Enzymes acetylation status

The family of HDAC enzymes has been named after their first substrate identified, i.e., the nuclear histone proteins. Histone proteins (H2A, H2B, H3 and H4) form an octamer complex, around which the DNA helix is wrapped in order to establish a condensed chromatin structure. The acetylation status of histones is in a dynamic equilibrium governed by histone acetyl transferases (HATs), which acetylate and HDACs which are responsible for the deacetylation of histone tails (Fig. 1). Inhibition of the HDAC enzyme promotes the acetylation of nucleosome histone tails, favoring a more transcriptionally competent chromatin structure, which in turn leads to altered expression of genes involved in cellular processes such as cell prohferation, apoptosis and differentiation. Inhibition of HDAC activity results in the activation of only a limited set of pre-programmed genes microarray experiments have shown that 2% of all genes are activated by structmally different HDAC inhibitors [1-5]. In recent years, a growing number of additional nonhistone HDAC substrates have been identified, which will be discussed in more detail below. 

The impact of small molecules on the acetylation status of histones has attracted the interest of the medicinal chemistry community for almost a decade now. Nevertheless, the fast and reversible increase in cellular histone acetylation in the presence of -butyrate was already recognized in 1977 by Riggs et al. (Fig. 3) [30]. Two years later, it was proven that n-butyrate, among some related and less active small linear aliphatic carboxylates, was a noncompetitive inhibitor of histone deacetylating enzymes [31-34]. More than ten years after the initial interest in -butyrate, Yoshida et al. showed that trichostatin A (TSA, Fig. 3), originally reported as an antifungal agent [35],... 

However, using "polymorphic" substrates with the isolated enzyme, no correlation could be found between Km and acetylator status only when "monomorphic" substrates (p-aminobenzoic acid and p-aminosalicylic acid) were used was this apparent (Table 5.15). It is notable that both these substrates are negatively charged at physiological pH. 

Rifampicin (a known potent liver enzyme inducer) increases the metabolism and clearance of the phenytoin from the body so that a larger dose is needed to maintain adequate serum levels. Isoniazid inhibits the liver microsomal enzymes that metabolise phenytoin, and as a result the phenytoin accumulates and its serum levels rise. Only those who are slow acetylators (slow metabolisers) of isoniazid normally attain blood levels of isoniazid that are sufficiently high to cause extensive inhibition of the phenytoin metabolism. Fast acetylators (fast metabolisers) remove the isoniazid too quickly for this to occur. Acetylator status is genetically determined. Thus some individuals will show a rapid rise in phenytoin levels, which eventually reaches toxic concentrations, whereas others will show only a relatively slow and unimportant rise to a plateau within, or only slightly above the therapeutic range. 

How many of the 14 NADPH needed to form one palmitate (Eq. 25.1) can be made in this way The answer depends on the status of malate. Every citrate entering the cytosol produces one acetyl-CoA and one malate (Figure 25.1). Every malate oxidized by malic enzyme produces one NADPH, at the expense of a decarboxylation to pyruvate. Thus, when malate is oxidized, one NADPH is produced for every acetyl-CoA. Conversion of 8 acetyl-CoA units to one palmitate would then be accompanied by production of 8 NADPH. (The other 6 NADPH required [Eq. 25.1] would be provided by the pentose phosphate pathway.) On the other hand, for every malate returned to the mitochondria, one NADPH fewer is produced. 

At least two enzymes compete for acetyl-CoA - the citrate synthase and 3-ke-tothiolase. The affinities of these enzymes differ for acetyl-CoA (Table l),and at low concentrations of it the citrate synthase reaction tends to dominate, provided that the concentration of 2/H/ is not inhibiting. The fine regulation of the citrate synthases of various poly(3HB) accumulating bacteria has been studied [ 14, 47, 48]. They appear to be controlled by cellular energy status indicators (ATP, NADH, NADPH) and/or intermediates of the TCA cycle. The 3-ketothio-lase has also been investigated [10-14,49, 50]. This enzyme is, above all, inhibited by CoASH [10,14,49]. This important feature will be further considered below. 

The extent to which a sulfonamide is acetylated depends upon the drug administered and the animal species. Acetylsulfathiazole is the principal metabolite found in the urine of cattle, sheep, and swine after enteral or parenteral administration of sulfathiazole. However, sheep can acetylate only 10% of the dose, while cattle can acetylate 32%, and swine 39%. When sulfamethazine was administered intravenously or orally to cattle, the animals eliminated 11% or 25% of the dose, respectively, in urine as N" -acetylsulfamethazine. The increased acetylation that occurred following tlie oral administration may be related to the increased exposure of sulfamethazine to liver enzymes following its absorption into the portal circulation. The acetylation rate may also be affected by the health status of an animal. Tims, cows suffering from ketosis in cows acetylate sulfonamides at much lower extent. 

Jhis article discusses the present status of the mechanism of carbamyl phosphate (carbamyl-P) formation and illustrates that the reagents acetyl phosphate (acetyl-P) and carbamyl-P can replace each other with a number of well defined and/or highly purified enzymes. 

Figure 47-SO The major metabolic pathways for the use of ammonia by the hepatocyte. Solid bars indicate the sites of primary enzyme defects in various metabolic disorders associated with hyperammonemia /) carbamyl phosphate synthetase I, (2) ornithine transcarbamylase, (3) argininosuccinate synthetase, (4) argininosuccinate lyase, (5) arginase, (6) mitochondrial ornithine transport, (7) propionyi CoA carboxylase, (fi) methylmalonyl CoA mutase, (9) L-lysine dehydrogenase, and (10) N-acetyl glutamine synthetase. Dotted lines indicate the site of pathway activation (+) or inhibition ( ). (From Flannery OB, Hsia YE, Wolf 6. Current status of /lyperommofiemjo syndromes. Hepatology 1982 2 495-506,)...

Genetic factors are particularly important in humans and can influence the response to the compound or the disposition of the compound and hence its toxicity. Several genetic factors affecting metabolism are known in which a non-functional or less functional form of the enzyme is produced in a particular phenotype, e.g. acetylator phenotype (N-acetyltransferase NAT2) hydroxylator status (cytochrome P-450 2D6) esterase deficiency (pseudocholinesterase). 

---