O-sulphate esters of L-serine, L-threonine and L-hydroxyproline

Chemoselective O-acylation of hydroxyamino acids and amino alcohols under acidic reaction conditions: History, scope and applications

Amino acids, whether natural, semisynthetic or synthetic, are among the most important and useful chiral building blocks available for organic chemical synthesis. In principle, they can function as inexpensive, chiral and densely functionalized starting materials. On the other hand, the use of amino acid starting materials routinely necessitates protective group chemistry, and in reality, large-scale preparations of even the simplest side-chain derivatives of many amino acids often become annoyingly strenuous due to the necessity of employing protecting groups, on one or more of the amino acid functionalities, during the synthetic sequence. However, in the case of hydroxyamino acids such as hydroxyproline, serine, threonine, tyrosine and 3,4-dihydroxyphenylalanine (DOPA), many O-acyl side-chain derivatives are directly accessible via a particularly expedient and scalable method not commonly applied until recently. Direct acylation of unprotected hydroxyamino acids with acyl halides or carboxylic anhydrides under appropriately acidic reaction conditions renders possible chemoselective O-acylation, furnishing the corresponding side-chain esters directly, on multigram-scale, in a single step, and without chromatographic purification. Assuming a certain degree of stability under acidic reaction conditions, the method is also applicable for a number of related compounds, such as various amino alcohols and the thiol-functional amino acid cysteine. While the basic methodology underlying this approach has been known for decades, it has evolved through recent developments connected to amino acid-derived chiral organocatalysts to become a more widely recognized procedure for large-scale preparation of many useful side-chain derivatives of hydroxyamino acids and related compounds. Such derivatives are useful in peptide chemistry and drug development, as amino acid amphiphiles for asymmetric catalysis, and as amino acid acrylic precursors for preparation of catalytically active macromolecular networks in the form of soluble polymers, crosslinked polymer beads or nanoparticulate systems. The objective of the present review is to increase awareness of the existence and convenience of this methodology, assess its competitiveness compared to newer and more elaborate procedures for chemoselective O-acylation reactions, spur its further development, and finally to chronicle the informative, but poorly documented history of its development.

THE METABOLISM OF L-SERYLGLYCINE O(35S)-SULPHATE IN THE RAT

1. The preparation of potassium l-serylglycine O-sulphate and the corresponding (35)S-labelled ester is described. 2. Intraperitoneal injection of potassium l-serylglycine O[(35)S]-sulphate to rats results in about 75% of the radioactivity of the dose appearing in the urine within 48hr. Almost 72% of the radioactivity recovered in the urine was in the form of inorganic [(35)S]sulphate. 3. Analysis of urines by paper chromatography showed the presence of unchanged l-serylglycine O[(35)S]-sulphate and several other unidentified (35)S-labelled materials. 4. It has been established that micro-organisms of the gastrointestinal tract do not play any significant role in the production of inorganic [(35)S]sulphate from the injected ester. 5. l-Serylglycine O-sulphate was hydrolysed by crude dipeptidase preparations from rat kidney and intestine to yield l-serine O-sulphate and glycine as the sole products.

Sulfate esters of hydroxy amino acids as stereospecific glutamate receptor agonists

Enantiomerically pure sulfate esters of the hydroxy amino acids homoserine, hydroxyproline and 4-hydroxypipecolic acid were synthesized and tested on alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptors present in the mice cortical wedge preparation and on NMDA receptors present in the myenteric plexus of the guinea pig with the aim of finding new possible endogenous ligands (either agonists or antagonists) for excitatory amino acid receptors. The linear and flexible compound S-homoserine sulfate caused a depolarization of both AMPA and NMDA receptors. In the cortex its agonist action had an EC50 of 150 microM for NMDA and 300 microM for AMPA receptors and in the myenteric plexus its EC50 was 600 microM. The stereoisomer R-homoserine sulfate did not depolarize the cortical wedges and failed to cause ileal contraction up to a concentration of 500 microM. Among the four possible stereoisomers of 4-hydroxyproline sulfate, which are rigid structures and may be regarded as cyclization forms of homoserine sulfate, t-S-hydroxyproline sulfate was a selective AMPA receptor agonist with an EC50 of 70 microM in the cortex. The other three isomers were not active as agonists up to 500 microM and none of them had antagonist activity. Finally, t-4-hydroxy-S-pipecolic acid-4-sulfate, a superior homologue of t-S-hydroxyproline sulfate, was found to be one of the most potent and selective NMDA receptor agonists so far described with an EC50 of 0.7 microM in the cortex and 250 microM in the myenteric plexus.(ABSTRACT TRUNCATED AT 250 WORDS)

Specific sulfonation of tyrosine, tryptophan and hydroxy-amino acids in peptides

1. ClSO3H in trifluoroacetic acid rapidly converts serine and threonine into O-sulfate ester derivatives while tyrosine and tryptophan are converted into arylsulfonic acids. 2. H2SO4 in trifluoroacetic acid reacts more slowly with serine, threonine and tyrosine while is not able to modify tryptophan. 3. All other amino acids are perfectly stable under the above reaction conditions. 4. Peptides containing susceptible amino acid residues are specifically converted into the corresponding sulfonated derivatives in high or quantitative yield.

