Antibody conjugation services: choosing the chemistry and the labelling ratio for custom antibody conjugation, what antibody fluorophore conjugation and hrp antibody conjugation each need of the starting material, when antibody labeling kits or a single antibody labeling kit beat a service, and preparing the antibody so the conjugation works

Getting a usable conjugate

1. Send the antibody in a compatible buffer. Amine chemistry is consumed by any primary amine in the buffer, so material in a stabiliser containing one cannot be conjugated without a buffer exchange. Carrier protein in the formulation will also be labelled. Establish the buffer requirement before shipping, since this is the commonest avoidable failure.
2. Choose the chemistry from what you can tolerate. Amine chemistry is simple and labels randomly, including within binding regions. Thiol chemistry targets reduced disulphides and is more controlled. Site-specific approaches, whether enzymatic or through engineered residues, give homogeneous products and require more preparation.
3. Set the labelling ratio deliberately. The degree of labelling is a trade between signal and function, and the optimum differs by label and by application. Ask the provider to target a range and to report the measured ratio for the batch rather than the nominal one.
4. Purify away free label. Unreacted label produces background that looks like non-specific binding and is frequently mistaken for a bad antibody. Confirm that removal is part of the service and that residual free label is measured.
5. Verify function after conjugation. Conjugation can reduce affinity, especially with random chemistry at a high ratio. Test the conjugate in the intended application against the unconjugated antibody rather than assuming the label was neutral.

Doing it in house or outsourcing

Kits make small-scale conjugation straightforward and are worth it for occasional work and for screening several labels quickly. Outsourcing makes sense at larger scale, for site-specific chemistry and for anything where the conjugate must be characterised properly.

For payload conjugates destined for regulated use, the chemistry, the analytics and the documentation are a different level of work entirely and belong with a provider set up for it.

Characterisation to ask for

The measured degree of labelling, the residual free label, aggregate content after conjugation, and concentration by a method valid in the presence of the label. Absorbance-based concentration can be badly wrong when a chromophore has been attached.

Keep an aliquot of the unconjugated antibody. Comparing conjugate and parent in your own assay is the only reliable way to see what the conjugation cost.

Common questions

Why did my antibody conjugation fail?

Most often the buffer. Amine chemistry is consumed by primary amines in common stabilisers, and carrier protein in the formulation gets labelled too. Establish the buffer requirement and exchange before shipping.

What degree of labelling should I target?

Enough for signal without impairing binding, which varies by label and application. Ask the provider to target a range and to report the measured ratio for your batch rather than a nominal figure.

Random or site-specific conjugation?

Random amine chemistry is simple and can label binding regions. Site-specific approaches give homogeneous conjugates with preserved binding and need more preparation and cost. Function sensitivity decides it.

Should I test the conjugate?

Yes, in the intended application against the unconjugated antibody, which means keeping an aliquot of the parent. Conjugation can reduce affinity and the loss is invisible without that comparison.

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