Some years later, the Vliegenthart group re-examined the double dioxygenation of
AA (Van Os et al., 1981). Along with the already reported formation of 8(S),15(S)-
diHPETE (60 %) (Figure 8), they found anotherbis-hydroperoxide whose structure
was established as 5(S),15(S)-diHPETE (40 %). A classical explanation given for
this apparently forbidden double dioxygenation is the fact that at more acidic
pH, the carboxylic acid function is at least transiently under its protonated form
allowing an inverse orientation of the substrate in the active site of the enzyme lead-
ing to oxygenation at position 5 or 8 (Figure 9).
If the reaction is left for longer (2 h), then small amounts of conjugated tetraenic
material identified as lipoxin A and B could be detected whose origin is probably a
further transformation of 5(S),15(S)-diHPETE (Sok et al., 1988).
Use of unnatural PUFAs as soybean LOX-1 substrates
Despite its narrow substrate specificity, SBLOX-1 combines numerous advantages
prone to seduce synthetic organic chemists (i.e. very high regio-, stereo- and enan-
tiospecificity). Indeed, some authors have prepared unnatural substrates to take ad-
vantages of both the specificity of the enzyme and the possibilities of organic synth-
esis.
For example, in 1989, Zhang and Kyler synthesized a series of methylene inter-
rupted (Z,Z)-dienic alcohols which upon esterification with adipic acid monomer
became a substrate of SBLOX-1 (Zhang and Kyler, 1989). After reduction of the
HPODs formed and hydrolysis, various chiral dienols were obtained which repre-
sented valuable chiral synthons (Figure 10).
All the tested compounds were substrates of SBLOX-1, the reaction times varying
from 1 h ( 8 )to24h( 7 ). The substrate:enzyme ratio (w/w) used ranged from 0.06 ( 1 )
to 3.0 ( 3 ), the regiospecificity (A:B) varied from 99 : 1 ( 7 , 8 )to77:23( 5 ), and the
enantiomeric excess of the major regioisomer (A) from 96 % ee ( 2 ) to more than
99 % ee ( 3 ).
16.3 Substrate and product specificities of lipoxygenases 347
Figure 9. Substrate orientation used to explain the double dioxygenation catalyzed by SBLOX-1
(Gardner, 1991; Lehman, 1994).