Buying a recombinant human protein that behaves the same next time: why the expression host changes glycosylation, folding and therefore activity even when the sequence is identical, what a certificate has to state in purity, endotoxin, aggregate content and specific activity before a lot can be compared with the last one, why carrier protein presence changes the effective concentration, where a charge variant analysis belongs for a protein whose heterogeneity matters, what click chemistry reagents add when a protein has to be labelled site specifically, and what to record so a lot change can be investigated rather than argued about

Specifying and qualifying a lot

1. Choose the host for the modification you need. Bacterial expression gives unmodified protein cheaply; mammalian and insect systems give glycosylation that may be essential for activity or binding. The host is part of the reagent identity.
2. Demand specific activity, not just purity. Purity says what fraction is your protein; specific activity says how much of it works. A lot can be pure and largely misfolded, and only the activity figure reveals it.
3. Check endotoxin and aggregate content. Endotoxin provokes responses easily mistaken for the protein's effect, and aggregates change potency and can be immunogenic. Both belong on the certificate.
4. Know whether carrier protein is present. Carrier stabilises dilute preparations and interferes with some assays and with conjugation. A carrier free version exists for those cases and is a different product.
5. Titrate every new lot. Match lots by activity in your own assay rather than by stated mass. It is one dose response and it prevents a step change in the middle of a study.
6. Measure heterogeneity where it matters. For proteins whose charge or glycan distribution affects function, a charge variant separation describes the lot in a way a purity figure cannot.

Activity is the specification

Mass is easy to measure and easy to quote, and it predicts biological effect poorly. Specific activity in a defined assay is what actually transfers between lots and between laboratories.

Where a supplier will not state it, treat the reagent as uncharacterised for quantitative work and titrate every lot yourself.

Record the lot with the experiment

For this class of reagent the lot is the most likely explanation of an unexpected result, and it is the field most often missing from a methods section.

Recording supplier, catalogue number and lot costs nothing and turns an argument into a lookup.

Common questions

Why does a new lot behave differently?

Specific activity differs between lots because folding, aggregation and modification differ. Matching by stated mass rather than by activity is the usual cause of a step change mid study.

Does the expression host matter if the sequence is the same?

Frequently yes. Glycosylation and disulphide formation differ between hosts and can change activity, stability and binding, so the host is part of the reagent specification.

Carrier free or with carrier?

Carrier stabilises dilute stocks and interferes with conjugation and with some assays. Choose deliberately and keep it consistent, because switching changes effective concentration.

Browse by service class

• Compare Gene synthesis on advertised prices