Transport of d-[1-14C]-amino acids into Chinese hamster ovary (CHO-K1) cells: implications for use of labeled d-amino acids as molecular imaging agents

One-pot radiosynthesis of O-[18F]fluoromethyl-D-tyrosine via intra-molecular nucleophilic 18F-fluorination with 1,2,3-triazolium triflate salt precursor

A radiolabeled amino acid O-[¹⁸F]fluoromethyl-D-tyrosine (D-[¹⁸F]FMT) has been reported to show high tumor uptake. However, introduction of [¹⁸F]fluoromethyl group was difficult and was an issue to be solved. We solved it by using a precursor containing 1,2,3-triazolium salt. D-[¹⁸F]FMT was synthesized from (R)-1-((4-(2-((tert-butoxycarbonyl)amino)-3-((3,4-dimethylbenzyl)oxy)-3-oxopropyl)phenoxy)methyl)-3-methyl-4-phenyl-1H-1,2,3-triazol-3-ium trifluoromethanesulfonate via intra-molecular ¹⁸F-fluorination and subsequent removal of the protecting groups. The total synthesis time was 65min (including purification) and the overall radiochemical yield was 9% based on the isolated product (not decay-corrected). The resulting D-[¹⁸F]FMT was obtained with high radiochemical purity (> 99%) and specific activity (100-150 GBq/μmol). D-[¹⁸F]FMT also achieved excellent results in pharmacological evaluation such as stability test and protein binding assay. We expect that this simple one-pot labeling method would help using D-[¹⁸F]FMT more widely.

A biocompatible in vivo ligation reaction and its application for noninvasive bioluminescent imaging of protease activity in living mice

The discovery of biocompatible reactions had a tremendous impact on chemical biology, allowing the study of numerous biological processes directly in complex systems. However, despite the fact that multiple biocompatible reactions have been developed in the past decade, very few work well in living mice. Here we report that D-cysteine and 2-cyanobenzothiazoles can selectively react with each other in vivo to generate a luciferin substrate for firefly luciferase. The success of this "split luciferin" ligation reaction has important implications for both in vivo imaging and biocompatible labeling strategies. First, the production of a luciferin substrate can be visualized in a live mouse by bioluminescence imaging (BLI) and furthermore allows interrogation of targeted tissues using a "caged" luciferin approach. We therefore applied this reaction to the real-time noninvasive imaging of apoptosis associated with caspase 3/7. Caspase-dependent release of free D-cysteine from the caspase 3/7 peptide substrate Asp-Glu-Val-Asp-D-Cys (DEVD-(D-Cys)) allowed selective reaction with 6-amino-2-cyanobenzothiazole (NH(2)-CBT) in vivo to form 6-amino-D-luciferin with subsequent light emission from luciferase. Importantly, this strategy was found to be superior to the commercially available DEVD-aminoluciferin substrate for imaging of caspase 3/7 activity. Moreover, the split luciferin approach enables the modular construction of bioluminogenic sensors, where either or both reaction partners could be caged to report on multiple biological events. Lastly, the luciferin ligation reaction is 3 orders of magnitude faster than Staudinger ligation, suggesting further applications for both bioluminescence and specific molecular targeting in vivo.

Nutritional and medicinal aspects of D-amino acids

This paper reviews and interprets a method for determining the nutritional value of D-amino acids, D-peptides, and amino acid derivatives using a growth assay in mice fed a synthetic all-amino acid diet. A large number of experiments were carried out in which a molar equivalent of the test compound replaced a nutritionally essential amino acid such as L-lysine (L-Lys), L-methionine (L-Met), L-phenylalanine (L-Phe), and L-tryptophan (L-Trp) as well as the semi-essential amino acids L-cysteine (L-Cys) and L-tyrosine (L-Tyr). The results show wide-ranging variations in the biological utilization of test substances. The method is generally applicable to the determination of the biological utilization and safety of any amino acid derivative as a potential nutritional source of the corresponding L-amino acid. Because the organism is forced to use the D-amino acid or amino acid derivative as the sole source of the essential or semi-essential amino acid being replaced, and because a free amino acid diet allows better control of composition, the use of all-amino-acid diets for such determinations may be preferable to protein-based diets. Also covered are brief summaries of the widely scattered literature on dietary and pharmacological aspects of 27 individual D-amino acids, D-peptides, and isomeric amino acid derivatives and suggested research needs in each of these areas. The described results provide a valuable record and resource for further progress on the multifaceted aspects of D-amino acids in food and biological samples.

Homocysteine transport by systems L, A and y+L across the microvillous plasma membrane of human placenta

Elevated maternal plasma levels of homocysteine (Hcy) are associated with pregnancy complications and adverse neonatal outcomes, suggesting placental transport of Hcy may impact on fetal development. However, such transport mechanisms have not been defined. In this study we characterise Hcy transport mechanisms across the microvillous plasma membrane (MVM) of the syncytiotrophoblast, the transporting epithelium of human placenta. Three candidate transport systems, systems L, A and y(+)L, were examined utilising competitive inhibition to investigate the effects of Hcy on the uptake of well-characterised radiolabelled substrates for each system into isolated MVM vesicles, and that of model substrates on 10 microm [(35)S]l-Hcy uptake. System L activity was inhibited by both l-Hcy and dl-Hcy, comparable to model substrates including 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid (BCH). System L constituted the major transport mechanism, with significant BCH inhibition (69%) of [(35)S]l-Hcy uptake. System A activity was also inhibited by l-Hcy and dl-Hcy with a smaller contribution (21%) to [(35)S]l-Hcy uptake. Inhibition by l-Hcy and dl-Hcy of system y(+)L activity was Na(+) sensitive with a significant inhibition constant (K(i)) shift observed following K(+) replacement; l-arginine reduced [(35)S]l-Hcy uptake by 19%. Kinetic modelling of [(35)S]l-Hcy uptake resolved two, Na(+)-independent, transport components (K(m) 72 microm and 9.7 mm). This study provides evidence for the involvement of systems L, A and y(+)L in placental Hcy transport. Such transport, by competing with endogenous amino acids for transporter activity, could have major implications for syncytiotrophoblast metabolism and function as well as fetal development.