O-Aminobenzoyl-S-nitrosohomocysteine, a fluorogenic probe for cell-surface thiol determinations via a microtiter plate assay

O-Aminobenzoyl-S-nitrosoglutathione: A fluorogenic, cell permeable, pseudo-substrate for S-nitrosoglutathione reductase

S-nitrosoglutathione reductase (GSNOR) is a multifunctional enzyme. It can catalyze NADH-dependent reduction of S-nitrosoglutathione (GSNO); as well as NAD⁺-dependent oxidation of hydroxymethylglutathione (HMGSH; an adduct formed by the spontaneous reaction between formaldehyde and glutathione). While initially recognized as the enzyme that is involved in formaldehyde detoxification, increasing amount of evidence has shown that GSNOR also plays a significant role in nitric oxide mediated signaling through its modulation of protein S-nitrosothiol signaling. In humans, GSNOR/S-nitrosothiols have been implicated in the etiology of several diseases including lung cancer, cystic fibrosis, asthma, pulmonary hypertension, and neuronal dysfunction. Currently, it is not possible to monitor the activity of GSNOR in live cells. In this article, we present a new compound, O-aminobenzoyl-S-nitrosoglutathione (OAbz-GSNO), which acts as a fluorogenic pseudo-substrate for GSNOR with an estimated K_m value of 320µM. The weak OAbz-GSNO fluorescence increases by approximately 14 fold upon reduction of its S-NO moiety. In live cell imaging studies, OAbz-GSNO is readily taken up by primary pulmonary endothelial cells and localizes to the same perinuclear region as GSNOR. The perinuclear OAbz-GSNO fluorescence increases in a time dependent manner and this increase in fluorescence is abolished by siRNA knockdown of GSNOR or by treatment with GSNOR-specific inhibitors N6022 and C3. Taken together, these data demonstrate that OAbz-GSNO can be used as a tool to monitor the activity of GSNOR in live cells.

Simple biosensor with high selectivity and sensitivity: thiol-specific biomolecular probing and intracellular imaging by AIE fluorogen on a TLC plate through a thiol-ene click mechanism

A handy, specific, sensitive bioprobe has been developed. Tetraphenylethene (TPE) was functionalized by a maleimide (MI) group, giving a TPE-MI adduct that was nonemissive in both solution and the solid state. It was readily transformed into a fluorogen showing an aggregation-induced emission (AIE) property by the click addition of thiol to its MI pendant. The click reaction and the AIE effect enabled TPE-MI to function as a thiol-specific bioprobe in the solid state. Thus, the spot of TPE-MI on a TLC plate became emissive when it had been exposed to L-cysteine, an amino acid containing a thiol group, but remained nonemissive when exposed to other amino acids that lack free thiol units. The thiol-activated emission was rapid and strong, readily detected by the naked eye at an analyte concentration as low as approximately 1 ppb, thanks to the "lighting up" nature of the bioprobing process. Similarly, the emission of TPE-MI was turned on only by the proteins containing free thiol units, such as glutathione. Clear fluorescence images were taken when living cells were stained by using TPE-MI as a visualization agent, affording a facile fluorescent maker for mapping the distribution of thiol species in cellular systems.

Fluorescent and colorimetric probes for detection of thiols

Due to the biological importances of thiols, such as cysteine, homocysteine and glutathione, the development of optical probes for thiols has been an active research area in recent few years. This critical review focuses on the fluorescent or colorimetric sensors for thiols according to their unique mechanisms between sensors and thiols, including Michael addition, cyclization with aldehyde, cleavage of sulfonamide and sulfonate ester by thiols, cleavage of selenium-nitrogen bond by thiols, cleavage of disulfide by thiols, metal complexes-oxidation-reduction, metal complexes-displace coordination, nano-particles and others (110 references).

A direct, continuous, sensitive assay for protein disulphide-isomerase based on fluorescence self-quenching

PDI (protein disulphide-isomerase) activity is generally monitored by insulin turbidity assay or scrambled RNase assay, both of which are performed by UV-visible spectroscopy. In this paper, we present a sensitive fluorimetric assay for continuous determination of disulphide reduction activity of PDI. This assay utilizes the pseudo-substrate diabz-GSSG [where diabz stands for di-(o-aminobenzoyl)], which is formed by the reaction of isatoic anhydride with the two free N-terminal amino groups of GSSG. The proximity of two benzoyl groups leads to quenching of the diabz-GSSG fluorescence by approx. 50% in comparison with its non-disulphide-linked form, abz-GSH (where abz stands for o-aminobenzoyl). Therefore the PDI-dependent disulphide reduction can be monitored by the increase in fluorescence accompanying the loss of proximity-quenching upon conversion of diabz-GSSG into abz-GSH. The apparent K(m) of PDI for diabz-GSSG was estimated to be approx. 15 muM. Unlike the insulin turbidity assay and scrambled RNase assay, the diabz-GSSG-based assay was shown to be effective in determining a single turnover of enzyme in the absence of reducing agents with no appreciable blank rates. The assay is simple to perform and very sensitive, with an estimated detection limit of approx. 2.5 nM PDI, enabling its use for the determination of platelet surface PDI activity in crude sample preparations.