Which chromogenic substrate gives purple product with ALP?
Alkaline phosphatase paired with nitroblue tetrazolium chloride chemistry generates insoluble formazan precipitate through reduction. NBT accepts electrons, converting from yellow soluble tetrazolium to dark purple-blue diformazan that deposits at enzymatic site. Often combined with 5-bromo-4-chloro-3-indolyl phosphate which yields indigo intermediate, enhancing localization. TMB is substrate for horseradish peroxidase, not alkaline phosphatase, giving blue. ONPG is hydrolyzed by beta-galactosidase to yellow ortho-nitrophenol. Para-nitrophenyl phosphate with phosphatase produces yellow soluble
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.
