FAT SYNTHESIS AND ITS REGULATION IN PLANTSⅥ.ISOTOP...
Y.S.Zhu，C.H.Liu and C.N.Shih
（Institute of Plantt Physiology，Academia Sinica）
Since the fat synthesis in rape seeds during development is accompanied by the increase in the activity of the HMP pathway and the decrease in the activity of the EMP pathway，as previously indicated in studies of enzyme and respiratory inhibitors，it was suggested that the fat synthesis by rape seeds closely depends upon the HMP pathway.In this study，the oxidation of glucose-1-C14 and glucose-6-C14 or glucose-u-C14 by rape seeds and the radio activity in C1/C6 or C1/Cu at different stages of the seed development were estimated.In addition，the products of conversion of labelled glucose were separated and determined.The isotope evidence further supports above suggestion.(https://www.daowen.com)
The seeds of different age were incubated in the Conway dish with glucose-1-C14，glucose-6-C14 or glucose-u-C14 in phosphate buffer（0.01 mol/L，pH 6.5）at 30℃（or 35℃，38℃）for the determination of respiration.After incubation the seeds were immediately killed by adding hot alcohol.Then the lipid-soluble fraction，alcohol-soluble fraction，sugars，organic acids，amino acids，starch，crude cellulose and proteins were separated for the radioactivity assay.The results of experiments indicate that the ratio in C1/C6 rises as the seeds are developing，reachs the maximum level in seeds approaching to ripeness，but sharply decreases in full-ripe seeds，parallel to the course of the fat accumulation.The ratio in C1/Cu shows similar changes during seed development.These changes are very striking and conform in pattern with those obtained in a previous study on the enzymes of the HMP pathway.It seems，that in period of fat accumulation the rise of C1/C6 ratio in seeds，being attributed to the relatively greater increase in the contribution of C-1 to CO2，is due to the increase in the activity of the HMP pathway，because no activity of glucose dehydrogenase，another enzyme responsible to the release of CO2 from C-1 of glucose，was observed in 30 days-old seeds.
The conversion of labelled glucose into lipid-soluble fraction，alcohol-soluble fraction（e.g.sugars，organic acids，amino acids），proteins，starch and crude cellulose was also demonstrated.It is especially interested that the incorporation of glucose-6-C14 into organic acids，lipid-soluble substances，proteins，starch and crude cellulose is greater than that of glucose-1-C14，consistent with the preferential release of C14O2from glucose-1-C14oxidized via the HMP pathway.It may be a by-evidence for the operation of the HMP pathway in maturing rape seeds.The contribution of the HMP pathway to the total glucose oxidation in rape seeds approaching to ripeness，as appreciated on the basis of amounts of incorporation of G-1-C14 and G-6-C14 into fat，is about 47%.
