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Equimolar mixing of Streptavidin-ZAP and Biotinylated Molecule

Question: I was wondering if you could elaborate on why the Streptavidin-ZAP product recommends to be used at an equimolar ratio with the targeting reagent, when it is capable of binding up to four biotins/molecule?

Answer: It’s a question we get asked sometimes and it’s a good question.

You are correct that streptavidin is capable of binding up to 4 biotin molecules.  However, when we created streptavidin-ZAP with the purpose of being a modular way of creating targeted toxins, we learned that the best general rule to follow was using a equimolar reaction.  In theory, it is a 1:1 ratio of targeting molecule to streptavidin-ZAP, where we are most likely seeing an average of 1:1, but there is also the possibility of mixed ratios.

The amount of publications using the equimolar approach gave the desired results whether they were using a small biotinylated peptide or whole IgG.  You’ll notice that depending on the MW of your biotinylated targeting agent, the amount of streptavidin-ZAP needed for the experiment can vary drastically and through in-house characterization, the equimolar approach still worked best. 

Another reason we recommend a 1:1 ratio is based on our experience with our other secondary conjugates. It may be intuitive to think that using a higher dose of targeting agent would induce more cell death, but we found the opposite effect, where the excess, un-reacted targeting agent competed with the conjugated material for surface binding sites, which in turn decreased the amount of saporin being delivered. We have a publication (PMCID: PMC8952126 ) that also describes this observation.  

Once you’ve created a baseline using the equimolar protocol and are more accustomed to how streptavidin-ZAP works in your application, please contact us if you feel more optimization is needed.  It will be easier to help trouble-shoot when we are all working off the same protocol.

Related Products: Streptavidin-ZAP


  1. Ancheta LR et al. Saporin as a commercial reagent: its uses and unexpected impacts in the biological sciences-tools from the plant kingdom. Toxins (Basel) 14(3):184, 2022.

DMSO% for peptide and Streptavidin-ZAP

Q: Instead of performing the reaction between our biotinylated peptide and Streptavidin-ZAP at the initially provided concentration of Strep-ZAP (20 µM), is it OK if the reaction is done at a 10-fold more dilute concentration? This request is to ensure we don’t have any solubility problems with our very tricky lipophilic peptide. Our protocol would be to first dilute Streptavidin-ZAP to 2 µM with PBS and then add the peptide in DMSO (10% final), and store the aliquoted resulting 1.82 µM solution?

A: In regards to your question, while keeping in mind your solubility concerns, we suggest that you:

  1. Proceed with diluting the Streptavidin-ZAP to 2 uM with PBS as you suggest, BUT, only react the amount of Streptavidin-ZAP necessary for the next step.
  2. Store the undiluted and unreacted Streptavidin-ZAP at -80°C until you’re ready for more conjugate.

We understand the solubility of the peptide is a concern, and rightfully so.  However, we also do not want to compromise the Streptavidin-ZAP during storage, considering its value.

Related Products: ZAP Conjugates

ZAP Internalization Kit Concentrations

Q: We have your ZAP internalization kit and I have a question regarding the concentrations used in the cytotoxicity assay. The Hum-ZAP used in the assay (mentioned in the PDF protocol) is 4.5 nM and the target agent was 10 nM to 1 fM. Is there a stoichiometric relation between Hum-ZAP and the target agent concentrations?

A: To answer your question simply, yes, there is a stoichiometric relation between a secondary conjugate and the targeting agent.

Q: If I use higher concentrations of the target antigen, then should I also increase the concentration of Hum-ZAP?

A: It may be intuitive to think that using a higher dose of primary antibody induces a higher amount of cell death, but as seen in the attached figure, at the highest concentration of 192-IgG (10 nM = Log -8) there is a lessened amount of killing, at a 25-fold lower concentration, as compared to the antibody. The explanation for this is that, at the higher concentrations of primary antibody, there is more unconjugated 192-IgG and fewer 192-IgG+Fab-ZAP complexes. The free 192-IgG then out-competes the conjugates for cell surface binding sites which, in turn, decreases the amount of Saporin being internalized, hence less cell death.

Related Products: ZAP Conjugates

Anti-Melanopsin Protocol

Q: We’re interested in trying out your melanopsin antibody (Cat. #AB-N38) using immunohistochemistry in mouse retina. Do you have a recommended protocol?

A:  This protocol has been utilized successfully with anti-melanopsin.

Panda S. et al. 2002. Melanopsin (Opn4) requirement for normal light- induced circadian phase shifting. Science 298(5601):2213-2216.

Related: Anti-Melanopsin (Cat. #AB-N38), Anti-Melanopsin, affinity-purified (Cat. #AB-N39)

Cytotoxicity Assay Protocols

One of the tests you can use to test your targeting agent for internalization is the in vitro Cytotoxicity Assay. Protocols to assist in preparing for, executing and interpreting results are now posted on our website.

There are several protocols available.

Preparing for a Cytotoxicity Assay using Secondary Conjugates. This protocol will be helpful when using our secondary antibody-saporin conjugates with your primary antibody. These include Anti-M-ZAP (Cat. #IT-30), Goat-ZAP (Cat. #IT-36), Hum-ZAP (Cat. #IT-22), Mab-ZAP (Cat. #IT-04), Rab-ZAP (Cat. #IT-05), and Rat-ZAP (Cat. #IT-26).

Preparing for a Cytotoxicity Assay using Streptavidin-ZAP. This protocol will be helpful when using our streptavidin-saporin conjugate (Streptavidin-ZAP, Cat. #IT-27) with your biotinylated targeting agent (peptide, ligand, cytokine, growth factor, antibody, etc.).

Concentration Calculation: Convert molarity to mg/ml and mg/ml to molarity. This protocol will help in determining the correct amount of material to use in your assay. There is also a link to an Online Calculator.

Cytotoxicity Assay for Targeted Toxins in vitro. This protocol includes photos of what your plates should look like during the assay process. It takes five days to complete this assay. Start on a Monday and develop on Friday. There are many factors that go into a successful cytotoxicity assay. This protocol should help you design and execute appropriately.

Preparing Cytotoxicity Data. This protocol will give an example of how to process the data from a Cytotoxicity Assay. ATS uses SOFTMax Pro software connected to a plate reader to determine the A490 value. Then we import this data into Prism software (GraphPad) to conduct further data analysis. Here is a figure generated with Prism.

We hope these protocols will be helpful to you in your research. If there are additional protocols or tutorials we can provide, please do not hesitate to ask.

This graph gives important information about how the potency of your targeted toxin. The ED50 is the Median Effective Dose (produces desired effect in 50% percent of population). The lower this number is, the more potent the targeted toxin.

Related: Protocols Listing, Targeted Toxins Catalog, Secondary Conjugates Catalog

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