Peptides in Cell-Based Assays: Solubility, Stability and Controls

What happens to a peptide between the vial and the well: solubility, losses to plastic, oxidation, proteolysis, counter-ions and endotoxin, and the controls that separate a real effect from an artefact.

Illustrative — laboratory plasticware. Cationic peptides can adsorb to container walls. OP Labs · Research

In a cell-based assay, the concentration you calculate and the concentration the cells see can differ. Peptides fall out of solution, stick to plastic, oxidise and get cut by proteases. They also bring counter-ions, and sometimes endotoxin, into the well. Each of these has a known mechanism, and most can be controlled.

Start with an accurate stock concentration, calculated from net peptide content and the free-peptide molecular weight (see HPLC Purity vs Net Peptide Content). Everything below is about keeping that number true.

Solubility follows charge

Net charge depends on the ionisable groups and their pK values, and the pH of minimum solubility tracks the isoelectric point (Shaw et al., 2001). Work out the net charge at your working pH from the sequence, and dissolve the peptide at a pH well away from its pI.

If a hydrophobic sequence needs an organic co-solvent such as DMSO, two consequences follow. The solvent needs its own control (below). And DMSO can oxidise cysteine thiols to disulfides (Tam et al., 1991), so a DMSO stock of a cysteine-containing peptide may not stay the molecule you dissolved.

Losses to plastic and glass

Cationic peptides adsorb to container walls, and the losses can be large. For three cationic membrane-active peptides at 1 µM, only 10–20% was recovered from borosilicate glass vials and polypropylene tubes after an hour. Relatively more is lost at low concentrations, and after successive transfers between containers at 5 µM, close to none remained (Kristensen et al., 2015).

The same study points to the main defences:

• Low-binding tubes lost less peptide.
• Keep working concentrations high for as long as possible, and dilute last.
• Keep the ratio of sample volume to container surface high.
• Surfactants reduce adsorption, but they can disrupt membranes, so use them with care in cell work.

Where it matters, measure the concentration in the final vessel rather than assuming it.

Oxidation

Methionine and cysteine are the residues most sensitive to oxidation (Stadtman & Levine, 2003). Mass spectrometry shows the common products: methionine sulfoxide adds 16 Da, an intramolecular disulfide removes 2 Da, and a disulfide-linked dimer appears at twice the mass less 2 Da. (Mass shifts calculated from atomic masses.) Make solutions of susceptible sequences fresh, and check them by MS if they have been stored.

Stability in the medium

If the medium contains serum, or the cells secrete proteases, measure stability rather than assume it. Incubate the peptide in the complete medium, sample across the assay time course, quantify the intact peptide by RP-HPLC and identify fragments by mass spectrometry. If most of the peptide is gone before your readout, the result may reflect its fragments rather than the intact peptide.

What comes into the well with the peptide

Counter-ions. Synthetic peptides are usually TFA salts, and TFA can interfere with cell-based work and inhibit proliferation (Sikora et al., 2020). Do the arithmetic: at 10 µM, a peptide carrying three TFA counter-ions brings 30 µM TFA into the well. Use an exchanged salt, or run a TFA-only control at the matching concentration.

Endotoxin. Endotoxins act through monocytes and macrophages and often complicate study of the peptide’s own biological effects. They are highly heat-stable and are not destroyed under regular sterilising conditions (Magalhães et al., 2007). Two consequences: know the endotoxin level of your material (EU/mg) before using LPS-responsive cells, and don’t rely on a heat-inactivated peptide as an endotoxin control.

Controls that make a result interpretable

A scrambled peptide is a good control only if it is designed as one. Random shuffling can produce a very different spatial or charge distribution, or carry an active motif across from the original sequence (Farkas et al., 2022, preprint). It also has exactly the same mass as the original, so confirm which one is in the tube by MS/MS, not by intact mass.