Sucrose Phosphate Synthase, a KeyEnzymeforSucrose Biosynthesis inPlants Protein Purification fromCornLeavesandImmunological Detection

Wehavepurified theprotein fortheenzymesucrose phosphate synthase (SPS)fromcorn(Zeamays)leaves. Partially purified SPSprotein wasusedtogenerate specific monoclonal antibodies. Thefollowing immunoaffinity chromatography allowed the isolation ofpureSPSprotein. Theapparent molecular massof theSPSpolypeptide is138kilodaltons. Byimmunoblot, anSPS antigen wasfoundtoaccumulate inmatureleaves. SPSprotein levels remain constant during theday/night cycle. Theobserved diurnal fluctuation ofextractable enzymeactivity, therefore, must becausedbymodification ofthespecific activity ofSPSinvivo. Thedisaccharide sucroseistheexport formofthephotoassimilate ofmostplants. Carbonfixed inthechloroplast exits astriose phosphates, andthese enterthepathway ofsucrose synthesis inthecytoplasm. Mostofthesucroseformedin matureleaves (source) isexported viathephloemtothe heterotrophic parts oftheplants (sinks). A keyenzyme ofthepathway ofcytoplasmatic sucrose synthesis, SPS'(EC.2.3.1.14), was discovered in1955by Leloir andCardini (11). Itcatalyzes thefollowing reaction: Fructose-6-phosphate