Biomedical subjects
Publications and source records attributed to H Ashihara.
Activity of adenine phosphoribosyltransferase and adenosine kinase was detected in purified spinach chloroplasts by using differential centrifugation and discontinuous Percoll density gradients. This is the first report of purine salvage enzymes being located in chloroplasts. The role of adenine and adenosine salvage in chloroplasts is discussed.
The metabolism of [2-14C]thymine and [2-14C]thymidine in the cotyledons and embryonic axes of black gram (Phaseolus mungo) seedlings was investigated. Both [2-14C]thymine and [2-14C]thymidine degraded extensively into [14C]CO2. The rate of release of [14C]CO2 from [2-14C]thymine was much greater than that from [2-14C]thymidine. Radioactivity from both precursors was also observed beta-ureidoisobutyric acid. This indicated that thymine was degraded by the reductive pathway of pyrimidine degradation. Small amounts of [2-14C]thymine and [2-14C]thymidine were salvaged for deoxyribonucleotide and DNA synthesis. The highest incorporation of [2-14C]thymine and [2-14C]thymidine into the DNA fraction was observed in 24 hour-old cotyledons where net DNA synthesis was not observed. These precursors seem to be utilised for DNA synthesis of organelles of the cotyledonary cells, probably mitochondria. In embronic axes, [2-14C]thymine is more effectively salvaged for DNA synthesis than [2-14C]thymine. The incorporation rate increased during the early phase of germination and attained its maximum at 48 h after which it decreased. No thymidine kinase activity was detected in either cotyledons or in the embryonic axes. Thymidine salvage seems to be catalysed by nucleoside phosphotransferase which is present both in the cotyledons and in the embryonic axes. This suggests that, in contrast to other pyrimidine and purine bases and nucleosides, no specific salvage system for thymine and thymidine is present in black gram seedlings.
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A seven fold increase in the rate of respiratory O2 uptake was observed 24 h after slicing of potato tuber disks. The maximum activity of pyrophosphate:fructose-6-phosphate phosphotransferase (PFP) was 5-7 times greater than that of ATP-dependent phosphofructokinase (PFK) in fresh or aged potato slices. Thus, PFP may participate in glycolysis which supplies respiratory substrate in potato tubers. The PFP activity of desalted extracts determined in the absence of fructose-2,6-bisphosphate (F2,6BP) increased by 4.5 fold 24 h after slicing. However, maximal PFP activity determined with saturating (1 microM) F2,6BP was not changed. The Ka values of PFP for F2,6BP was lowered from 33 to 7 nM after 24 h of aging treatment. This increased susceptibility of the PFP activity to its allosteric activator, F2,6BP, may be involved in the increased respiration in wounded disks of potato tubers. Immunoblotting experiments indicated that both the alpha (66 kDa) and the beta (60 kDa) subunits of PFP were present in fresh or 24 h aged tuber slices.