Inhibition of soybean lipoxygenase-1 by a diaryl-N-hydroxyurea by reduction of the ferric enzyme

An unusual isotope effect on substrate inhibition in the oxidation of arachidonic acid by lipoxygenase

Soybean lipoxygenase catalyzes the oxidation of arachidonic acid to 15S-HPETE. The reaction displays strong substrate inhibition with unlabeled substrate but no discernible substrate inhibition with arachidonic acid labeled with deuterium at C13, the site of hydrogen/deuterium atom abstraction. The unusual behavior is due primarily to a large kinetic isotope effect on Km,O2 as a result of the strong selection against deuterium in the abstraction step.

Caseins and casein hydrolysates. 1. Lipoxygenase inhibitory properties

Whole casein from bovine origin, the different casein subtypes alpha, beta, and kappa, and the related dephosphorylated proteins were assayed as modulators of soybean lipoxygenase 1 activity and were found to inhibit it. To define the lipoxygenase inhibitory domain, whole casein and beta-casein were digested by proteases (trypsin, clostripain, and subtilisin). The beta-casein tryptic digest and the tryptic and subtilisin digests of whole casein retained their inhibitory properties. The tryptic beta-casein digest was the most potent inhibitor of lipoxygenase activity and was further fractionated by FPLC or HPLC. The collected peptides inhibited the lipoxygenase-catalyzed reaction to different extents. The active fractions were analyzed by ESI-MS, and the sequences of several lipoxygenase inhibitory peptides, corresponding mainly to the C-terminal moiety of beta-casein, were identified.