PHYSICAL AND KINETIC EVIDENCE FOR AN ASSOCIATION BETWEEN SUCROSE-PHOSPHATE SYNTHASE AND SUCROSE-PHOSPHATE PHOSPHATASE

The possible formation of a multienzyme complex between sucrose (Suc)- phosphate synthase (SPS) and Suc-phosphate phosphatase (SPP) was exami ned by measuring the rates of Suc-6-phosphate (Suc-6-P) synthesis and hydrolysis in mixing experiments with partially purified enzymes from spinach (Spinacia oleracea) and rice (Oryza sativa) leaves. The additi on of SPP to SPS stimulated the rate of Suc-6-P synthesis. SPS inhibit ed the hydrolysis of exogenous Suc-6-P by SPP when added in the absenc e of its substrate (i.e. UDP-glucose) but stimulated SPP activity when the SPS substrates were present and used to generate Suc-6-P directly in the reaction. Results from isotope-dilution experiments suggest th at Suc-6-P was channeled between SPS and SPP. A portion of the SPS act ivity comigrated with SPP during native polyacrylamide gel electrophor esis, providing physical evidence for an enzyme-enzyme interaction. Ta ken together, these results strongly suggest that SPS and SPP associat e to form a multienzyme complex.