Serpin A-8 (Recombinant)

Frequently Asked Questions

What is the expected molecular weight of recombinant Serpin A-8 / AGT on SDS-PAGE?

The unglycosylated AGT polypeptide predicts ~52 kDa from its 476-amino-acid sequence (UniProt P01019). Because this recombinant is expressed in HEK293 cells, N-linked glycosylation adds substantial mass, and the predominant band typically migrates at 55–65 kDa under denaturing, reducing SDS-PAGE conditions — consistent with the endogenous hepatic protein. Minor smearing or a diffuse upper band is normal and reflects heterogeneous glycan occupancy. If you need a sharp, glycan-independent reference band, PNGase F treatment prior to loading collapses the signal to ~52 kDa.

Is this recombinant AGT the full-length precursor or the des-angiotensin processed form?

The recombinant is the full-length 476-amino-acid secreted precursor, including the N-terminal angiotensin I sequence (residues 1–10 of the mature peptide) flanked by the Leu10–Val11 renin cleavage site. It is not pre-cleaved. This is the relevant substrate form for renin activity assays: the intact Leu10–Val11 bond is required for renin recognition and Ang I release. The reactive-centre loop encodes the angiotensin precursor sequence, so unlike classical inhibitory serpins, no spontaneous RCL cleavage or inhibitory complex formation is expected under normal storage conditions.

What enzyme cleaves recombinant Serpin A-8 and what product is generated?

Renin (EC 3.4.23.15) is the physiological and preferred experimental protease: it cleaves the Leu10–Val11 bond to release the decapeptide angiotensin I (Ang I). Downstream, ACE removes the C-terminal His-Leu dipeptide to yield the octapeptide angiotensin II (Ang II), and ACE2 or endopeptidase 3.4.15.1 can generate Ang-(1-7). AGT is strictly a substrate, not an inhibitor — no serpin-protease covalent complex (RCL loop-inserted form) should be expected. Other aspartyl proteases such as cathepsin D can cleave AGT at secondary sites but are not relevant to canonical RAS biochemistry.

How do I set up a renin activity assay using recombinant Serpin A-8 as substrate?

Use this AGT recombinant at 1–4 µM as the substrate in a standard direct radioimmunoassay or ELISA-based renin activity assay, or couple it to an Ang I ELISA readout. Assay buffer: 50 mM sodium acetate pH 6.0, 0.1% BSA, 1 mM PMSF (to suppress non-specific proteolysis) at 37 °C for 60–120 min. Because HEK293-derived AGT carries native glycosylation, it more closely replicates endogenous substrate kinetics than E. coli-expressed variants. Run a no-renin control to confirm baseline Ang I is below assay detection threshold before interpreting renin-dependent generation.

What buffer is recombinant Serpin A-8 supplied in, and does it interfere with downstream renin or ELISA assays?

The protein is supplied in 50 mM Tris-HCl pH 7.5, 150 mM NaCl, 10% glycerol. For renin activity assays run at pH 6.0, dilute at least 1:5–1:10 into the assay buffer so that residual Tris and glycerol do not shift the reaction pH or viscosity. Glycerol above ~2% can modestly reduce renin kcat, so dilution is recommended. For ELISA capture assays, the buffer is generally compatible at ≤1:10 dilution. Avoid introducing this protein into assays that use HEPES as the sole buffer without pH re-verification, as mixing can shift apparent pH.

What starting concentration of recombinant Serpin A-8 should I use for Km determination with human recombinant renin?

For Michaelis-Menten kinetics with human recombinant renin, a substrate titration spanning 0.25–20 µM AGT is a practical starting range; published Km values for human plasma AGT with renin fall in the 1–3 µM range depending on glycosylation state and assay pH (optimum pH ~6.0). Begin with 8–10 concentration points in duplicate. Fix renin at a concentration giving ≤20% substrate conversion at the lowest AGT point to maintain initial-rate conditions. Because glycosylation affects substrate affinity, results with this HEK293-expressed protein will more closely match physiological Km than bacterial recombinants.

Can I use recombinant Serpin A-8 as a Western blot positive control for the matched RP-SerpinA8 antibody?

Yes — this is one of the primary intended uses. Load 20–50 ng of recombinant AGT per lane alongside your cell lysate or plasma samples. Under reducing SDS-PAGE, expect a band at 55–65 kDa (glycosylated form). The matched rabbit polyclonal antibody RP-SerpinA8 (/anti-serpin-a8-rabbit-polyclonal-antibody) was raised and validated in the same laboratory, ensuring the recombinant and antibody are confirmed compatible. This pairing is particularly useful when validating RP-SerpinA8 against new cell line or tissue lysates where endogenous AGT expression may be low or uncertain.

How much recombinant Serpin A-8 should I load to get a clean band for antibody RP-SerpinA8 validation?

For Western blot validation of RP-SerpinA8, 20–50 ng per lane typically produces a strong, clean band at 55–65 kDa with standard ECL detection. If you are titrating primary antibody concentration (recommended range for RP-SerpinA8 is 1:500–1:2,000), load a fixed 30 ng of this recombinant as the positive control lane. At 50 ng you may see minor background from glycoform heterogeneity; this is expected and does not indicate degradation. Including both the recombinant control and a positive tissue lysate (e.g., human liver) in the same blot is the most informative validation setup.

What are the storage and handling recommendations to maintain activity of recombinant Serpin A-8 after receipt?

Upon receipt, centrifuge briefly to collect contents, then aliquot into single-use volumes sized for your experiment — repeated freeze-thaw cycles measurably reduce renin substrate activity. Store aliquots at -20 °C; for aliquots to be used within 1 week, storage at 4 °C is acceptable. The supplied buffer (50 mM Tris-HCl pH 7.5, 150 mM NaCl, 10% glycerol) is formulated for stability, so do not dilute the stock more than necessary before freezing. Purity is >90% by SDS-PAGE and endotoxin is <0.1 EU/µg by LAL, making it suitable for cell-based RAS stimulation experiments without additional endotoxin removal steps.