CHAPTER 9 Inhibition of Amino Acid Decarboxylases* William Gilbert Clark I. Introduction 316 II. Pyridoxal Kinase 318 III. Apoenzyme-Coenzyme Dissociation 319 IV. Transport, Uptake, Binding, Release 320 V. Inanition 322 VI. Tissue Damage, Growth, Neoplasms, Organectomy 322 VII. Apoenzyme Synthesis 323 VIII. Inhibition by Apoenzyme Antibody 326 IX. Metals, Chelators, and Metal Complexers 327 X. Cyanide 330 XI. Pyridoxal-5-Phosphate (Codecarboxylase), Vitamin B6, Be-Deficien- cy, Be-Antagonists 331 A. Ββ-Deficiency and Antagonists 331 B. B6-P04 Hydrazones 334 C. Toxopyrimidine 335 D. Steroids 337 XII. Activators, Stabilizers, and Cofactors Other Than Be-P04 337 A. Metals 337 B. Surfactants 337 C. Solvents 337 D. Phosphates and Arsenate 338 E. Miscellaneous 338 XIII. Carbonyl Reagents and Inhibitors Acting on Coenzyme Other Than Cyanide, Substrate Analogues, and Pyridoxine Antagonists 339 A. Hydroxylamine, Hydrazides, Semicarbazide, Sulfite, Hydrazine, Oximes, etc 339 B. Cycloserine (4-Amino-3-isoxazolidone) 343 C. Penicillamine 345 D. Sulfonylureas 345 E. Cysteine, 2,3-Dimercapto-l-propanol (BAL), Glutathione 346 F. Ascorbic Acid 346 * Supported by Grants from the National Mental Health Association, U.S. Public Health Service, American Heart Association, Los Angeles County Heart Association, U.S. Army Chemical Center, American Cancer Society, Office of Naval Research, Helen Hay Whitney, Jr. Foundation, Life Insurance Medical Research Fund. 315 316 W. G. CLARK XIV. Hormones 346 A. Insulin 347 B. Pituitary 347 C. Adrenal Cortex 347 D. Thyroid 349 E. Sex Hormones 352 XV. Miscellaneous 354 A. Antibiotics 354 B. Antihistamines 354 C. Reserpine 354 D. Tranquilizers 354 E. Tetrahydroisoquinolines 354 F. Folic Acid Antagonists 355 XVI. Substrate Analogues 356 A. Bacterial and Plant Decarboxylases 356 B. Mammalian (and Fowl) Decarboxylases in vitro 357 C. Mammalian Dopa Decarboxylase in vitro 358 D. Mammalian 5-HTP Decarboxylase in vitro 358 E. Glutamic Acid Decarboxylase in vivo 359 F. Dopa Decarboxylase in vivo 359 G. Quinones and Potential Quinoids 360 H. Ketonuria 361 I. α-Alkyl Substrate Analogues 363 References 366 I. INTRODUCTION Although enzymic decarboxylation plays a minor role quantitatively ni metabolism fo amino acids, ti si na important one because fo the critical nature, marked pharmacological activity, and function fo the end products. The physiological significance fo most, fi not all, fo these end products still remains ot eb clarified, but many fo them probably are essential for a num­ ber fo homeostatic regulatory and adaptive mechanisms ni all living organ­ isms. tI was not until 1936-1937 that amino acid decarboxylases were de­ scribed yb Okunuki (1937) ni plants (glutamic acid decarboxylase) and yb Werle (1936) and Holtz and Heise (1937, 1938) [histidine and 3,4-dihy- droxyphenylanine (dopa) decarboxylase]. This and other work, including data no inhibitors fo these decarboxylases, have been reviewed yb Gale (1940b, 1946, 1953), Holtz (1941), Storck (1951), Janke (1951), Karrer (1947), Mardashew (1949), Schales (1951), Mèister (1955, 1957), Werle (1943a,b, 1947, 1951), and Ruiz and Zaragoza (1959). It si the purpose fo this review ot discuss inhibition fo the amino acid decarboxylases ni general, with some emphasis no more recent contribu­ tions (up ot early 1961 ni most cases). Inhibition yb general enzyme "poisons" ro dénaturants in vitro, such sa trichloroacetate and alkylat- 9. INHIBITION OF AMINO ACID DECARBOXYLASES 317 ing agents, will not be included. Clark (1959) and Clark and Pogrund (1961) recently reviewed the subject of dopa decarboxylase inhibition in vitro and in vivo, and Sourkes and D'lorio discuss the subject in Volume II of this treatise. Since many inhibitors of these enzyme systems exert their effects directly or indirectly through pyridoxal-5-phosphate (B6-P04), the coenzyme of amino acid decarboxylases, some aspects of the vitamin B6-dependent enzymes in general must be considered. Ref­ erence should be made to the extensive reviews available of the pyri- doxine-dependent enzymes by Blaschko (1945a), Gunsalus (1951), Wil­ liams et al. (1950), Sherman (1954), Tower (1956, 1959, 1960), Umbreit (1954), Meister (1957), Mathews (1958), Snell (1958), Snell and Jenkins (1959), Siliprandi (1960), Roberts and Eidelberg (1959), Roberts et al (1960), Braunstein (I960), and Axelrod and Martin (1961). Braunstein's review, "Pyridoxal Phosphate" (1960), and that of Snell (1959), "Chemical Structure in Relation to Biological Activities of Vitamin B6," are particu­ larly exhaustive. The amino acid decarboxylases described so far catalyze the following reactions: (1 Glycine —> methylamine (2; L-Alanine —> ethylamine (3: L-Serine —> ethanolamine (4; α-Aminobutyric acid —• propylamine (5; L-Methionine —> 3-methylthiopropylamine (β: L-Valine —> isobutylamine (7; L-Norvaline —• butylamine (s: L-Leucine —> isoamylamine (9: L-Isoleucine —* 3-methylbutylamine do: L-Aspartic acid —> L-alanine (11 L-Arginine —> agmatine (12: L-Histidine —> histamine (13 L-Aspartic acid —• /3-alanine (14 L-Ornithine —> putrescine (15 L-Lysine —> cadaverine (16 meso-δ-6-Diaminopimelic acid —> L-lysine (17 α-Aminomalonic acid —* glycine (is: δ-Hydroxy-L-lysine —» hydroxycadaverine (i9: L-Glutamic acid —> 7-aminobutyric acid (20 Allo-jS-hydroxy-L-glutamic acid —• 7-amino-/3-hydroxybutyric acid (21 7-Hydroxy-L-glutamic acid —• a-hydroxy-7-aminobutyric acid (22 7-Methylene-L-glutamic acid —> 7-amino-a-methylene butyric acid (23 L-Cysteic acid —> taurine (24 L-Cysteinesulfinic acid —> hypotaurine (25: L-Tryptophan -> tryptamine (26 4-Hydroxy-L-tryptophan —* 4-hydroxytryptamine (27 5-Hydroxy-L-tryptophan (^δ-ΗΤΡ") -> 5-hydroxytryptamine (serotonin, "5HT") 318 W. G. CLARK (28) 6-Hydroxy-L-tryptophan —» 6-hydroxytryptamine (29) α-Methyl-L-tryptophan —» a-methyltryptamine (30) a-Methyl-5-hydroxy-L-tryptophan —» a-methyl-5-hydroxytryptamine (31) a. L-Phenylalanine —> phenylethylamine Ring-substituted hydroxy-L-phenylalanines to corresponding amines, e.g. : b. L-Tyrosine —» tyramine c. Diiodo-L-tyrosine —* diiodotyramine (Werle, 1947; not confirmed) d. 2-Hydroxy-L-phenylalanine (o-tyrosine) —> 2-hydroxyphenylethylamine (o-tyramine) e. 3-Hydroxy-L-phenylalanine (ra-tyrosine) —• ra-tyramine f. a-Methyl-3-hydroxy-L-phenylalanine (α-methyl-m-tyrosine) —> a-methyl- 3-hydroxyphenylethylamine (a-methyl-ra-tyramine) g. 3,4-Dihydroxy-L-phenylalanine (dopa) —• 3,4-dihydroxytyramine (dopa­ mine) (32) a. L-Phenylserine —• phenylethanolamine b. 2-Hydroxy-L-phenylserine —• 2-hydroxyphenylethanolamine c. 3-Hydroxy-L-phenylserine —> 3-hydroxyphenylethanolamine d. 4-Hydroxy-L-phenylserine —• 4-hydroxyphenylethanolamine (octopamine, norsynephrine) e. 3,4-Dihydroxy-L-phenylserine (dops) —> 3,4-dihydroxyphenylethanol- amine (arterenol, norepinephrine) Some of these reactions have been shown to be catalyzed by one and the same enzyme, and possibilities of this kind should be borne in mind in considering this section. Further discussion of this point, classifying and documenting the individual enzymes by their distribution in micro­ organisms, plants, and animals, the structures and stereospecificity of their substrates, their kinetics, apoenzyme-coenzyme affinity and dissoci­ ation must be sought in the reviews cited. Since the introduction of tracer techniques and amine catabolic enzyme inhibitors, many decarboxylations formerly thought to be absent in animals are being announced. II. PYRIDOXAL KINASE Snell (1959) and McCormick et al (1961; McCormick and Snell, 1961) have discussed codecarboxylase kinase (pyridoxal phosphokinase) and have reviewed the literature. Chevillard and Thoai (1951) and Thoai and Chevillard (1951a,b) showed that Mg+ + and Mn+ + activate it, ATP or ADP are necessary, and thiamine inhibits it. Hurwitz (1952, 1953, 1955) showed that some pyridoxine analogues inhibit tyrosine decarboxylase in bacteria, while others inhibit the phosphorylation of pyridoxal in the presence of ATP. Some adenine and purine derivatives inhibit competi­ tively and several metal cations activate (cf. McCormick et al., 1961; McCormick and Snell, 1961). Hurwitz suggested that the adenines and purines act through the activating metallic ions. McCormick and Snell 9. INHIBITION OF AMINO ACID DECARBOXYLASES 319 (1959, 1961) and McCormick et al. (1960, 1961) showed that purified pyridoxal phosphokinase from brain is markedly inhibited by a variety of condensation products formed from pyridoxal and hydroxylamine, O-sub- stituted hydroxylamines, hydrazine, and substituted hydrazines; they questioned former explanations of the convulsive effects of such carbonyl reagents which causally related the seizures to lowered brain 7-amino­ butyric acid (gaba) through interaction with B6-PO4. Dubnick and co­ workers (1960c) postulated that pyridoxal hydrazones are formed from hydrazines and B6-P04 in vivo and exert toxic effects by inhibiting the phosphokinase (cf. Balzerei al., 1960a,b,c; and Baxter and Roberts, 1960). Recently, Wada and Snell (1961), Turner and Happold (1961), and Wada et al. (1961) described an enzyme which oxidizes pyridoxine and pyri- doxamine phosphates to B6-PO4. Evidently, the primary pathway of the formation of B6-P04 is phosphorylation by the kinase, followed by the action of the oxidase. The oxidase, a flavoprotein with riboflavin-5'- phosphate as a cofactor, is sensitive to thiol reagents, heavy metals, and some phosphorylated analogues of Be, especially pyridoxal phosphate oxime. Greenberg et al. (1959) observed that chlorpromazine had little or no effect