The uptake of L-glutamate by the retina

The accumulation of L[14C]glutamate by the isolated rat retina has been studied. When retinae were incubated at 37 degrees C in a medium containing L-[14C]glutamate, tissue/medium ratios of about 40:1 were achieved after 60 min. The labelled L-glutamate was rapidly metabolised and after 10 min about 50% of the radioactivity in the tissue amino acids was present as glutamine, aspartate, and 4-aminobutyrate (GABA). The process responsible for L-glutamate uptake showed many of the properties of an active uptake system: it was temperature sensitive, sodium dependent, inhibited by metabolic inhibitors and showed saturation kinetics. The saturable uptake process could be resolved into two components; a 'high' affinity process (apparent Km = 21 muM, Vmax = 35 nmoles/min/g tissue) and a 'low' affinity process (Km = 630 muM, Vmax = 881 nmoles/min/g tissue). The 'high' affinity and 'low' affinity uptake processes for L-glutamate appeared to have identical properties in the retina. The uptake of L-glutamate was not specific and was inhibited by other acidic amino acids including D-glutamate but not by neutral or basic amino acids. The retinal uptake of L-glutamate is not likely to be due to a homoexchange phenomenon because the retina was capable of achieving a large net uptake of glutamate and the efflux of L-[14C]glutamate from the tissue was not increased by the addition of non-radioactive L-glutamate to the incubation medium. Autoradiographic studies indicated that the sites for glutamate uptake are largely in the neuroglial Muller cells.

Induction of primary alkysulphatases and metabolism of sodium hexan-1-yl sulphate by Pseudomonas C12B

Sodium hexan-1-yl sulphate and certain related alkyl sulphate esters have been shown to serve as inducers of the formation of primary alkylsulphatases (designated as P1 and P2) in Pseudomonas C12B. When the organism is grown on sodium hexan-1-yl [(35)S]sulphate as the sole source of sulphur or as the sole source of carbon and sulphur only the P2 alkylsulphatase is formed and inorganic (35)SO(4) (2-) is liberated into the media. Cell extracts contain this anion as the major (35)S-labelled metabolite although two unidentified labelled metabolites as well as choline O-[(35)S]sulphate occur in trace quantities in some extracts. Dialysed cell extracts are capable of liberating inorganic (35)SO(4) (2-) from sodium hexan-1-yl [(35)S]sulphate without the need to include cofactors known to be required for the bacterial degradation of n-alkanes. The collective results suggest that sodium hexan-1-yl sulphate can act as an inducer of P1 alkylsulphatase formation without the need for prior metabolic modification of the carbon moiety of the ester.

The formation of choline O-sulphate by Pseudomonas C12B and other Pseudomonas species

Pseudomonas C(12)B and other Pseudomonas species released larger amounts of a (35)S-labelled metabolite into the medium when cultured on growth-limiting concentrations of Na(2)SO(4) as opposed to growth in SO(4) (2-)-sufficient media. The metabolite was found at all stages of the culture cycle of Pseudomonas C(12)B and maximum quantities occurred in stationary-phase culture supernatants. The metabolite was not detected when the bacterium was cultured on growth-limiting concentrations of potassium phosphate. The amount of the metabolite present in the medium greatly exceeded that which could be extracted from intact cells and, except for choline chloride, it was independent of the carbon source used for growth. If choline chloride was present in high concentration, then larger amounts of the metabolite were found in the culture medium. The metabolite was not detected extracellularly or intracellularly when the bacterium was grown in SO(4) (2-)-deficient media containing 5mm-l-cysteine. The same metabolite was also synthesized in vitro only when Pseudomonas C(12)B extracts were incubated with choline chloride, ATP, MgCl(2) and Na(2) (35)SO(4). The metabolite-forming system was not subject to repression by Na(2)SO(4) and was completely inhibited by 0.5mm-l-cysteine and activated by Na(2)SO(4) (up to 1.0mm). The metabolite was identified as choline O-sulphate by electrophoresis, chromatography and isotope-dilution analysis. Another (35)S-labelled metabolite was also detected in culture supernatants, but was not identified.

Carbon and sulphur utilization during growth of Pseudomonas fluorescens on potassium D-glucose 6-O-sulphate as the sole sulphur source

Pseudomonas fluorescens N.C.I.B. 8248, cultured on potassium d-glucose 6[(35)S]-O-sulphate as the sole sulphur source, liberated the 6-O-sulphate ester of d-gluconate into the culture medium. Extracts of bacteria grown under this cultural condition oxidized d-glucose 6-O-sulphate to yield the gluconate ester. Results suggest the involvement of a glucose dehydrogenase-like enzyme. The gluconate ester was apparently not oxidized further to any significant extent; however, it served as substrate for a desulphating enzyme found in extracts. Growth on d-glucose 6-O-sulphate as the sole source of sulphur was not associated with the appearance of a true glycosulphatase. Collectively, these results suggest that d-gluconate 6-O-sulphate, rather than the glucose ester, supplied the necessary sulphur for growth. Oxidative activities toward d-glucose 6-O-sulphate, d-glucose, d-gluconate 6-O-sulphate and d-gluconate found in extracts of P. fluorescens adapted to grow on d-glucose 6-O-sulphate as the sole source of carbon and sulphur are presented for comparative purposes.

