What is the most suitable molar ratio for using HRP labeled streptavidin for ELISA?

HRP labeled streptavidin is not recommended to be blindly fixed at a 1:1 or 1:2 ratio. Streptomycin is a tetramer with high steric hindrance, and a high HRP labeling rate may reduce its biotin binding ability or form oligomers. Starting from 1:1 to 1:2 in experience, perform ELISA chessboard titration to compare signal-to-noise ratio and sensitivity. The 1:4 ratio in the literature may not be suitable for your system, as there may be differences in the batch numbers of activated HRP, buffer, and streptavidin. It is recommended to divide the same batch of SA into three tubes, with ratios of 1:1, 1:2, and 1:4 respectively. After labeling, purify and package the tubes for testing, selecting the proportion with low background and good linear signal growth.

Molecular sieves are more reliable. Ultrafiltration centrifugation is suitable for liquid exchange and concentration, but its resolution for removing free HRP and aggregates is limited, especially for small volumes of 0.5mg SA that are easily lost on the membrane. After labeling, it is recommended to first centrifuge at low speed to remove insoluble substances, then replace the sample with a suitable buffer solution using an ultrafiltration centrifuge tube, and finally pass it through Superdex 200 or Superose 6 molecular sieves to collect different molecular weight peaks; This can separate the SA-HRP conjugate, free HRP, and polymer. If only a small amount is done once, the desalination column in the commercial labeled purification kit can also be used, which is more reliable than pure ultrafiltration.

For biotin-avidin amplified ELISA, the labeling conditions are more critical than the ratio. SA contains amino/ sulfhydryl groups; choose the corresponding activated HRP. Avoid amine-containing labeling buffers, and adjust the pH according to the activated group. When labeling 0.5 mg SA on a small scale, first set two gradients of HRP molar ratio at 1:1 and 1:2, and immediately perform column purification after labeling. For validation, coat the plate with a known concentration of biotinylated capture antibody or biotin-BSA, examine the signal curves of the conjugate at different dilutions, and select the condition with the highest signal-to-noise ratio. Sensitivity and batch-to-batch consistency are more important than simply increasing the HRP molar ratio.

For a 0.5 mg SA dosage, the amount used is very small. It is recommended to purchase small-spec maleimide-activated HRP to avoid the need to activate non-activated HRP yourself. When purchasing, directly search for "maleimide-activated HRP" or "HRP conjugation kit." Thermo, Sigma, Roche, and others all offer small-package lyophilized powder; you need to confirm the activated group and suitable protein with the supplier regarding the catalog number. For example, Sigma's non-activated HRP is commonly P8375, but it requires self-activation and is not suitable for your small-scale needs. I would recommend using a commercial conjugation kit, which includes activated HRP and buffer, has clear steps, and reduces catalog number matching and process optimization.