PJB-1999-8
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 Kact 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. Pi inhibition of SPS was dependent upon suubstrate conditions; being more pronounced in "limiting" conditions. Mannose and Pi feeding experiments rindicated that rice SPS was activated by mannose in darkness and the activated enzyme was not sensitive to Pi. In contrast, SPS from leaves fed with Pi was substantially inhibited when Pi 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 Pi sensitive in the dark.
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