STEREOSPECIFICITY OF (-PINORESINOL AND (+)-LARICIRESINOL REDUCTASES FROM FORSYTHIA-INTERMEDIA())

Pinoresinol/lariciresinol reductase catalyzes the first known example of a highly unusual benzylic ether reduction in plants; its mechanism of hydride transfer is described. The enzyme was found in Forsythia in termedia and catalyzes the presumed regulatory branch-points in the pa thway leading to benzylaryltetrahydrofuran, dibenzylbutane, dibenzylbu tyrolactone, and aryltetrahydronaphthalene lignans. Using [7,7'-H-2(2) ]-pinoresinol and [7,7'-H-2(3)]lariciresinol as substrates, the hydrid e transfers of the highly unusual reductase were demonstrated to be co mpletely stereospecific (>99%). The incoming hydrides were found to ta ke up the pro-R position at C-7' (and/or C-7) in lariciresinol and sec oisolariciresinol, thereby eliminating the possibility of random hydri de delivery to a planar quinone methide intermediate. As might be expe cted, the mode of hydride abstraction from NADPH was also stereospecif ic: using [4R-H-3] and [4S-H-3]NADPH, it was found that only the 4 pro -R hydrogen was abstracted for enzymatic hydride transfer.