Kazuko Oba

The occurrence and some properties of 3-hydroxy-3-methylglutaryl coenzyme A reductase in sweet potato roots infected by Ceratocystis fimbriata☆☆☆

Abstract Fresh sweet potato ( Ipomoea batatas , Lam. cv. Norin 1) root tissue had a very low activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase and the activity increased only slightly after the tissue was sliced and incubated. However, when infected by Ceratocystis fimbriata Ell. and Halst., enzyme activity increased rapidly, and reached a maximum in 2 days, thereafter, the activity decreased rapidly. Formation of terpenes such as ipomeamarone followed the increase in enzyme activity, and both changes showed a parallel relationship, indicating that the enzyme participates in the formation of terpenes such as ipomeamarone in response to the infection. Optimal activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase occurred at pH 7·3 to 7·5. The relationship between reaction rate and S -3-hydroxy-3-methylglutaryl coenzyme A concentration indicated substrate inhibition at a concentration above 250 μ m . The Lineweaver-Burk plot was non-linear and the Hill coefficient was 0·81. The differential centrifugation and sucrose density gradient centrifugation showed high enzyme activity associated with microsomes, and lesser activity with mitochondria.

The fate of the phytoalexin ipomeamarone: furanoterpenes and butenolides from Ceratocystis fimbriata-infected sweet potatoes

Abstract Nine new sesquiterpenes related biosynthetically to ipomeamarone, the well-known sweet potato phytoalexin, have been isolated from Ceratocystis fimbriata -infected sweet potato root tissue and the structures determined. Their biosynthetic relationship with previously identified furanoterpenes is discussed.

Enzymatic reduction of dehydroipomeamarone to ipomeamarone in sweet potato root tissue infected by Ceratocystis fimbriata

Abstract The activity of the enzyme that catalyses the reduction of dehydroipomeamarone to ipomeamarone was detected mainly in the 105 000 g precipitated fraction of the extract from Ceratocystis fimbriata-infected sweet potato root tissue. The enzyme in this fraction showed optimal pH in the range of 7·5 to 8·0, and the Km value of the enzyme for dehydroipomeamarone was 45·0 μ m . The enzyme required NADPH for activity. The SH-group of the enzyme participated in the activity. According to the substrate competing experiment, it appeared that the enzyme might also catalyse the reduction of 4-hydroxydehydromyoporone to 4-hydroxymyoporone. The enzyme was not present in fresh tissue, but formed in adjacent non-infected tissue in response to C. fimbriata inoculation, with a lag of about 12 h, and showed peak activity around the second day, followed by the accumulation of furano-sesquiterpenes such as ipomeamarone. The enzyme was also formed in response to cut injury, although the activity appeared later and was less than in C. fimbriata-infected tissue.

Metabolism of ipomeamarone in sweet potato root slices before and after treatment with mercuric chloride or infection with Ceratocystis fimbriata

Abstract Furanoterpenes produced in sweet potato ( Ipomoea batatas ) root tissue in response to fungal infection or injury with mercuric chloride were metabolized to compounds negative to Ehrlichs reagent. The metabolism of ipomeamarone in sweet potato root tissue was enhanced in response to cut injury. The enhanced metabolism of ipomeamarone in the cut tissue was partly prevented by HgCl 2 treatment. The initial metabolite of ipomeamarone was identified as ipomeamaronol, 15-hydroxyipomeamarone. A time-course analysis of the metabolism of [ 14 C]ipomeamarone by cut tissue indicated that ipomeamarone was converted to ipomeamaronol, which was further metabolized to unknown compounds.

Synthesis of poly(A)+ RNA and poly(A)− RNA in slices of sweet potato root

Abstract Poly(A) + RNA and poly(A) − RNA from sweet potato root tissue were examined for translational activity using a wheat germ cell-free system. The activity of poly(A) + RNA was 20 times higher than that of poly(A) - RNA. Time course studies of in vivo radioactive labelling in tissue slices indicated the appearance of two peaks of poly(A) + RNA synthesis, the first at ca 6 hr after slicing and the second at about ca 24 hr, and one of poly(A) − RNA synthesis at ca 12 hr. The synthetic rates of both poly(A) + RNA and poly(A) − RNA at 24 hr were not changed by subsequent treatment with mercuric chloride. In the 6 hr incubation period, i.e. the period of the highest increase in poly(A) + RNA labeling, no significant change in the content of poly(A) + RNA occurred, although the poly(A) - RNA content increased. Larger MW populations of poly(A) + RNA were synthesized during the early stage (6 hr) of incubation after slicing, but smaller MW populations were formed during the later stage (24 hr) of incubation.