Purification and photoaffinity labeling of sucrose phosphate synthase from spinach leaves

• Published in: Archives of biochemistry and biophysics Vol. 281; no. 2; p. 212
• Main Authors: Salvucci, M E, Drake, R R, Haley, B E
• Format: Journal Article
• Language: English
• Published: United States 01-09-1990
• Subjects: Affinity Labels; Azides - pharmacology; Glucosyltransferases - isolation & purification; Isoelectric Point; Molecular Weight; Uridine Diphosphate Glucose - analogs & derivatives; Uridine Diphosphate Glucose - pharmacology; Uridine Diphosphate Sugars - pharmacology; Vegetables - enzymology
• ISSN: 0003-9861
• DOI: 10.1016/0003-9861(90)90434-Z

Sucrose phosphate synthase (SPS) was isolated from spinach leaves by precipitation with polyethylene glycol, ion-exchange and hydrophobic interaction chromatography, and rate zonal centrifugation. The enzyme was purified more than 600-fold to a specific activity of 57 mumol/min/mg protein. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that a 120-kDa polypeptide was enriched through purification and was the major polypeptide in the final SPS preparation. The 120-kDa polypeptide was photoaffinity labeled with the substrate analog, 5-azidouridine [beta-32P]5'-diphosphate-glucose ([beta-32P]5-N3UDP-Glc). Covalent incorporation of 5-N3UDP-Glc into the 120-kDa polypeptide exhibited an apparent Kd of 74 microM, similar to the apparent Ki for inhibition of SPS activity by unphotolyzed 5-N3UDP-Glc. Competition experiments showed that photolabeling of the 120-kDa polypeptide by 5-N3UDP-Glc was reduced in the presence of UDP-Glc, exhibiting an apparent Ki value that was similar to the apparent Km (UDP-Glc) of 2.9 mM for the purified enzyme. The relative molecular mass of the SPS holoenzyme was 253,000, and the isoelectric point of the 120-kDa subunit was 5.2. The data confirmed the identity of the 120-kDa polypeptide as the SPS subunit, established the structure of the active enzyme as a dimer, and demonstrated active-site labeling of SPS by a photoaffinity analog of the substrate.