NADP+-dependent isocitrate dehydrogenase activity in marine plankton

NADP+-isocitrate dehydrogenase (NADP-IDH), as one of the most active intracellular CO2-producing enzymes, was measured in marine plankton by adapting an enzyme assay to the 0.7-50 μm, 50-200 μm, and 200-2000 μm size fraction of the Canary Islands coastal plankton community. The variability of NADP-IDH activity in relation to pH, temperature, dilution of the sample, centrifugation or substrates was measured. In our hands, the maximum NADP-IDH activity (Vmax) in marine plankton samples was attained in 0.1 M phosphate buffer at pH 8.2 (pKa1 = 7.6 and pKa2 = 8.8), by adding 6 mM MgCl2, 0.3 mM NADP+ and 2 mM DL-trisodium isocitrate. The optimum temperature in these subtropic mesozooplankton samples was 28 °C, with an Arrhenius energy of activation (Ea) of 20.4 kcal mol−1 (85.4 J mol−1), and an Arrhenius collision frecuency factor (A) of 2.9·1011 mol NADPH s−1 (kg of protein)−1. The apparent Michaelis-Menten Km values for the substrates in crude homogenates at pH 8.5 and 18 °C were 271 ± 63 μM for isocitrate and 18 ± 3 μM for NADP+. This enzyme, in addition to its high CO2-producing activity, is also important in regulating other CO2-producing enzymes. Thus, it can be used to calculate: (1) respiration in marine samples; (2) carbon flux in the water column; (3) metabolic adaptations to environmental changes; (4) roles of the plankton components of the food chain; and (5) the reactive oxygen species (ROS) scavenging capacity of the marine plankton resources.