Zhonghua Jia

MECHANISM AND STRUCTURAL REQUIREMENTS FOR TRANSFORMATION OF SUBSTRATES BY THE (S)-ADENOSYL-L-METHIONINE : DELTA 24(25)-STEROL METHYL TRANSFERASE FROM SACCHAROMYCES CEREVISIAE

The mechanism of action and active site of the enzyme (S)-adenosyl-L-methionine:delta 24(25)-sterol methyl transferase (SMT) from Saccharomyces cerevisiae strain GL7 have been probed with AdoMet, (S)-adenosyl-L-homocysteine, a series of 35 sterol substrates as acceptor molecules and a series of 10 substrate and high energy intermediate (HEI) sterol analogues as inhibitors of biomethylation. The SMT was found to be selective for sterol, both regio- and stereochemically. The presence of an unhindered 24,25-bond, an equatorially-oriented polar group at C-3 (which must act as a proton acceptor) attached to a planar nucleus and a freely rotating side chain were obligatory structural features for sterol binding/catalysis; no essential requirement or significant harmful effects could be found for the introduction of an 8(9)-bond, 14 alpha-methyl or 9 beta,19-cyclopropyl group. Alternatively, methyl groups at C-4 prevented productive sterol binding to the SMT. Initial velocity, product inhibition, and dead-end experiments demonstrated a rapid-equilibrium random bi bi mechanism. Deuterium isotope effects developed from SMT assays containing mixtures of [3-3H]zymosterol with AdoMet or [methyl-2H3]AdoMet confirmed the operation of a random mechanism, kappa H/kappa D = 1.3. From this combination of results, the spatial relationship of the sterol substrate to AdoMet could be approximated and the topology of the sterol binding to the SMT thereby formulated.

Triterpene glycosides from the bark of Anthocephalus cadamba

Two triterpenoid glycosides, glycosides A and B were isolated from the bark of Anthocephalus cadamba and defined as 3-O-[α-L-rhamnopyranosyl]-quinovic acid-28-O-[β-D-glucopyranosyl] ester and 3-O-[β-D-glucopyranosyl]-quinovic acid-28-O-[β-D-glucopyranosyl] ester respectively.

Phytosterol biosynthesis: Isotope effects associated with biomethylation formation to 24-alkene sterol isomers

Abstract Changes in 24-alkene sterol product distribution resulting from deuterium substitution on the coenzyme methyl group of AdoMet and on the sterol acceptor molecule at C-23 and C-24 were used to determine kinetic isotope effects for the terminating deprotonations involved in sterol biomethylation catalyzed by (S)-adenosyl-L-methionine-Δ 24 -sterol methyl transferase (SMT) enzyme. By this method 24(28)-methylene cycloartanol and cyclosadol were shown to be synthesized by two different SMT enzymes.

Antifungal Sterol Biosynthesis Inhibitors

During the course of the last decade, the development of SBIs, and particularly sterol biomethylation inhibitors, has been based on the rational design approach. Successful though this approach has been in elucidating sterol biomethylation enzymology, its limitations are becoming apparent from the findings that: (i) 24,25-double bond metabolism gives rise to cholesterol and ergosterol in a mechanistically similar manner, (ii) 25-azasterols are harmful to human physiology, and (iii) side-chain modified sterols designed to inhibit the SMT enzyme in S. cerevisiae may be ineffective or operate by another kinetic mechanism in a related organism, rendering it therapeutically nonuseful. Nevertheless, it may be possible to ultimately capitalize on the unique aspects of sterol biomethylation chemistry and enzymology to design taxa-specific inhibitors. With increased understanding of the structure and function of SMT enzymes in different fungi, it should be possible to prepare novel mechanism-based inactivators to control SMT activity uniquely and with high specific activity.

Synthesis of Rationally Designed Mechanism-Based Inactivators of the (S)-Adenosyl-L-methionine: Δ24(25)− Sterol Methyl Transferase

Abstract A series of 26,27-cyclopropylidine side chain modified sterols were prepared for the first time from the known aldehydes by Wittig olefination with the ylide from cyclopropyltriphenylphosphonium bromide in butyllithium. Two novel by-products were detected; sterols with nine carbon side chains which lack the terminal isopropyl group and with double bonds in positions 23,24 or 24,25. The structures of the compounds were confirmed by a combination of chromatographic (GLC and HPLC) and spectral (IR, 1H, 13C-NMR) methods. #Visiting Professor from the Department of Pharmacognosy, Beijing Medical University, Beijing 100083, China

A Minor Acylated Triterpenoid Saponin from the Seeds of Pithecellobium dulce

A novel acylated triterpenoid saponin, designated pithecelloside, is isolated from the seeds of Pithecellobium dulce and characterised as 3-O-[α-L-arabinopyranosyl-(1 → 2)-α-L-arabinopyranosyl-(1 → 6)-β-D-glucopyranosyl]-21β-O-[(2′E)-6′-hydroxy-2′,6′-dimethylocta-2′,7′-dienoyl]acacic acid 1.