Substrate capacity considerations in developing kinase assays

In developing a screening assay for a serine/threonine kinase, we evaluated various formats of an in-plate enzyme-linked immunosorbent assay (ELISA), as well as solution-phase kinase assays using either ELISA or AlphaScreen detection. Substrate was available both as a biotinylated 15-residue peptide and as a 25-residue peptide containing the same sequence expressed as a glutathione S-transferase fusion protein. When increasing concentrations of either of these substrates were coated directly onto ELISA plates, the rates of the kinase reactions progressively increased. In contrast, when the biotin-peptide was captured onto NeutrAvidin-coated plates, the finite peptide binding capacity of the plates limited the amount of substrate that could be incorporated into the assay system and thereby limited the rate of the reaction at a given kinase concentration. Solution-phase kinase reactions can tolerate high substrate concentrations; however, analysis of kinase reaction samples containing biotin-peptide concentrations higher than the binding capacity of NeutrAvidin-coated plates resulted in an inability to detect differences between reactions run at different substrate concentrations. For AlphaScreen detection following solution-phase kinase reactions, limitations in the binding capacity of the donor and acceptor beads caused loss of signal for substrate concentrations above the maximum binding capacity. Overall, the solution-phase assays required significantly more kinase than the in-plate assays (1-4 microg/ml versus <100 ng/ml, respectively). These studies demonstrate that the amount of substrate that can be incorporated into an assay system substantially affects the rate of the kinase reaction and therefore the amount of kinase required for the assay.