In Western blotting, a laboratory technician uses an anti-phosphoserine antibody to detect a 52 kDa phosphoprotein. The membrane shows no band despite positive controls working. Which troubleshooting step specifically addresses the risk of phosphatase activity dephosphorylating the target during sample preparation?

• A Adding SDS to the lysis buffer to denature endogenous phosphatases
• B Running the gel at higher voltage to minimize sample heating and phosphatase activity
• C Including sodium orthovanadate and sodium fluoride (phosphatase inhibitors) in the lysis buffer ✓
• D Using a non-fat milk blocking buffer instead of BSA

Explanation

Phosphoproteins are labile because endogenous protein phosphatases (serine/threonine phosphatases PP1, PP2A, PP2B, and tyrosine phosphatases) remain active during cell lysis and sample preparation, rapidly dephosphorylating the target epitope. Sodium orthovanadate (inhibits protein tyrosine phosphatases and serine/threonine phosphatases) and sodium fluoride (inhibits serine/threonine phosphatases) must be added to the lysis buffer to preserve the phosphorylation state. The phosphatase inhibitor cocktail should be freshly prepared and added immediately before cell lysis. SDS denatures proteins but cannot prevent dephosphorylation that occurs before SDS reaches the enzymes. The blocking buffer choice affects nonspecific binding, not phosphorylation status.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.