Front Matter

Other methods such as microemulsion systems (Piazza, 1992) have been devel-

oped to overcome the low solubility of PUFAs in water, but do not offer great benefits

over the other methods described in Sections 16.2.3–16.2.5.

16.2.6 Soybean lipoxygenase-1 use: some conclusions

It has long been the impression of workers in the field of LOXs that to conduct

lipoxygenation, the PUFA substrate must be dissolved in the reaction medium. How-

ever, based on studies performed on ‘under-pressure lipoxygenation’, this is clearly

not the case, as LA concentrations as high as 0.1 M could be used. At these con-

centrations, we are dealing with dispersed ‘pieces of soap’ rather than with dis-

solved substrates. Nevertheless, a near-quantitative conversion of the PUFA in

HPOD could be achieved, a finding which could be explained as follows. It is ne-

cessary to maintain a sufficient quantity of dissolved oxygen in the medium to allow

an optimal catalytic activity of the enzyme during the entire course of the reaction,

thus lowering substrate inhibition (Berry et al., 1997; 1998). Indeed, it is well known

that in the absence of oxygen, SBLOX-1 can catalyze an anaerobic reaction between

its substrate and its product. This reaction is thought to generate radicals that are

deleterious to the enzyme, leading to its irreversible inactivation. Thus, correct oxy-

genation of the reaction medium is vital in order to conduct high substrate concen-

tration lipoxygenation. As seen in Sections 16.2.2 and 16.2.3, this can be achieved

either by oxygen bubbling or pressurization combined with strong agitation. Hence,

while correct oxygenation is maintained, the enzyme will maintain its optimal cat-

alytic activity, with side chain reactions being reduced and product specificity en-

hanced. Another key to the success of high substrate concentration lipoxygenation is

the finding that the hydroperoxides formed act as surfactants, thus solubilizing in-

creasing amounts of substrate as the reaction proceeds. The optimal basic pH (9–10)

of SBLOX-1 is another advantage, as fatty acids are more soluble as salts in aqueous

environments.

High substrate concentration lipoxygenation is now a well-established method for

the synthesis of n-6(S)-PUFA-HPODs, on a multigram scale using SBLOX-1 as

catalyst, as long as a sufficient oxygenation is maintained during the course of

the reaction. In the following Sections, the diversity of substrate and product spe-

cificities shown by LOXs that render them attractive biocatalysts in the lipid bio-

technological field will be discussed further.

16.2 How to use lipoxygenases 343