POTATO (SOLANUM-TUBEROSUM VAR DESIREE) TUBER 5-LIPOXYGENASE SELECTIVITY FOR THE PHYSICOCHEMICAL PROPERTIES OF LINOLEIC-ACID

The dependence of potato 5-lipoxygenase (LOX) activity on the physicoc hemical properties of linoleic acid (LA) was studied. pH and beta-cycl odextrin (beta-CD) were used as tools to investigate the effect of the physicochemical state of LA on LOX kinetic properties in vitro. The L A concentration dependence of LOX activity was best fitted by using th e Hill equation. It was found that the decrease of LOX activity at hig h pH corresponds to a pK(a) lower than the pK(a) of LA; thus, such dec rease was assigned to some ionizable side chain group of LOX related t o the active center. At a fixed LA concentration, the presence of beta -CD led to a decrease in the LOX reaction rate, which was due to its e ffect on K and the Hill constant since V-max was not affected. Experim ents in the presence of beta-CD revealed that LA monomers were also us ed as substrate, although less efficiently than aggregates. The differ ent activities exhibited against monomers and aggregates is the reason for the observed apparent substrate cooperativity, which can be inter preted as an aggregate-induced enzyme activation. The effect of beta-C D on LOX activity could be explained on the basis of the specific inte raction between LA and beta-CD and the equations derived for such inte raction developed in a previous work.