Buccutite™ Fluorescent Protein and Tandem Dye Antibody Labeling Kits

Prepare High-Performance PE, APC & Tandem Conjugates for Flow Cytometry

---

The use of phycobiliprotein labeled antibody conjugates has important applications in flow cytometry. Their usefulness is attributed to their brightness and their ability to create spectral separation when used as tandem dyes for multicolor analysis. A common approach for making PE, APC, or tandem antibody conjugates is to use a succinimidyl 4-(N-maleimidomethyl) cyclohexane1-carboxylate (SMCC) crosslinking method.

This method uses a heterobifunctional crosslinker SMCC. SMCC crosslinkers contain N-hydroxysuccinimide (NHS) ester and maleimide groups, which allow covalent conjugation of amine (-NH₂) and sulfhydryl (-SH) containing molecules. NHS esters react with primary amines at pH 7-9, while maleimides react with sulfhydryl groups at pH 6.5-7.5. Because the rate of NHS esters hydrolytic degradation increases with pH, the NHS ester reaction usually occurs before or simultaneous with the maleimide reaction. For users not adept at conjugation chemistry, this may be challenging.

Although SMCC is a well-established method, it has its disadvantages. One of the significant drawbacks includes poor conjugation efficiency, with a good yield at approximately 30% recovery. Secondly, SMCC requires the addition of dithiothreitol (DTT), which may significantly impact antibody immunoreactivity. Lastly, because the efficiency of SMCC-based reactions is typically low, very high input (i.e., PE and antibody) is required for successful conjugation.

Simplified Workflow

---

An alternative to SMCC conjugation is AAT Bioquest's novel Buccutite™ technology. Buccutite™ conjugation is a quick and straightforward technique for labeling PE, APC, or tandem dyes to antibodies. It utilizes two separate linkers, Buccutite™ MTA and Buccutite™ FOL. These linkers are independently labeled to the phycobiliprotein and antibody of interest, and when mixed, will bind strongly together to produce a bioconjugate.

To improve workflow and reduce hands-on time, Buccutite™ phycobiliprotein labeling kits are supplied with the fluorescent protein pre-activated with the Buccutite™ FOL linker. After antibody activation with Buccutite™ MTA, linker, both components are mixed, and antibody conjugates are produced. The following is an overview of the protocol for Buccutite™ Rapid PE Antibody Labeling Kit (Cat No. 1310)

1. Add 5 µl Reaction Buffer (Component C) into the antibody (100 µL)
2. Add the antibody solution into Buccutite™ MTA vial (Component B)
3. Incubate at room temperature
4. Remove free Buccutite™ MTA by spin column
5. Mix with 50 µL Buccutite™ FOL-Activated PE (Component A)
6. Incubate at room temperature

Buccutite™ vs. SMCC Conjugation

---

Buccutite™ offers several advantages over SMCC-based conjugation. First, Buccutite™ conjugation efficiency is relatively high, with final recovery more than double (>60%) that SMCC conjugation. Second, the Buccutite™ reaction is efficient. Conjugation can occur at low concentrations of reactants requiring a minimal sample concentration of 0.5 mg/mL. Lastly, Buccutite™ conjugates are highly stable and can be stored at 4 °C for at least 12 months (see below for data).

Mouse monoclonal antibody conjugates (GXM) were prepared using Buccutite™ and SMCC conjugation in the following comparison. The conjugates were used to perform cell stains for flow cytometry (Figure 1). Results illustrate similarities in both conjugates' positive stain and stain indexes, highlighting Buccutite™ as a successful alternative to SMCC.

Figure 2 illustrates the high labeling efficiency of the Buccutite™ conjugation reaction comparable to the SMCC method. More importantly, we discovered that even without the column purification step, Buccutite™ conjugation kits produced fluorescent conjugates far superior to those of unpurified SMCC conjugates. Unpurified SMCC conjugates saw a decrease in stain index by 50%.

Buccutite™ Conjugates vs. Commercial Conjugates

---

PE-Cy7 (Cat No. 2616) is a popular tandem dye commonly used in combination with FITC (Cat No. 135), PE (Cat No. 2558), and other tandem dyes for multi-parameter flow cytometry. PE-Cy7 consists of a PE phycobiliprotein labeled with the cyanine dye, Cy7 (Cat No. 161). When excited by the 488 laser, PE functions as a donor transferring energy to Cy7 via Förster resonance energy transfer (FRET) to emit fluorescence at 780 nm. This difference of 292 nm between the maximal excitation and maximal emission is referred to as Stoke's shift. Large Stoke's shift of tandem dyes, such as PE-Cy7, is extremely valuable when designing multicolor flow cytometry panels because it provides considerable spectral separation and minimizes crosstalk.



In the following study, PE-Cy7-streptavidin conjugates prepared using Buccutite™ Rapid PE-Cy7 Tandem Antibody Labeling Kit (Cat No. 1317) were compared with PE-Cy7-streptavidin conjugates purchased from BioLegend. The performance of both conjugates was tested in HL-60 cells. Cells were stained with 1 µg/mL CD45-Biotin or mouse IgG biotin (control) and then stained with PE-Cy7-streptavidin conjugates. Results illustrate similar performance in staining, indicating Buccutite™ conjugation technology as a valuable alternative for conjugate production.

 

Stability of Buccutite™ Reagents

---

A key advantage of Buccutite™ conjugation kits is the stability of its components. Both the Buccutite™ MTA linker and the pre-activated PE, APC, or tandem dye can be lyophilized into powder form and stored at 4°C for at least 12 months without compromising product integrity. Stability was tested using CD45 antibody conjugates labeled with Buccutite™ FOL-Activated PE (Component A in Cat No. 1310) and Buccutite™ FOL-Activated APC (Component A in Cat No. 1311) stored at 4°C for 12 months. CD45 antibodies were pre-activated with Buccutite™ MTA (Component B in Cat No. 1310 and Cat No. 1311) and reacted with reconstituted Buccutite™ FOL-Activated PE and Buccutite™ FOL-Activated APC. For control, CD45 antibodies were also labeled with freshly prepared Buccutite™ FOL-Activated PE and Buccutite™ FOL-Activated APC. Jurkat cells were stained with the conjugates without further purification, and performance is shown in Figure 5. The brightness and signal-to-background ratio showed little variation between fresh Buccutite™ FOL-Activated PE/APC and Buccutite™ FOL-Activated PE/APC stored at 4°C for 12 months.

 

Conclusion

---

• Pre-activated phycobiliprotein and the tandems are stable for at least 12 months with similar activity in Buccutite™ reactions, without significant changes in performance.
• Buccutite™ Ab conjugation kit is suitable to make Ab conjugate with good performance.
• The conjugates prepared with Buccutite™ technology are compatible with all other commercial conjugates and also compatible with each other in cell surface staining for analysis and sorting.

Available Buccutite™ Labeling Kits

---

Buccutite™ Rapid Antibody Conjugation Kits are available for labeling antibodies and proteins with PE, APC, or Tandem dyes. Each kit's phycobiliprotein or tandem dye is pre-activated with Buccutite™ FOL and can be directly conjugated to proteins of interest activated with Buccutite MTA. The Buccutite™ FOL-activated dye readily reacts with Buccutite™ MTA-containing molecules under extremely mild neutral conditions without any catalyst required. Compared to commonly used SMCC.