The significance of nitrogen limited condition in the initiation of lipid biosynthesis in Aurantiochytrium sp. SW1

摘要

The manner of the onset of lipid synthesis in Aurantiochytrium sp. SW1 as well as the possible role of NAD~+ dependent isocitrate dehydrogenase (NAD~+: ICDH) in the initiation of lipid biosynthesis were studied. The initiation of lipid synthesis in the microalgae was not associated with the cessation of growth, but commence at the early phase of growth. Substantial amount of lipid (30 %, g/g biomass) was accumulated during the active growth phase at 48 h with growth rate decreasing from 0.11 g/L/h during active growth to 0.02 g/L/h in the limited growth phase. At that period the activity of NAD~+: ICDH was still detectable although it slightly decreased to 20 nmol/min/mg in 48 h from 25 nmol/min/mg at 24 h. Analysis of ammonium sulfate fractionated of NAD~+: ICDH activity showed that NAD~+: ICDH was not completely dependent on adenosine monophosphate (AMP) for its activity, although the presence of AMP increased the enzyme's affinity towards its substrate (isocitrate) indicated by the low K_m value of the enzyme for isocitrate. While citrate acts as inhibitor of the enzyme only at high concentration. The probable implications of these properties to the regulation of lipid are discussed.