Pyridoxal-dependent enzymes

Ornithine decarboxylase is a pyridoxal dependent enzyme. In its catalytic cycle, it normally converts ornithine (7) to putrisine by decarboxylation. If it starts the process with eflornithine instead, the key imine anion (11) produced by decarboxylation can either alkylate the enzyme directly by displacement of either fluorine atom or it can eject a fluorine atom to produce viny-logue 12 which can alkylate the enzyme by conjugate addidon. In either case, 13 results in which the active site of the enzyme is alkylated and unable to continue processing substrate. The net result is a downturn in the synthesis of cellular polyamine production and a decrease in growth rate. Eflornithine is described as being useful in the treatment of benign prostatic hyperplasia, as an antiprotozoal or an antineoplastic substance [3,4]. 

Suicide substrates have not been applied yet to proteases as extensively as they have to other classes of enzymes such as pyridoxal dependent enzymes (42, 43). The potential high selectivity obtainable with suicide substrates certainly makes pursuit of such inhibitors a worthy goal. 

Silverman and associates explored a variety of potential inactivators for GABA [y-aminobutyric acid, H3T (CH2)3COOH] transaminase, another pyridoxal-dependent enzyme. In the reaction of the enzyme with 4-amino-5-fluoropentanoic acid, Silverman and Invergo wrote the mechanism in equation 25 for the covalent interaction of the enzyme with the inactivator161. The mechanism, dubbed the enamine mechanism, was earlier suggested by Metzler s group162, who had also proposed, as a test, alkaline treatment of the inactivated enzyme that would result in the release of the coenzyme-bound modified inactivator. 

All terpenoid indole alkaloids are derived from tryptophan and the iridoid terpene secologanin (Fig. 2b). Tryptophan decarboxylase, a pyridoxal-dependent enzyme, converts tryptophan to tryptamine (62, 63). The enzyme strictosidine synthase catalyzes a stereoselective Pictet-Spengler condensation between tryptamine and secologanin to yield strictosidine. Strictosidine synthase (64) has been cloned from the plants C. roseus (65), Rauwolfla serpentine (66), and, recently, Ophiorrhiza pumila (67). A crystal structure of strictosidine synthase from R. serpentina has been reported (68, 69), and the substrate specificity of the enzyme can be modulated (70). 

Aspartate Aminotransferase Is a Member of a Large and Versatile Family of Pyridoxal-Dependent Enzymes... 

Penicillamine, both L-penicillamine and the racemic mixture, strongly inhibit pyridoxal-dependent enzymes, cause pyridoxine deficiency in animal experiments, and are neurotoxic. Although this effect is much weaker with D-penicillamine, a few case reports have shown that D-penicillamine can also occasionally cause a polyneuropathy, as either a toxic or an allergic reaction (69-72). Rarely, an optic neuropathy (73) or a polyradiculoneuropathy (Guillain-Barre syndrome) (74,75) can occur. 

L-penicillamine strongly inhibits pyridoxal-dependent enzymes and causes pyridoxine deficiency in animals. Although D-penicillamine is much less active in this respect, there is some reduction in pyridoxine, and in a report of penicillamine-associated polyneuropathy, pyridoxine supplements for patients receiving D-penicillamine were advised (69). 

A mechanistic proposal for this pyridoxal-dependent enzyme is outlined in Fig. 17 [64]. No mechanistic studies have been reported. 

When purified, the DO PA decarboxylase of rat liver has an absorption spectrum similar to that of other pyridoxal-dependent enzymes. In this case, the co-enzyme seems to be very tightly bound to the apo-enzyme, but addition of an excess of pyridoxal phosphate still causes an increase in the enzyme activityii . It was therefore suggested that pyridoxal phosphate is a prosthetic group of this enzyme, and that when present in excess it acts as a co-enzyme. The 5-HTP decarboxylase of rat kidney was found to be potentiated by pyridoxal phosphate, but the effect was shown only when the tissue had been repeatedly frozen and thawed. These observations provide some evidence that pyridoxal phosphate is the co-enzyme for non-specific histidine decarboxylase. 

Pyridoxine (Vitamin Bg) IX), pyridoxal (A), and pyridoxamine XI), which are normally available in the diet, must be converted in vivo to the active co-enzyme pyridoxal phosphate XII) as shown in Figure 4.1. Inhibition of pyridoxal phosphate production in vivo results in reduced activity of pyridoxal-dependent enzymes. The extent of the inhibition may vary from one enzyme system to another, depending on the affinities of the apo-cnzymes for the co-enzyme. 

Pyridoxal phosphate is a co-enzyme for numerous enzymes, notably amino acid decarboxylases, amino acid transaminases, histaminase and probably diamine oxidase Ais.iw. As most of the evidence on which the mechanism of action of pyridoxal-dependent enzymes is based has been obtained from studies of the non-enzymic interaction of pyridoxal with amino acids, these non-enzymic reactions will be considered first in some detail. 

Carbonyl reagents, including cyanide, hydroxylamine, semicarbazide, hydrazine and substituted hydrazines inhibit non-specific histidine decarboxylase by combining with the co-enzyme pyridoxal phosphate. Such compounds, of course, inhibit other pyridoxal-dependent enzymes. A list of these and other compounds which inhibit non-specific histidine decarboxylase has been compiled by Schayer . 

The histidine decarboxylase activity of tissues can be raised or lowered by changing the hormonal state of the animal or by subjecting it to certain stressful stimuli. Administration of thyroid hormones to rats produces a marked increase in the specific histidine decarboxylase activity of the glandular mucosa of the stomach " , while the activity of the nonspecific enzyme in the liver is lowered . Studies of the action of thyroid hormones on other pyridoxal-dependent enzymes in rat liver suggest that, in this organ at least, these changes arise from corresponding alterations in both pyridoxal phosphate and apo-enzyme synthesis . 

The monoamine oxidase inhibitors have other actions the hydrazides inhibit pyridoxal-dependent enzymes and thus lower the y-aminobutyric acid content of brain. There is also evidence that they prevent the release of noradrenaline from brain binding sites . 

The enzyme is a pyridoxal-dependent enzyme, but the cofactor binds loosely to the enzyme. Extensive dialysis of an enzyme preparation yields almost inactive enzyme in the absence of pyridoxal phosphate but activity is fully restored by addition of the cofactor... 

Because L-vinylglycine also irreversibly inactivates ACC synthase in a time-dependent manner when the enzyme is incubated with L-vinylglycine and pyridoxal phosphate in the absence of AdoMet and because ACC synthase, like other pyridoxal-dependent enzymes, may catalyse p.-y-elimination of AdoMet to produce L-vinylglycine, it has been proposed that the mechanism-based inactivation proceeds through the formation of a vinylglycine-enzyme complex which is inactive (Fig. 3). Detailed kinetic studies indicate that the... 

This arrangement is found in aldolase, rioflavin, niacin, thiamin and pyridoxal dependent enzymes. Here we shall limit ourselves to models of NAD (nicotinamide-adeninedinueleotide). In structure (2) we see the anion... 

Alternatively, j8,y-unsaturated amino acids, y-acetylenic amino acids, or amino acids having a leaving group at the j8-position are potentially good inhibitors of pyridoxal-dependent enzymes involved in amino acid metabolism. The first two become activated either by carbanion formation... 

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