Metabolism of nicotinamide, adenine and inosine in developing microspore-derived canola (Brassica napus) embryos

The metabolic fate of [carbonyl-14C]nicotinamide, [8-14C]adenine and [8-14C]inosine was examined in microspore-derived canola (Brassica napus) embryos at different developmental stages: globular stage (day 10, stage 1), early cotyledonary stage (day 20, stage 2), late cotyledonary stage (day 25, stage 3), and fully developed stage (day 35, stage 4). Uptake of [8-14C]nicotinamide by the embryos was always rapid. A lower uptake rate was found for [8-14C]adenine and [8-14C]inosine, especially at stages 1 and 2. [Carbonyl-14C]nicotinamide was converted to nicotinic acid and further metabolized to pyridine nucleotides (NAD/NADP). Some radioactivity was also associated to nicotinic acid glucoside. [8-14C]adenine was efficiently utilized for the synthesis of adenine nucleotides and RNA. A small fraction of adenine was degraded to CO2 via ureides. Up to 40% of [8-14C]inosine was salvaged to nucleotides and RNA, although degradation of [8-14C]inosine to CO2 was pronounced. At stage 1, highest salvage activities of nicotinamide, adenine and inosine were observed. In contrast, the lowest purine salvage and highest purine catabolism were found in stage 3 embryos. These results suggest that both nicotinamide and purine salvage for NAD/NADP and purine nucleotides synthesis are extremely high in the globular stage (stage 1). These activities decrease gradually until the late cotyledonary stage (stage 3), before increasing again in the fully developed embryos (stage 4). Overall it appears that nicotinamide and purine salvage are required in support of active growth during the initial phases of embryogenesis and at the end of the maturation period, in preparation for post-embryonic growth.