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STUDY OF SOME KINETIC PROPERTIES OF SUCROSE PHOSPHATE SYNTHASE FROM RICE LEAVES

MIAN WAJAHAT HUSSAIN*

Abstract: Rice (Orya sativa 1. cv. IR-30) was grown hydroponically in growth chambers at 28/23°C day/night temperatures with 14-h photoperiod and quantum irradiance of 450 umol m-2 S-l. Sucrose phosphate ;ynthase (SPS) activity in desalted extracts from 21 days old leaves was measured in saturating and "limiting" ( Pi included) substrate conditions. Evidence is presented that SPS activity via UDP estimation underestimates !the enzyme activity in rice. Substrate UDP-Glc and Fru-6-P saturation curves in presence of G1c-6-P were hyperbolic with apparent Km(UDP-G1c) of 2.7 mM and SO.5 (Fru-6-P) of 1.2 mM. G1c-6-P activated SPS rand apparent K_act was 3.2 mM. Diel SPS profiles indicated that rice SPS was activated in light and this activation was more marked under "limiting" assay conditions. P_i inhibition of SPS was dependent upon suubstrate conditions; being more pronounced in "limiting" conditions. Mannose and P_i feeding experiments rindicated that rice SPS was activated by mannose in darkness and the activated enzyme was not sensitive to P_i. In contrast, SPS from leaves fed with P_i was substantially inhibited when P_i 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 form is more active in light while P_i sensitive in the dark.

Department of Botany, University of Florida, Gainesville, FL 32611, USA.
Department of Botany, Government College, Lahore, Pakistan.