The resolution of racemic alpha-amino acids

d-Aspartic acid: An endogenous amino acid with an important neuroendocrine role

D-Aspartic acid (d-Asp), an endogenous amino acid present in vertebrates and invertebrates, plays an important role in the neuroendocrine system, as well as in the development of the nervous system. During the embryonic stage of birds and the early postnatal life of mammals, a transient high concentration of d-Asp takes place in the brain and in the retina. d-Asp also acts as a neurotransmitter/neuromodulator. Indeed, this amino acid has been detected in synaptosomes and in synaptic vesicles, where it is released after chemical (K(+) ion, ionomycin) or electric stimuli. Furthermore, d-Asp increases cAMP in neuronal cells and is transported from the synaptic clefts to presynaptic nerve cells through a specific transporter. In the endocrine system, instead, d-Asp is involved in the regulation of hormone synthesis and release. For example, in the rat hypothalamus, it enhances gonadotropin-releasing hormone (GnRH) release and induces oxytocin and vasopressin mRNA synthesis. In the pituitary gland, it stimulates the secretion of the following hormones: prolactin (PRL), luteinizing hormone (LH), and growth hormone (GH) In the testes, it is present in Leydig cells and is involved in testosterone and progesterone release. Thus, a hypothalamus-pituitary-gonads pathway, in which d-Asp is involved, has been formulated. In conclusion, the present work is a summary of previous and current research done on the role of d-Asp in the nervous and endocrine systems of invertebrates and vertebrates, including mammals.

Distribution and substrate specificity of benzylpenicillin acylase

Benzylpenicillin acylase, which hydrolyzes benzylpenicillin to 6-aminopenicillanic acid, was found to be widely distributed among members of the Schizomycetes, particularly in gram-negative bacteria, and in the genus Nocardia. The hydrolysis of a series of biosynthetic and semisynthetic penicillins by freeze-dried cells of a strain of Nocardia and of Proteus was studied. Benzylpenicillin was the preferred substrate; all departures from the benzylpenicillin side-chain structure led to reduction of substrate activity (the greater the departure, the greater the reduction in activity). Penicillin amides and methyl esters were also hydrolyzed, as were suitable N-acyl derivatives of 7-aminocephalosporanic acid. Occurrence of an enzyme activity which hydrolyzes benzylpenicillinamide to benzylpenicillin was detected in certain strains of yeasts.

PAPAIN-CATALYSED HYDROLYSIS OF SOME HIPPURIC ESTERS. A NEW MECHANISM FOR PAPAIN-CATALYSED HYDROLYSIS

1. The Michaelis-Menten parameters for the papain-catalysed hydrolysis of a number of alkyl, aryl and alkyl-thiol esters of hippuric acid have been determined. 2. For all the aryl esters and most of the alkyl esters studied, the catalytic constant, k(0), is 2-3sec.(-1) and most probably represents deacylation of the common intermediate, hippuryl-papain. 3. Two alkyl esters and hippurylamide, however, have catalytic rate constants, k(0), less than 2-3sec.(-1). It is possible to interpret all the available kinetic data in terms of a three-step mechanism in which an enzyme-substrate complex is first formed, followed by acylation of the enzyme through an essential thiol group, followed by deacylation of the acyl-enzyme. 4. The logarithm of the ratio of the Michaelis-Menten parameters, which reflect the acylation rate constant, for four aryl esters of hippuric acid studied give a linear Hammett plot against the substituent constant, sigma. Arguments are presented that indicate acid as well as nucleophilic catalysis in the acylation process and that the most likely proton donor is an imidazolium ion. 5. It is suggested that this imidazolium ion is part of the same histidine residue that has been tentatively implicated in the deacylation process (Lowe & Williams, 1965b). 6. A new mechanism is proposed for the papain-catalysed hydrolysis of N-acyl-alpha-amino acid derivatives.

T.l.c. enantiomeric separation of amino acids

Resolution of enantiomers is very important particularly in the fields of asymmetric synthesis, mechanistic studies, geochronology, studies of structure-function relationship of proteins, pharmacology, and medicine. Various chromatographic methods have replaced the classical fractional crystallization, seeding and enzymatic procedures. Of these, t.l.c. provides a direct, simple, and inexpensive method for resolution of enantiomers of amino acids and their derivatives. Ligand exchange, ion exchange, and molecular inclusion complexation have been the basis of t.l.c. resolution of enantiomers of amino acids and their derivatives. The innovation of new plate types, and methods of development and detection have renewed interest in the direct resolution of enantiomers of amino acids, their derivatives and a variety of other compounds by t.l.c. The present report provides an overview of some of the more recent approaches to the direct t.l.c. resolution of amino acids and their derivatives together with special advantages and scope of t.l.c.

Stereoselective binding of copper(II), zinc(II), cobalt(II), and ni(II) to the optically active amino acids beta-(2-pyridyl)-alpha-alanine and beta-(6-methyl-2-pyridyl)-alpha-alanine, analogs of histidine and phenylalanine

In order to study the metal ion binding of optically active beta-(2-pyridyl)-alpha-alanine, NH2CH(CH2C5H4N)CO2H, Pyala, and beta-(6-methyl-2-pyridyl)-alpha-alanine, NH2CH(CH2C5H3NCH3)CO2H, Mepyala, these pyridine analogs of histidine were synthesized and resolved; absolute configurations were determined for the isolated enatiomers. Protonation constants and formation constants for the binding of L-Pyala, D,L-Pyala, D-Mepyala, and D,L-Mepyala with Cu(II), Ni(II), Co(II), and Zn(II) were determined by potentiometrictitration. They show that the formation constant (Kx) for the reaction, M(L-Pyala) + D-Pyala in equilibrium M(L-Pyala)(D-pyala), is larger (up to 8.7 times larger) than that (K2) for the coordination of the same enantiomer, M(L-Pyala) + L-Pyala in equilibrium M(L-Pyala)2. Although the difference between Kx and K2 can be explained on a statistical basis for Cu(II), the larger differences for Ni(II), Co(II), and Zn(II) demonstrate that their M(L-Pyala)complexes bind D-Pyala more favorably than they do L-Pyala. A similar trend was found for the optical isomers of Mepyala except that the stereoselectivity is less than in the Pyala system. This presumably results from the reduced coordinating ability of the 6-methylpyridyl group as compared to the less crowded pyridyl donor. Using ideas previously applied to the analogous histidine system, the observed stereoselectivities may be explained in terms of the structures of the complexes.

Toxicity and blood ammonia rise resulting from intravenous amino acid administration in man: the protective effect of l-arginine

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