INTRACELLULAR LOCATION OF OLEATE DESATURASE AND ASSOCIATED CONSTITUENTS IN DEVELOPING SUNFLOWER (HELIANTHUS-ANNUUS) SEEDS

The location of sunflower seed oleate desaturase (ODS; EC 1.3.1.35) ac tivity and its putative component cytochrome b has been investigated i n fractions obtained by isopycnic centrifugation of seed homogenates p repared in the absence and in the presence of added Mg2+ ions. Electro n microscopy showed that the addition of mM Mg2+ resulted in maximal r etention of ribosome association with the separated microsomes, whilst omission of this ion from the homogenisation medium yielded preparati ons containing denuded vesicles. Both ODS activity and cytochrome b we re present in the fractions containing smooth vesicles which co-sedime nted with the ER marker enzyme NCR, in fractions of density 1.09 g ml( -1) in the latter and with vesicles having a slightly less than maxima l population of associated ribosomes, sedimenting at a density of 1.18 g ml(-1), in the former preparations. Whilst the enzymes LPCAT and CP T, which may be concerned in the formation of the phosphatidylcholine substrate of ODS, co-sedimented with the desaturase enzyme in the smoo th microsomal preparations of the homogenates prepared in the absence of Mg2+ addition of this ion resulted in preparations showing a polydi sperse distribution of both enzymes and the marker NCR in which neglig ible CPT activity and 38% of the LPCAT activity was present in the fra ctions containing ODS activity.