微生物轉換 Steviol 及 ent-16β-hydroxybeyeran-19-oic acid
Stevioside is a sweet glycoside commercially extracted from the leaves of Stevia rebaudiana. It is now used as a natural sweet ening agent in Japan. For the biological activity, stevio-side could lower blood pressure in spontaneously hypertensive rats. St eviol (1), the aglycone part of stevioside, is one of the major metabolites of stevioside during its enzymatic hydrolysis. It coul d stimulate insulin secretion via a direct action on β cells. ent-16β-Hydroxybeyeran-19-oic acid (3) is from the reduction of is osteviol (2). In our preliminary experiment, ent-16β-hydroxybeyeran-19-oic acid has the ability to lower blood pressure in spo ntaneously hypertensive rats. Steviol and ent-16β-hydroxybeyeran-19-oic acid are with different skeletons of tetracyclic diterp enoids. As a part of an ongoing program to study the bioconversion of diterpenoids with different skeletons by microorganism s, the microbial meta-bolism of these two diterpenoids is investigated.
By screening twenty-seven microorganisms, Bacillus megaterium, Mucor recurvatus, Cunninghamella elegans and Asper-gill us niger were selected for the preparative-scale microbial transformations of these two diterpenoids. Microbial transfor-matio ns of steviol produced ent-7α,13-dihydroxykaur-16-en-19-oic acid (4), ent-13,16β,17-trihydroxykauran-19-oic acid (5), ent-1 3-hydroxy-7-ketokaur-16-en-19- oic acid (6), ent-7α,11β,13-trihydroxykaur-16-en-19-oic acid (7), and ent-13-hydroxy- kaur- 16-en-19-β-D-glucopyranosyl ester (8). Microbial transfor-mations of ent-16β-hydroxybeyeran-19-oic acid produced ent-7α,1 6β-dihydroxybeyeran-19-oic acid (9), ent-7α-hydroxy-16-ketobeyeran-19-oic acid (10), ent-1β,7α-dihydroxy-16-keto- beyera n-19-oic acid (11), ent-1b,7α,16β-trihydroxybeyeran-19-oic acid (12), ent-16β-hydroxybeyeran-19-β-D-glucopyranosyl ester (13) and ent-7α,16β-dihydroxybeyeran-19-β-D-gluco-pyranosyl ester (14). Among them, metabolites 7, 12, 13, and 14 are the new compounds. The structures of metabolites are estab-lished on the basis of HRFABMS, 1D and 2D NMR, and enzymatic hydrolysis.
Utilizing microbial transformation as in vitro models to study the metabolism of steviol and ent-16β-hydroxybeyeran-19-oic a
cid, we find that the hydroxylation at 7β position is a common occurrence for the microbes with kaurene and beyerane skeleto
ns. Glucosidation is also found in the microbial transformations of both substrates by B. megaterium. These isolated metabolit
es will be used as reference standards for monitoring our continuing studies on the mammalian metabolism of steviol and ent-
16β-hydroxybeyeran-19-oic acid.
Microbial Transformations of Steviol and ent-16β-hydroxybeyeran- 19-oic acid
