Study of some kinetic properties of sucrose phosphate synthase from rice leaves

Rice (Orya sativa L. cv. IR-30) was grown hydroponically in growth chambers at 28/23 degrees C day/night temperatures with 14-h photoperiod and quantu m irradiance of 450 mu mol m-2 S-1. Sucrose phosphate synthase (SPS) activi ty in desalted extracts from 21 days old leaves was measured in saturating and "limiting" (P-1 included) substrate conditions. Evidence is presented t hat SPS activity via UDP estimation underestimates the enzyme activity in r ice. Substrate UDP-Glc and Fru-6-P saturation curves in presence of Glc-6-P were hyperbolic with apparent Km(UDP-Glc) of 2.7 mM and S0.5 (Fru-6-P) of 1.2 mM. Glc-6-P activated SPS and apparent K-act was 3.2 mM. Diel SPS profi les indicated that rice SPS was activated in light and this activation was more marked under "limiting" assay conditions. P-1 inhibition of SPS was de pendent upon substrate conditions: being more pronounced in "limiting" cond itions. Mannose and P-1 feeding experiments indicated that rice SPS was act ivated by mannose in darkness and the activated enzyme was not sensitive to P-1. In contrast, SPS from leaves fed with P-1 was substantially inhibited when P-1 was included in the assay medium. The data suggests that rice SPS may exist in different forms. It may also be possible that P insensitive f orm is more active in light while P-1 sensitive