their breakdown.35 An example is the antienzyme that controls ornithine decarboxylase, a key enzyme in the synthesis of the polyamines that are essential to growth.36,37 As with all cell constituents, the synthesis of enzymes and other proteins is balanced by degradation. 

The proportion of 1,4-anhydroribitol formed by treatment of teichoic acids and synthetic poly(ribitol phosphate) with alkali is small, and the major hydrolytic pathway involves the cyclic phosphate sequence. No 1,4-anhydroribitol glycosides have been observed in the alkaline hydrolyzates of teichoic acids possibly, the presence of a glycosyl substituent makes the reaction sterically less favorable than when such substituents are absent. 

The present site-selective scission proceeds by an hydrolytic pathway, as is the case in DNA scission by nucleases. Thus, the scission fragments can be recombined with various oligonucleotides by using DNA ligase. Figure 7.6 depicts a typical example. 

Under conditions not favoring dehydration alternative hydrolytic pathways predominate.233 Thus, if the carbinolamine derived from compound (101) is treated with aqueous base, the main product is the octalone (102).234 This annelation clearly has great potential in synthesis, and it has already been exploited in, for example, the construction of the AB ring system of steroids (Stork and McMurry235 see also Ohashi236). 

Figure 9.3. The two main pathways for metabolism of PTX-2 in shellfish. The oxidative pathway has so far been confirmed only in P. yessoensis. The hydrolytic pathway appears to occur in all other shellfish species studied, including mussels, clams, and other species of scallop.

PCB arene oxides can also generate dihydrodiols via a hydrolytic pathway mediated by microsomal epoxide hydrolase, although metabolism to monohydroxy-metabolites is more commonly observed [74,75]. OH-PCBs are susceptible to further metabolism, i. e. conjugation reaction with glucuronic acid or sulfate, which increases the water solubility and facilitates excretion. Glucuronic acid and sulfate conjugates of several PCB congeners have been determined in bile and urine from experimental animals exposed to the PCBs [50,76,77]. Biliary excretion is the preferred pathway for PCB metabolites, whereas only a small portion is excreted via the urine [77]. 

The alternative sphcing mode is the hydrolytic pathway (Fig. 2b). As the name indicates, a water molecule is the nucleophile that attacks the 5 -splice site in this reaction mode, which generates a linear intron. The second sphcing step is identical to that of the branching pathway. 

A very interesting feature of group II introns is their modularity the intronic domains can be provided separately to form a functional ribozyme (9). For example, the exD123 construct consists of a 5 -exon and Dl, D2, and D3. This construct alone is umeactive, but addition of the catalytically essential domain D5 in trans generates an active two-piece ribozyme that cleaves off the 5 -exon it is therefore a mimic for the hydrolytic pathway of the first step of splicing. Similarly, an intron construct... 

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