Iodate Reduction Activity in Nitrate Reductase Extracts from Marine Phytoplankton
Abstract:The scheme of an established method for measuring nitrate reductase activity (NRA) was adapted for determining the rate of the induced reduction of iodate to iodide by this enzyme extracted from marine phytoplankton. The enzyme extract was exposed to a mixture of 125 IO3 –and stable iodate. In the presence of added NADH (reduced nicotinamide adenine dinucleotide) at pH 8 and 20°C, the enzyme induced the reduction of iodate to iodide. The iodide formed was selectively removed from solution in one aliquot by its adsorption onto charcoal as palladium (II) iodide. Iodide plus iodate was similarly removed in a separate aliquot after iodate had been reduced to iodide with hydrazine. The radioactivities associated with iodide alone and iodide plus iodate on the charcoal then were determined with a NaI (Tl) gamma counting system. The rate of reduction of iodate, as given by the rate of iodide production, was estimated from the radioactivities associated with these two fractions of iodine. The NRA in a separate sub-sample of the same enzyme extract was measured in parallel. The reduction of iodate to iodide was observed in the enzyme extracted from four natural marine phytoplankton assemblages and from a culture of Skeletonema costatum (Grev.) Cleve. In the natural assemblages, the iodate re- duction activity, IRA, ranged between 0.008 and 0.019 n mole IO3 – μg-chl a –1 h–1 and was linearly related to NRA in n mole NO2 – μg-chla –1h–1 so that: IRA = 0.012 NRA + 0.09, r2= 0.73. These results suggest that nitrate reductase can induce the reduction of both nitrate to nitrite and iodate to iodide. However, nitrate is favored over iodate by about two orders of magnitude.
Document Type: Research Article
Publication date: 2005-01-01
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