The purification and properties of the L-serine O-sulphate degrading system of pig liver

1. The enzyme system from pig liver responsible for the alphabeta-elimination of l-serine O-sulphate was purified 1000-fold. 2. Isoelectric focusing produced two enzymically active fractions with isoelectric points at pH5.6 and 5.9 respectively. 3. Osmometry and gel filtration showed both enzymes to possess molecular weights of approx. 54000. 4. The separate activities exhibited similar amino acid compositions.

Sulphur utilization during growth of pseudomonas fluorescens on potassium D-glucose 6-O-sulphate

Pseudomonas fluorescens N.C.I.B. 8248 was adapted to grow on potassium d-glucose 6-O-sulphate as the sole carbon and sulphur source. Adapted bacteria grew optimally at 37 degrees C on 1.6% (w/v) sulphate ester and growth coincided with the disappearance of the ester from the culture medium at a rate of 2.4mg/h per ml. Three sulphated compounds were detected in the culture fluid at the termination of growth. One of these was present in traces only and has not been identified. The second was present in somewhat greater amounts and was identified as the 6-O-sulphate ester of d-gluconate, and the major metabolite was identified as d-glycerate 3-O-sulphate. Sulphur utilization by the organism was not associated with the appearance of a glycosulphatase enzyme in the cells. However, a novel enzyme system (or systems) was present that liberated inorganic (35)SO(4) (2-) ions from dipotassium d-gluconate 6[(35)S]-O-sulphate and from dipotassium dl-glycerate 3[(35)S]-O-sulphate. Activity towards the latter substrate could not be detected when the adapted or parent Pseudomonas strain was cultured on d-glucose and potassium sulphate as respective carbon and sulphur sources. Some properties of the enzyme acting on the glycerate ester are recorded.

The pyridoxal- and pyridoxal 5'-phosphate-catalysed non-enzymic degradations of l-serine o-sulphate and related compounds

1. l-Serine O-sulphate and l-threonine O-sulphate are degraded in the presence of pyridoxal 5'-phosphate to yield equimolar amounts of the corresponding keto acid, ammonia and sulphate. 2. Pyridoxal catalyses the same reactions at a faster rate. 3. One of a number of bi- or ter-valent metal ions must be present for these degradations to proceed. The reaction rates are dependent on a number of factors including pH and the nature of the metal ion used. 4. Studies with related sulphate esters indicate that the alpha-hydrogen atom and the amino group are essential for activity. 5. Spectral changes during the pyridoxal and pyridoxal 5'-phosphate catalysis of l-serine O-sulphate breakdown suggest the formation of a Schiff base. 6. The mechanism for these reactions appears to be in accordance with the general mechanism proposed for pyridoxal-catalysed alphabeta-elimination reactions.

The glycosulphatase of Trichoderma viride

The growth of the mould Trichoderma viride on a defined medium containing either potassium d-glucose 6-O-sulphate or potassium d-galactose 6-O-sulphate as sole sources of both carbon and sulphur is marked by the production of an enzyme system capable of liberating inorganic SO(4) (2-) ions from either of the sulphate esters. The enzyme is not produced when the organism is grown with glucose (or galactose) and potassium sulphate or with glucose and methionine as sole sources of carbon and sulphur. Experimental conditions are described whereby inorganic SO(4) (2-) ions liberated from potassium glucose 6-O-sulphate by the growing mould appear in the culture medium after a constant lag period of 21-24hr. The enzyme has been shown to be a simple glycosulphatase that is active towards the 6-O-sulphate esters of d-glucose and d-galactose but not towards potassium glucose 3-O-sulphate. The properties of the crude glycosulphatase show the enzyme to be appreciably different from analogous molluscan enzymes that can degrade monosaccharide sulphate esters.

Studies on the enzymic degradation of L-serine O-sulphate by a rat liver preparation

1. The enzyme system of rat liver responsible for the degradation of l-serine O-sulphate was purified 300-fold and the optimum conditions for the activity were determined. 2. Inorganic sulphate, pyruvate and ammonia were found to be the products of enzyme action on lserine O-sulphate, being formed in equivalent amounts under all conditions examined. No free l-serine was detected as a product of enzyme action. 3. The enzyme preparation was free from other serine-metabolizing systems such as O-phospho-l-serine phosphatase and l-serine dehydratase. 4. The enzyme has a very narrow substrate specificity and is inactive towards a wide variety of related sulphate esters and amino acids. 5. Pyridoxal 5'-phosphate is capable of catalysing the non-enzymic breakdown of l-serine O-sulphate in the presence of metal salts to yield inorganic sulphate, pyruvate and ammonia as products. 6. The possible role of pyridoxal 5'-phosphate as a coenzyme in the enzymic degradation of l-serine O-sulphate is discussed.