Acetylation of p-aminobenzoic acid

Acetylated MS-222 was found in much higher concentrations in the urine than in the blood of rainbow trout. This suggests that the kidney concentrated the drug metabolite, or that MS-222 was acetylated in the kidney and excreted in the urine (28). Weber (31) stated that acetylation of p-aminobenzoic acid and sulfamethazine is catalyzed by most of the tissues in the body. He showed that in rabbits the acetylation of these amines by the kidney of rabbit is a small percentage of the total acetylation capability, but that the kidney is involved in this biotransformation. 

The activity of acetyltransferase as measured by the rate of acetylation of p-aminobenzoic acid was significantly reduced in liver biopsy samples from subjects with cirrhosis compared to controls [79]. 

Decreased acetylation of p-aminobenzoic acid, sulfonamides, and other drugs, thus reflecting reduced availability of acetyl CoA for these reactions. 

Table 3 Extent of Urinary Excretion of p-Aminobenzoic Acid (PABA) and Its Acetyl Metabolite as a Function of Route of Administration and Ingestion of Fat... 

Table 3 The extent of urinary excretion of p-aminobenzoic acid and its acetyl metabolite as a function of rate and route of administration in one subject...

Acetylated conjugates. Phase I Biotransformation products that contain amino, hydroxyl, or sulfur groups may be acetylated, producing conjugates that can be excreted in the urine. An example of a Phase I product that would be acetylated is p-aminobenzoic acid, producing p-(N-acetamido)benzoic acid. 

Although the two forms of the enzyme have preferred substrates, there is overlap between them such that no substrate seems to be exclusively acetylated by one or the other. Some preferred NAT1 substrates are p-aminobenzoic acid and p-aminosalicylic acid and sulfanilamide, whereas preferred substrates for NAT2 include isoniazid, hydralazine, procainamide, and dapsone. 

Figure 5.23 Frequency distribution for the acetylation of (A) procainamide (B) and p-aminobenzoic acid in human subjects. Data represents excretion of acetylated product in the urine 6 hours after dosing. Source From Ref. 28.

There are no functional tests of pantothenic acid nutritional status that are generally applicable. Deficiency of pantothenic acid impairs the ability to acetylate a variety of drugs, such as p-aminobenzoic acid, but this has not been developed as an index of vitamin status. The capacity to acetylate drugs is genetically determined neither experimental pantothenate deficiency nor the administration of supplements affects the determination of fast or slow acetylator status (Pietrzik et al., 1975 Vas et al., 1990). 

The NBT-PABA test of pancreatic function is based on the hydrolysis, by chymotrypsin, of a synthetic tripeptide— N-benzoyl-l-tyrosyl-p-aminobenzoic acid. The tripeptide, variously called NBT-PABA, BTP, or bentiromide, is administered orally together with a test meal to stimulate pancreatic secretion. BTP is specifically hydrolyzed by chymotrypsin in the duodenum to release PABA, which is subsequently absorbed in the intestinal tract and metabolized in the liver to hippurate and to PABA glucuronide and PABA acetylate. These arylamines are then excreted by the Iddney. In the presence of low chymotrypsin, as found in pancreatic insufficiency, less peptide is hydrolyzed, and therefore less chromogen is excreted in the urine or found in serum. Thus the amount of PABA detected in serum or urine is an indirect measure of chymotrypsin activity in duodenal content. 

For CES, there appear to be no regional differences in the small intestine for the N-acetyltransferase (NAT) activity [108]. The presence ofboth NAT isozymes has been demonstrated in the human gut mucosa by using the prototypical substrates p-aminobenzoic acid (NAT1) and sulfamethazine (NAT2) [109, 110]. The active metabolites 5-aminosalicylic acid and sulfapyridine of the prodrug sulfasalazine undergo extensive presystemic acetylation in the small intestine [111]. 

Examination of liver N-acetyltransferase obtained from rapid acetyla-tor rabbits on polyacrylamide-agarose gel electrophoresis has shown that two anodal migrating components with enzymic activity are regularly present (Szabadi, 1970). The slower migrating component accounts fdr most of the activity (ca. 67-83%). Both components possess sulfamethazine and p-aminobenzoic acid acetylating activity. Preliminary comparative studies of the electrophoretic patterns for rapid and slow acetylator rabbits have also been carried out. These studies, which are described in Section III,D, provide some clues about the possible nature of each of these electrophoretic components. 

A comparative study of the capacity of these tissues to acetylate sulfamethazine and p-aminobenzoic acid was carried out also. The results obtained are typified by the data for liver and spleen given in Fig. 7. 

Fig. 7. Distribution of sulfamethazine and p-aminobenzoic acid acetylating activity in rabbit liver (black bars) and spleen (white bars). Upper graph substrate, sulfamethazine lower graph substrate, p-aminobenzoic add.

No systematic studies of the relative ability to acetylate p-aminoben-zoic acid and sulfamethazine have been carried out with human tissues. Juchau et al. (1968) have observed, however, that p-aminobenzoic acid is acetylated by preparations of human immature placentas (9000 g supernatant fraction), while acetylation of isoniazid could not be detected. 

The concept that enzymic acetylation is a stepwise process involving the formation and breakdown of an acetylated enzyme intermediate has been suggested by several investigators. Bessman and Lipmann (1953) observed that a pigeon liver preparation reversibly catalyzed the acetyl exchange between 4-acetaminoazobenzene-4 - sulfonic acid and various aryl amines including aniline, p-aminobenzoic acid, and sulfanilamide. They deduced that an acetyl-enzyme compound was formed as an intermediate... 

The transfer of the acetyl group to the amine is catalysed by a number of different enzymes. In man the rates of acetylation of isoniazid. sulphamethazine, and hydralazine are bimodally distributed into fast acetylators and slow acetyl-ators (see also Sect. B, Chap. 10). On the other hand, there is no such genetic variability in the acetylation of p-aminosalicylic add or p-aminobenzoic acid. 

These derivatives may be obtained by reduction of the appropriate nitro derivative catalytically or with a metal-acid system, or by Beckmann or Hofmann rearrangements of suitable acyl or carboxamido derivatives. 4-Aminobenzo[6]thiophene has also been prepared by means of a Bucherer reaction with 4-hydroxybenzo[6 Jthiophene. Several 5-aminobenzo[6]thiophenes have been prepared by cyclization reactions of p-acetamino-phenylthio derivatives. 6-Acetaminobenzo[6 Jthiophenes may be obtained from the corresponding 6-acetyl derivative by Schmidt or Beckmann rearrangements. 7-Aminobenzo[6 ]thiophene can also be prepared from 7-hydroxybenzo[6 ]thiophene by a Bucherer reaction (70AHC(ll)l77). 

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